Review your Application Architecture Today !


Application architecture review can be defined as reviewing the current security controls in the application architecture. This helps a user to identify potential security flaws at an early stage and mitigate them before starting the development stage. Poor design of architecture may expose the application to many security loopholes. It is preferable to perform the architecture review at the design stage, as the cost and effort required for implementing security after development is high.

This document can be considered as the secure design guideline for the architects or as a checklist for penetration tester to perform application architecture review as a part of the overall security assessment.

The following diagram shows some of the primary issues that must be addressed at the designing stage.

While doing the architecture review we can primarily focus on the following areas:

  1. Application Architecture Documents
  2. Deployment and Infrastructure Considerations
  3. Input Validation
  4. Authentication
  5. Authorization
  6. Configuration Management
  7. Session Management
  8. Cryptography
  9. Parameter Manipulation
  10. Exception Management
  11. Auditing & Logging
  12. Application Framework and Libraries

Additional category or points under any category can be added as per the requirement. Let’s have a look at each area separately:

Application Architecture Documents:

The first thing to look for is the availability of the application architecture document. Every application should have a properly documented architecture diagram with a high-level explanation of the above points and a network connectivity diagram showing how different component are placed and secured.

Deployment and Infrastructure Considerations:

Review the infrastructure on which the application is deployed. This can include reviewing the network, system, infrastructure performance monitoring, etc.

Some of the points which should be taken into consideration are as follows:

  • Components required for the application: What is the OS supporting the application, hardware requirement, etc.?
  • Restrictions applied on the firewall: Review the firewall policies defined for the application. What type of traffic is allowed and what type of traffic is blocked?
  • Port and Service requirement: An application may communicate with other application as well. Identify which ports and services are required to be open for the application.
  • Component Segregation: Components of the application should be segregated from each other. For example, the application server and database server should not reside in the same machine.
  • Disable clear text protocol: Ports running clear text services like (HTTP, FTP, etc.) should be closed and not used for any part of the application.

Input Validation

Weak input validation is one of the main reasons for application security weakness. Proper input validation can help in preventing many attacks like Cross-Site Scripting, SQL Injection, etc. The validation should be applied on every input field (including the hidden form field) of all the pages. The best practice is to use a centralized approach.

Some of the points which should be taken into consideration are as follows:

  • Mechanism to validate the user inputs: Check if the application is validating the user input or processing the input as it is.
  • Bypassing the validation: Check how the user input is being validated. Is it possible to bypass the validation, for example, encoding the input? Identify if input validation depends on the application framework. Check whether there is any vulnerability in the framework through which a user can bypass the validation.
  • Centralized approach: If custom implementation to validate the user input is present, check whether the approach is centralized.
  • Validating across all the tiers: As a best practice, validation should be applied on all the layers, i.e. business layer, data layer, etc.
  • Addressing SQL Injection issue: Input validation helps in mitigating SQL Injection issue to some extent. Check whether the application is safe against the SQL Injection vulnerability by using parameterized query in the back end.


Authentication is the act of verifying a user’s identity. In the application, it is achieved by providing the username and password. Weak authentication mechanism can result in bypassing the login process and accessing the application. This can lead to a major compromise. The application should be designed with strong authentication.

Some of the points which should be taken into consideration are as follows:

  • Authentication control on the server side: Make sure the credentials are verified at the server side and not on the client side. Client-side validation can be easily bypassed.
  • Secure channel for authentication: Login credentials should always be sent through an encrypted channel. Credentials going through a clear text channel can be easily sniffed by the attacker.
  • Check whether the login page is served over HTTP protocol. Check whether the application can be accessible on any other port where SSL certificate is not implemented.
  • Change password page: Check whether the Old Password field is present in the Change Password page and verified as well.
  • Strong password policy: An application should be configured to accept strong passwords only. Weak passwords can be brute forced easily.
  • Authentication cookie: Check if SSL is implemented on an entire application and the authentication cookie is not sent in clear text on any page.
  • Service account: Service account is an account under which a service application is running. A service account is required for the application and communicating with the database should have a restrictive set of privilege.
  • Default framework password: Many application framework comes with a default password. Make sure the password is changed to a non-guessable strong password.


Authorization determines which resources can be accessed by the authenticated user. Weak authorization control can lead to Privileges Escalation attacks.

Some of the points which should be taken into consideration are as follows:

  • Privilege escalation and spoofing: Privilege escalation happens when a user gets access to more resources than they are allowed or when the user can perform additional actions than they are allowed. Check the controls present when the user tries to escalate his/her privilege through manipulating the request or by directly accessing the unauthorized page/resource.
  • Direct object reference: Check whether the application provides direct access to the objects based on the user-supplied input. This may allow the attacker to access the resources belonging to other users by bypassing the authorization. For example, downloading the invoices/statement of other users.

Configuration Management

Weak configuration should be avoided. Any sensitive information being stored in the configuration file can be extracted by the attacker.

Some of the points which should be taken into consideration are as follows:

  • Secure Hardening: Make sure all the components required for the application are updated and the latest patches are applied on them. The default configuration should be changed wherever possible.
  • Sensitive Data: Sensitive data like database connection string, encryption key, admin credentials or any other secret should not be stored as clear text in the code. Check whether the configuration file is secured against the unauthorized access.
  • Persistent cookie: Storing sensitive data as plaintext in a persistent cookie should be avoided. The user can see and modify the clear text data. Check if the application is storing the clear text data in a persistent cookie.
  • Passing sensitive data using GET protocol: The GET protocol sends the data in the query string. Sensitive information going in a GET request can be accessed from the browser history or logs.
  • Disable unused methods: Verify that the application accepts only GET and POST methods. Other methods like TRACE, PUT, DELETE, etc. should be disabled.
  • Sensitive data over HTTP: Communication between the components, such as between application server and database server, should be encrypted.

Session Management

The session is the track of the user activities. Strong session management plays an important role in the overall security of the application. Weakness in the session management may lead to serious attacks.

Some of the points which should be taken into consideration are as follows:

  • Use framework’s default session management: Custom session management can have multiple vulnerabilities. Ensure there is no custom manager is being used and the application framework’s default session management is used.
  • Ensure session management best practices are followed:
    • The session ID is random, long and unique.
    • The session is invalidating after logout.
    • The session ID is changing on successful authentication and re-authentication.
    • The session ID is not going in URL.
    • The session is timed out after a certain period of inactivity.
    • The session ID is going insecure channel like SSL.
    • Check whether cookie attributes (HttpOnly, Secure, path and domain) are secured.


Applications frequently use cryptography to secure the stored data or to protect the data in transit over an insecure channel.

Some of the points which should be taken into consideration are as follows:

  • Custom Implementation: Designing a dedicated encryption mechanism may lead to weaker protection. Secure cryptographic service provided by the platform should be used. Check what type of encryption is being used in the application.
  • Encryption Key management: Check whether there is any policy on encryption key management, i.e., Key generation, distribution, deletion, and expiration.
  • Securing encryption keys: Encryption keys are used as an input to encrypt or decrypt the data. If encryption keys are compromised, the encrypted data can be decrypted and hence, will be no longer be secure.
  • Key recycle policy: The key should be recycled after a certain period. Using the same key for a long time is not a safe practice.

Parameter Manipulation

With Parameter Manipulation attacks, the attacker modifies the data going from application to the web server. This can result in unauthorized access to services.

Some of the points which should be taken into consideration are as follows:

  • Validate all inputs from the client: Validation applied on the client side may reduce the load on the server but relying only on client-side validation is not a safe practice. Client-side validation can be bypassed using a proxy tool. Check whether validation is applied on the server as well.
  • Do not rely on HTTP header: Security decision in the application should not be based on the HTTP header. If the application is serving any page by checking only the “referrer” header, then an attacker can bypass this by changing the header in a proxy tool.
  • Encrypt the cookie: Cookies can have data that is being used on the server to authorize the user. This type of data should be protected against unauthorized manipulation attacks.
  • Sensitive data in view state: View state in an ASP.NET application can have sensitive data that is required to take the authorization decision on the server. Data in view state can be tempered if MAC (message authentication code) is not enabled. Check whether view state is protected using MAC.

Exception Management

Insecure exception handling can expose the valuable information, which can be used by the attacker to fine-tune his attack. Without exception management, information such as stack trace, framework details, server details, SQL query, internal path and other sensitive information can be exposed. Check whether centralized exception management is in place, with minimum information being displayed.

Auditing & Logging

Log files contain the record of the events. These events can be a success or failed login attempt, data retrieval, modification, deletion, network communication, etc. The logs should be monitored in real time.

Some of the points which should be taken into consideration are as follows:

  • Logging enabled: Check if logging is enabled for application and platform level as well.
  • Log events: Logs should be generated for all security-level important events like successful and failed authentication, data access, modification, network access, etc. The log should include time of the event, user identity, location with machine name, etc. Identify which events are being logged.
  • Logging sensitive data: An application should not log sensitive data like user credentials, password hashes, credit card details, etc.
  • Storage, security, and analysis:
    • The log file should be stored on a different partition than the one on which the application is running. The log file should be copied and moved to permanent storage for retention.
    • The log files must be protected against the unauthorized access, modification or deletion.
    • The log file should be analyzed on a regular interval.

Application Framework and Libraries

Make sure that the application framework and libraries are up to date and relevant patches are applied on them. Verify that there is no default password being used in the framework (admin/admin, tomcat/tomcat, etc.). Check whether the old or vulnerable framework is in use.


The above points represent the key areas for secure designing of the application. Implementing these points at the designing stage can reduce the overall cost and effort to secure the application. If the application is already deployed, secure architecture review is an important part of the overall security assessment and can help in fixing the existing vulnerabilities and improving the future design.

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What is Merkle Tree in Blockchain?

What’s A Merkle Tree?

If you’re a newcomer to the blockchain world, you may have come across the phrase “Merkle Tree” and felt a little lost. While Merkle Trees are not a widely-understood concept, they’re also not terribly complicated.

So, what’s a Merkle Tree? To put it very simply, a Merkle Tree is a method of structuring data that allows a large body of information to be verified for accuracy extremely quickly and efficiently.

Since Merkle Trees are such a crucial component of blockchain technology, it’s worth gaining an in-depth understanding of them. This post will help you do just that. Let’s get started.

All About Merkle Trees

The story of Merkle Trees begins way back in 1979 with a person named Ralph Merkle. While in grad school at Stanford University, Merkle wrote an academic paper called “A Certified Digital Signature.” In this essay, Merkle described a new, extremely efficient method of creating proofs. In other words, he designed a process for verifying data that would allow computers to do their work much, much faster than ever before.

Merkle’s idea, now better known as a Merkle Tree, revolutionised the world of cryptography and, by extension, the way that encrypted computer protocols function. In fact, Merkle Trees are mentioned repeatedly in Satoshi Nakamoto’s 2008 essay  that introduced Bitcoin to the world. They’re also used extensively in Bitcoin’s foundational code.

So, what exactly are Merkle Trees? First, it’s important to note that each transaction on a blockchain has its own unique transaction ID. With most blockchains, each transaction ID is a 64-character code (SHA2 Hash) that takes up 256 bits (32 bytes) of memory.

When you consider that blockchains are typically made up of hundreds of thousands of blocks, with each block containing as many as several thousand transactions, you can imagine how quickly memory space becomes a problem.

As such, it’s optimal to use as little data as possible when processing and verifying transactions. This minimises CPU processing times while also ensuring the highest level of security.

Well, that’s exactly what Merkle Trees do. To put it very simply, Merkle Trees take a huge number of transaction IDs and put them through a mathematical process that results in a single, 64-character code (SHA2 Hash).

This code is extremely important because it allows any computer to quickly verify that a specific transaction took place on a particular block as efficiently as possible. This code is called the Merkle Root.

What’s A Merkle Root?

The single code that a Merkle Tree produces is called the Merkle Root. Each block in a blockchain has exactly one. And, as we just mentioned, the Merkle Root is a crucial piece of data because it allows computers to verify information with incredible speed and efficiency.

Let’s dive a little deeper. How is a Merkle Root produced? The first step is to organise all of the data inputs.

Merkle Trees, by design, always group all of the inputs into pairs. If there is an odd number of inputs, the last input is copied and then paired with itself. This holds true for all the transaction IDs written onto a block of a blockchain.


EvenNumber: 12345678 == 11223344 & 55667788

OddNumber: 1234567 == 12345677 == 11223344 & 55667777

For instance, let’s suppose that a single block contains a total of 512 transactions. The Merkle Tree would begin by grouping those 512 transactions IDs into 256 pairs. Then, those 256 pairs of transaction IDs would go through a mathematical process, called a hashing function or hashing algorithm, that would result in 256 new, 64-character alphanumeric codes.

The same exact process would occur again. Those 256 new codes would be paired up and turned into 128 codes. The process would be repeated, cutting the number of codes in half each time, until only a single code remained. That single code is our Merkle Root.

An Example Of A Merkle Tree

To make this concept clear, let’s look at a very simple example of a Merkle Tree. Imagine that there were 8 transactions performed on one particular block. In reality, transaction IDs are 64 characters long, but for the sake of simplicity, let’s pretend that they’re only 8 characters long. To make things even easier, let’s use only numbers (and ignore letters altogether).

So, in this example, our eight transactions IDs will be:

  • 11111111
  • 22222222
  • 33333333
  • 44444444
  • 55555555
  • 66666666
  • 77777777
  • 88888888

Now let’s suppose that the method for hashing transaction IDs together is to take the first, third, fifth, and seventh digits from each of the two IDs being combined, and then simply push those numbers together to form a new, 8-digit code.

Of course, in reality, the mathematics behind hashing algorithms is far more complicated than this. But for this simple demonstration, this elementary system will suffice.

This is what our Merkle Tree would look like:

Notice that the number of codes is cut in half each step down the Merkle Tree. We start with 8 transaction IDs and, after just 3 steps, end up with a single code—the Merkle Root. In this example, our Merkle Root is the code in the bottom box: 12345678.

The primary benefit of Merkle Trees is that they allow extremely quick verification of data. If we want to validate a single transaction ID, we wouldn’t need to double-check every single transaction on the block. Rather, we would only need to verify that particular “branch” of our Merkle Tree.

Efficiency And Speed: The Benefits Of Merkle Trees

Let’s suppose that we want to validate a transaction ID in our current example. Suresh says that he paid Praveen a certain sum of Bitcoin and tells us that the transaction ID is 88888888. He also sends us 3 hashes: 77777777, 55556666, and 11223344. That’s all the info that needs to be sent or received to verify Suresh’s payment to Praveen.

These three hashes, along with the transaction ID in question and this particular block’s Merkle Root, are the only data needed to verify Suresh’s payment to Praveen. This is far less data than what would be required to verify the entire Merkle Tree. As a result, the verification process is much faster and far more efficient for everyone.

Here’s how it works. We already have the block’s Merkle Root, so Suresh doesn’t need to send us that. He sends us his transaction ID and the 3 additional hashes we listed above. He also sends a tiny bit of information about the order and placement in which to use the hashes. Now, all we have to do is run the hashing algorithm on the set of data Suresh provided.

We start by hashing the first code 77777777 with the transaction ID 88888888, which gives us the result 77778888. Suresh didn’t send us this code but he didn’t need to because we’re using the same hashing algorithm as him. Therefore, we receive the exact same results.

We then take the second code Suresh sent us, 55556666, and hash it with the new code 77778888 we just derived. This, of course, produces the number 55667788.

Finally, we hash the third code Suresh gave us, 11223344, with the other new code we received, 55667788, and we end up with the correct Merkle Root: 12345678.

Notice that we only need 3 codes from Suresh and only had to run the hashing algorithm three times to see that Suresh’s transaction is valid. That means our computer has done less than half the work that would’ve been required to verify the entire Merkle Tree. The original Merkle Tree diagram has 15 numbers and the hashing algorithm needs to be run 7 times. But more than half of that tree isn’t necessary to verify Suresh’s transaction!

This procedure is sufficient to verify that Suresh did, in fact, pay Praveen that certain sum of Bitcoin because we derived numbers that, when hashed together with the other codes Suresh sent us, produced the same Merkle Root that we already knew to be true for this particular block.

Suresh can’t fake a transaction because that would require finding a fake transaction ID and an additional set of fake codes that, when put through the hashing function, would produce the true Merkle Root. The chances of this happening are so astronomically small that we can confidently say it’s impossible.

In this simple example, the savings of computing power might not seem substantial. However, when you consider that blocks in a blockchain might contain several thousand transactions, it’s easy to see how Merkle Trees increase efficiency so dramatically.

In short, that’s the main benefit of a Merkle Tree. It allows computers to verify information extremely efficiently and with far less data than what would be required without the Merkle Tree.

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The RIGHT way of Password Hashing !

If you’re a web developer, you’ve probably had to make a user account system. The most important aspect of a user account system is how user passwords are protected. User account databases are hacked frequently, so you absolutely must do something to protect your users’ passwords if your website is ever breached. The best way to protect passwords is to employ salted password hashing. This page will explain why it’s done the way it is.

There are a lot of conflicting ideas and misconceptions on how to do password hashing properly, probably due to the abundance of misinformation on the web. Password hashing is one of those things that’s so simple, but yet so many people get wrong. With this page, I hope to explain not only the correct way to do it, but why it should be done that way.

IMPORTANT WARNING: If you are thinking of writing your own password hashing code, please don’t!. It’s too easy to screw up. No, that cryptography course you took in university doesn’t make you exempt from this warning. This applies to everyone: DO NOT WRITE YOUR OWN CRYPTO! The problem of storing passwords has already been solved. Use either use either phpass, the PHP, C#, Java, and Ruby implementations in defuse/password-hashing, or libsodium.

If for some reason you missed that big red warning note, please go read it now. Really, this guide is not meant to walk you through the process of writing your own storage system, it’s to explain the reasons why passwords should be stored a certain way

What is password hashing?

hash("hello") = 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
hash("hbllo") = 58756879c05c68dfac9866712fad6a93f8146f337a69afe7dd238f3364946366
hash("waltz") = c0e81794384491161f1777c232bc6bd9ec38f616560b120fda8e90f383853542

Hash algorithms are one way functions. They turn any amount of data into a fixed-length “fingerprint” that cannot be reversed. They also have the property that if the input changes by even a tiny bit, the resulting hash is completely different (see the example above). This is great for protecting passwords, because we want to store passwords in a form that protects them even if the password file itself is compromised, but at the same time, we need to be able to verify that a user’s password is correct.

The general workflow for account registration and authentication in a hash-based account system is as follows:

  1. The user creates an account.
  2. Their password is hashed and stored in the database. At no point is the plain-text (unencrypted) password ever written to the hard drive.
  3. When the user attempts to login, the hash of the password they entered is checked against the hash of their real password (retrieved from the database).
  4. If the hashes match, the user is granted access. If not, the user is told they entered invalid login credentials.
  5. Steps 3 and 4 repeat every time someone tries to login to their account.

In step 4, never tell the user if it was the username or password they got wrong. Always display a generic message like “Invalid username or password.” This prevents attackers from enumerating valid usernames without knowing their passwords.

It should be noted that the hash functions used to protect passwords are not the same as the hash functions you may have seen in a data structures course. The hash functions used to implement data structures such as hash tables are designed to be fast, not secure. Only cryptographic hash functions may be used to implement password hashing. Hash functions like SHA256, SHA512, RipeMD, and WHIRLPOOL are cryptographic hash functions.

It is easy to think that all you have to do is run the password through a cryptographic hash function and your users’ passwords will be secure. This is far from the truth. There are many ways to recover passwords from plain hashes very quickly. There are several easy-to-implement techniques that make these “attacks” much less effective. To motivate the need for these techniques, consider this very website. On the front page, you can submit a list of hashes to be cracked, and receive results in less than a second. Clearly, simply hashing the password does not meet our needs for security.

The next section will discuss some of the common attacks used to crack plain password hashes.

How Hashes are Cracked

  • Dictionary and Brute Force Attacks

    Dictionary Attack

    Trying apple        : failed
    Trying blueberry    : failed
    Trying justinbeiber : failed
    Trying letmein      : failed
    Trying s3cr3t       : success!
    Brute Force Attack

    Trying aaaa : failed
    Trying aaab : failed
    Trying aaac : failed
    Trying acdb : failed
    Trying acdc : success!
  • The simplest way to crack a hash is to try to guess the password, hashing each guess, and checking if the guess’s hash equals the hash being cracked. If the hashes are equal, the guess is the password. The two most common ways of guessing passwords are dictionary attacks and brute-force attacks.A dictionary attack uses a file containing words, phrases, common passwords, and other strings that are likely to be used as a password. Each word in the file is hashed, and its hash is compared to the password hash. If they match, that word is the password. These dictionary files are constructed by extracting words from large bodies of text, and even from real databases of passwords. Further processing is often applied to dictionary files, such as replacing words with their “leet speak” equivalents (“hello” becomes “h3110”), to make them more effective.A brute-force attack tries every possible combination of characters up to a given length. These attacks are very computationally expensive, and are usually the least efficient in terms of hashes cracked per processor time, but they will always eventually find the password. Passwords should be long enough that searching through all possible character strings to find it will take too long to be worthwhile.There is no way to prevent dictionary attacks or brute force attacks. They can be made less effective, but there isn’t a way to prevent them altogether. If your password hashing system is secure, the only way to crack the hashes will be to run a dictionary or brute-force attack on each hash.
  • Lookup Tables

    Searching: 5f4dcc3b5aa765d61d8327deb882cf99: FOUND: password5
    Searching: 6cbe615c106f422d23669b610b564800:  not in database
    Searching: 630bf032efe4507f2c57b280995925a9: FOUND: letMEin12 
    Searching: 386f43fab5d096a7a66d67c8f213e5ec: FOUND: mcd0nalds
    Searching: d5ec75d5fe70d428685510fae36492d9: FOUND: p@ssw0rd!

    Lookup tables are an extremely effective method for cracking many hashes of the same type very quickly. The general idea is to pre-compute the hashes of the passwords in a password dictionary and store them, and their corresponding password, in a lookup table data structure. A good implementation of a lookup table can process hundreds of hash lookups per second, even when they contain many billions of hashes.

    If you want a better idea of how fast lookup tables can be, try cracking the following sha256 hashes with CrackStation’s free hash cracker.

  • Reverse Lookup Tables

    Searching for hash(apple) in users' hash list...     : Matches [alice3, 0bob0, charles8]
    Searching for hash(blueberry) in users' hash list... : Matches [usr10101, timmy, john91]
    Searching for hash(letmein) in users' hash list...   : Matches [wilson10, dragonslayerX, joe1984]
    Searching for hash(s3cr3t) in users' hash list...    : Matches [bruce19, knuth1337, john87]
    Searching for hash(z@29hjja) in users' hash list...  : No users used this password

    This attack allows an attacker to apply a dictionary or brute-force attack to many hashes at the same time, without having to pre-compute a lookup table.

    First, the attacker creates a lookup table that maps each password hash from the compromised user account database to a list of users who had that hash. The attacker then hashes each password guess and uses the lookup table to get a list of users whose password was the attacker’s guess. This attack is especially effective because it is common for many users to have the same password.

  • Rainbow Tables

    Rainbow tables are a time-memory trade-off technique. They are like lookup tables, except that they sacrifice hash cracking speed to make the lookup tables smaller. Because they are smaller, the solutions to more hashes can be stored in the same amount of space, making them more effective. Rainbow tables that can crack any md5 hash of a password up to 8 characters long exist.

Next, we’ll look at a technique called salting, which makes it impossible to use lookup tables and rainbow tables to crack a hash.

Adding Salt

hash("hello")                    = 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
hash("hello" + "QxLUF1bgIAdeQX") = 9e209040c863f84a31e719795b2577523954739fe5ed3b58a75cff2127075ed1
hash("hello" + "bv5PehSMfV11Cd") = d1d3ec2e6f20fd420d50e2642992841d8338a314b8ea157c9e18477aaef226ab
hash("hello" + "YYLmfY6IehjZMQ") = a49670c3c18b9e079b9cfaf51634f563dc8ae3070db2c4a8544305df1b60f007

Lookup tables and rainbow tables only work because each password is hashed the exact same way. If two users have the same password, they’ll have the same password hashes. We can prevent these attacks by randomizing each hash, so that when the same password is hashed twice, the hashes are not the same.

We can randomize the hashes by appending or prepending a random string, called a salt, to the password before hashing. As shown in the example above, this makes the same password hash into a completely different string every time. To check if a password is correct, we need the salt, so it is usually stored in the user account database along with the hash, or as part of the hash string itself.

The salt does not need to be secret. Just by randomizing the hashes, lookup tables, reverse lookup tables, and rainbow tables become ineffective. An attacker won’t know in advance what the salt will be, so they can’t pre-compute a lookup table or rainbow table. If each user’s password is hashed with a different salt, the reverse lookup table attack won’t work either.

In the next section, we’ll look at how salt is commonly implemented incorrectly.

The WRONG Way: Short Salt & Salt Reuse

The most common salt implementation errors are reusing the same salt in multiple hashes, or using a salt that is too short.

Salt Reuse

A common mistake is to use the same salt in each hash. Either the salt is hard-coded into the program, or is generated randomly once. This is ineffective because if two users have the same password, they’ll still have the same hash. An attacker can still use a reverse lookup table attack to run a dictionary attack on every hash at the same time. They just have to apply the salt to each password guess before they hash it. If the salt is hard-coded into a popular product, lookup tables and rainbow tables can be built for that salt, to make it easier to crack hashes generated by the product.

A new random salt must be generated each time a user creates an account or changes their password.

Short Salt

If the salt is too short, an attacker can build a lookup table for every possible salt. For example, if the salt is only three ASCII characters, there are only 95x95x95 = 857,375 possible salts. That may seem like a lot, but if each lookup table contains only 1MB of the most common passwords, collectively they will be only 837GB, which is not a lot considering 1000GB hard drives can be bought for under $100 today.

For the same reason, the username shouldn’t be used as a salt. Usernames may be unique to a single service, but they are predictable and often reused for accounts on other services. An attacker can build lookup tables for common usernames and use them to crack username-salted hashes.

To make it impossible for an attacker to create a lookup table for every possible salt, the salt must be long. A good rule of thumb is to use a salt that is the same size as the output of the hash function. For example, the output of SHA256 is 256 bits (32 bytes), so the salt should be at least 32 random bytes.

The WRONG Way: Double Hashing & Wacky Hash Functions

This section covers another common password hashing misconception: wacky combinations of hash algorithms. It’s easy to get carried away and try to combine different hash functions, hoping that the result will be more secure. In practice, though, there is very little benefit to doing it. All it does is create interoperability problems, and can sometimes even make the hashes less secure. Never try to invent your own crypto, always use a standard that has been designed by experts. Some will argue that using multiple hash functions makes the process of computing the hash slower, so cracking is slower, but there’s a better way to make the cracking process slower as we’ll see later.

Here are some examples of poor wacky hash functions I’ve seen suggested in forums on the internet.

md5(md5(salt) + md5(password))
sha1(str_rot13(password + salt))
md5(sha1(md5(md5(password) + sha1(password)) + md5(password)))

Do not use any of these.

Note: This section has proven to be controversial. I’ve received a number of emails arguing that wacky hash functions are a good thing, because it’s better if the attacker doesn’t know which hash function is in use, it’s less likely for an attacker to have pre-computed a rainbow table for the wacky hash function, and it takes longer to compute the hash function.

An attacker cannot attack a hash when he doesn’t know the algorithm, but note Kerckhoffs’s principle, that the attacker will usually have access to the source code (especially if it’s free or open source software), and that given a few password-hash pairs from the target system, it is not difficult to reverse engineer the algorithm. It does take longer to compute wacky hash functions, but only by a small constant factor. It’s better to use an iterated algorithm that’s designed to be extremely hard to parallelize (these are discussed below). And, properly salting the hash solves the rainbow table problem.

If you really want to use a standardized “wacky” hash function like HMAC, then it’s OK. But if your reason for doing so is to make the hash computation slower, read the section below about key stretching first.

Compare these minor benefits to the risks of accidentally implementing a completely insecure hash function and the interoperability problems wacky hashes create. It’s clearly best to use a standard and well-tested algorithm.

Hash Collisions

Because hash functions map arbitrary amounts of data to fixed-length strings, there must be some inputs that hash into the same string. Cryptographic hash functions are designed to make these collisions incredibly difficult to find. From time to time, cryptographers find “attacks” on hash functions that make finding collisions easier. A recent example is the MD5 hash function, for which collisions have actually been found.

Collision attacks are a sign that it may be more likely for a string other than the user’s password to have the same hash. However, finding collisions in even a weak hash function like MD5 requires a lot of dedicated computing power, so it is very unlikely that these collisions will happen “by accident” in practice. A password hashed using MD5 and salt is, for all practical purposes, just as secure as if it were hashed with SHA256 and salt. Nevertheless, it is a good idea to use a more secure hash function like SHA256, SHA512, RipeMD, or WHIRLPOOL if possible.

The RIGHT Way: How to Hash Properly

This section describes exactly how passwords should be hashed. The first subsection covers the basics—everything that is absolutely necessary. The following subsections explain how the basics can be augmented to make the hashes even harder to crack.

The Basics: Hashing with Salt

Warning: Do not just read this section. You absolutely must implement the stuff in the next section: “Making Password Cracking Harder: Slow Hash Functions”.

We’ve seen how malicious hackers can crack plain hashes very quickly using lookup tables and rainbow tables. We’ve learned that randomizing the hashing using salt is the solution to the problem. But how do we generate the salt, and how do we apply it to the password?

Salt should be generated using a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG). CSPRNGs are very different than ordinary pseudo-random number generators, like the “C” language’s rand() function. As the name suggests, CSPRNGs are designed to be cryptographically secure, meaning they provide a high level of randomness and are completely unpredictable. We don’t want our salts to be predictable, so we must use a CSPRNG. The following table lists some CSPRNGs that exist for some popular programming platforms.

Platform CSPRNG
PHP mcrypt_create_ivopenssl_random_pseudo_bytes
Dot NET (C#, VB) System.Security.Cryptography.RNGCryptoServiceProvider
Ruby SecureRandom
Python os.urandom
Perl Math::Random::Secure
C/C++ (Windows API) CryptGenRandom
Any language on GNU/Linux or Unix Read from /dev/random or /dev/urandom

The salt needs to be unique per-user per-password. Every time a user creates an account or changes their password, the password should be hashed using a new random salt. Never reuse a salt. The salt also needs to be long, so that there are many possible salts. As a rule of thumb, make your salt is at least as long as the hash function’s output. The salt should be stored in the user account table alongside the hash.

To Store a Password

  1. Generate a long random salt using a CSPRNG.
  2. Prepend the salt to the password and hash it with a standard password hashing function like Argon2, bcrypt, scrypt, or PBKDF2.
  3. Save both the salt and the hash in the user’s database record.

To Validate a Password

  1. Retrieve the user’s salt and hash from the database.
  2. Prepend the salt to the given password and hash it using the same hash function.
  3. Compare the hash of the given password with the hash from the database. If they match, the password is correct. Otherwise, the password is incorrect.

In a Web Application, always hash on the server

If you are writing a web application, you might wonder where to hash. Should the password be hashed in the user’s browser with JavaScript, or should it be sent to the server “in the clear” and hashed there?

Even if you are hashing the user’s passwords in JavaScript, you still have to hash the hashes on the server. Consider a website that hashes users’ passwords in the user’s browser without hashing the hashes on the server. To authenticate a user, this website will accept a hash from the browser and check if that hash exactly matches the one in the database. This seems more secure than just hashing on the server, since the users’ passwords are never sent to the server, but it’s not.

The problem is that the client-side hash logically becomes the user’s password. All the user needs to do to authenticate is tell the server the hash of their password. If a bad guy got a user’s hash they could use it to authenticate to the server, without knowing the user’s password! So, if the bad guy somehow steals the database of hashes from this hypothetical website, they’ll have immediate access to everyone’s accounts without having to guess any passwords.

This isn’t to say that you shouldn’t hash in the browser, but if you do, you absolutely have to hash on the server too. Hashing in the browser is certainly a good idea, but consider the following points for your implementation:

  • Client-side password hashing is not a substitute for HTTPS (SSL/TLS). If the connection between the browser and the server is insecure, a man-in-the-middle can modify the JavaScript code as it is downloaded to remove the hashing functionality and get the user’s password.
  • Some web browsers don’t support JavaScript, and some users disable JavaScript in their browser. So for maximum compatibility, your app should detect whether or not the browser supports JavaScript and emulate the client-side hash on the server if it doesn’t.
  • You need to salt the client-side hashes too. The obvious solution is to make the client-side script ask the server for the user’s salt. Don’t do that, because it lets the bad guys check if a username is valid without knowing the password. Since you’re hashing and salting (with a good salt) on the server too, it’s OK to use the username (or email) concatenated with a site-specific string (e.g. domain name) as the client-side salt.

Making Password Cracking Harder: Slow Hash Functions

Salt ensures that attackers can’t use specialized attacks like lookup tables and rainbow tables to crack large collections of hashes quickly, but it doesn’t prevent them from running dictionary or brute-force attacks on each hash individually. High-end graphics cards (GPUs) and custom hardware can compute billions of hashes per second, so these attacks are still very effective. To make these attacks less effective, we can use a technique known as key stretching.

The idea is to make the hash function very slow, so that even with a fast GPU or custom hardware, dictionary and brute-force attacks are too slow to be worthwhile. The goal is to make the hash function slow enough to impede attacks, but still fast enough to not cause a noticeable delay for the user.

Key stretching is implemented using a special type of CPU-intensive hash function. Don’t try to invent your own–simply iteratively hashing the hash of the password isn’t enough as it can be parallelized in hardware and executed as fast as a normal hash. Use a standard algorithm like PBKDF2 or bcrypt. You can find a PHP implementation of PBKDF2 here.

These algorithms take a security factor or iteration count as an argument. This value determines how slow the hash function will be. For desktop software or smartphone apps, the best way to choose this parameter is to run a short benchmark on the device to find the value that makes the hash take about half a second. This way, your program can be as secure as possible without affecting the user experience.

If you use a key stretching hash in a web application, be aware that you will need extra computational resources to process large volumes of authentication requests, and that key stretching may make it easier to run a Denial of Service (DoS) attack on your website. I still recommend using key stretching, but with a lower iteration count. You should calculate the iteration count based on your computational resources and the expected maximum authentication request rate. The denial of service threat can be eliminated by making the user solve a CAPTCHA every time they log in. Always design your system so that the iteration count can be increased or decreased in the future.

If you are worried about the computational burden, but still want to use key stretching in a web application, consider running the key stretching algorithm in the user’s browser with JavaScript. The Stanford JavaScript Crypto Library includes PBKDF2. The iteration count should be set low enough that the system is usable with slower clients like mobile devices, and the system should fall back to server-side computation if the user’s browser doesn’t support JavaScript. Client-side key stretching does not remove the need for server-side hashing. You must hash the hash generated by the client the same way you would hash a normal password.

Impossible-to-crack Hashes: Keyed Hashes and Password Hashing Hardware

As long as an attacker can use a hash to check whether a password guess is right or wrong, they can run a dictionary or brute-force attack on the hash. The next step is to add a secret key to the hash so that only someone who knows the key can use the hash to validate a password. This can be accomplished two ways. Either the hash can be encrypted using a cipher like AES, or the secret key can be included in the hash using a keyed hash algorithm like HMAC.

This is not as easy as it sounds. The key has to be kept secret from an attacker even in the event of a breach. If an attacker gains full access to the system, they’ll be able to steal the key no matter where it is stored. The key must be stored in an external system, such as a physically separate server dedicated to password validation, or a special hardware device attached to the server such as the YubiHSM.

I highly recommend this approach for any large scale (more than 100,000 users) service. I consider it necessary for any service hosting more than 1,000,000 user accounts.

If you can’t afford multiple dedicated servers or special hardware devices, you can still get some of the benefits of keyed hashes on a standard web server. Most databases are breached using SQL Injection Attacks, which, in most cases, don’t give attackers access to the local filesystem (disable local filesystem access in your SQL server if it has this feature). If you generate a random key and store it in a file that isn’t accessible from the web, and include it into the salted hashes, then the hashes won’t be vulnerable if your database is breached using a simple SQL injection attack. Don’t hard-code a key into the source code, generate it randomly when the application is installed. This isn’t as secure as using a separate system to do the password hashing, because if there are SQL injection vulnerabilities in a web application, there are probably other types, such as Local File Inclusion, that an attacker could use to read the secret key file. But, it’s better than nothing.

Please note that keyed hashes do not remove the need for salt. Clever attackers will eventually find ways to compromise the keys, so it is important that hashes are still protected by salt and key stretching.

Other Security Measures

Password hashing protects passwords in the event of a security breach. It does not make the application as a whole more secure. Much more must be done to prevent the password hashes (and other user data) from being stolen in the first place.

Even experienced developers must be educated in security in order to write secure applications. A great resource for learning about web application vulnerabilities is The Open Web Application Security Project (OWASP). A good introduction is the OWASP Top Ten Vulnerability List. Unless you understand all the vulnerabilities on the list, do not attempt to write a web application that deals with sensitive data. It is the employer’s responsibility to ensure all developers are adequately trained in secure application development.

Having a third party “penetration test” your application is a good idea. Even the best programmers make mistakes, so it always makes sense to have a security expert review the code for potential vulnerabilities. Find a trustworthy organization (or hire staff) to review your code on a regular basis. The security review process should begin early in an application’s life and continue throughout its development.

It is also important to monitor your website to detect a breach if one does occur. I recommend hiring at least one person whose full time job is detecting and responding to security breaches. If a breach goes undetected, the attacker can make your website infect visitors with malware, so it is extremely important that breaches are detected and responded to promptly.

Frequently Asked Questions

What hash algorithm should I use?

DO use:

DO NOT use:

  • Fast cryptographic hash functions such as MD5, SHA1, SHA256, SHA512, RipeMD, WHIRLPOOL, SHA3, etc.
  • Insecure versions of crypt ($1$, $2$, $2x$, $3$).
  • Any algorithm that you designed yourself. Only use technology that is in the public domain and has been well-tested by experienced cryptographers.

Even though there are no cryptographic attacks on MD5 or SHA1 that make their hashes easier to crack, they are old and are widely considered (somewhat incorrectly) to be inadequate for password storage. So I don’t recommend using them. An exception to this rule is PBKDF2, which is frequently implemented using SHA1 as the underlying hash function.

How should I allow users to reset their password when they forget it?

It is my personal opinion that all password reset mechanisms in widespread use today are insecure. If you have high security requirements, such as an encryption service would, do not let the user reset their password.

Most websites use an email loop to authenticate users who have forgotten their password. To do this, generate a random single-use token that is strongly tied to the account. Include it in a password reset link sent to the user’s email address. When the user clicks a password reset link containing a valid token, prompt them for a new password. Be sure that the token is strongly tied to the user account so that an attacker can’t use a token sent to his own email address to reset a different user’s password.

The token must be set to expire in 15 minutes or after it is used, whichever comes first. It is also a good idea to expire any existing password tokens when the user logs in (they remembered their password) or requests another reset token. If a token doesn’t expire, it can be forever used to break into the user’s account. Email (SMTP) is a plain-text protocol, and there may be malicious routers on the internet recording email traffic. And, a user’s email account (including the reset link) may be compromised long after their password has been changed. Making the token expire as soon as possible reduces the user’s exposure to these attacks.

Attackers will be able to modify the tokens, so don’t store the user account information or timeout information in them. They should be an unpredictable random binary blob used only to identify a record in a database table.

Never send the user a new password over email. Remember to pick a new random salt when the user resets their password. Don’t re-use the one that was used to hash their old password.

What should I do if my user account database gets leaked/hacked?

Your first priority is to determine how the system was compromised and patch the vulnerability the attacker used to get in. If you do not have experience responding to breaches, I highly recommend hiring a third-party security firm.

It may be tempting to cover up the breach and hope nobody notices. However, trying to cover up a breach makes you look worse, because you’re putting your users at further risk by not informing them that their passwords and other personal information may be compromised. You must inform your users as soon as possible—even if you don’t yet fully understand what happened. Put a notice on the front page of your website that links to a page with more detailed information, and send a notice to each user by email if possible.

Explain to your users exactly how their passwords were protected—hopefully hashed with salt—and that even though they were protected with a salted hash, a malicious hacker can still run dictionary and brute force attacks on the hashes. Malicious hackers will use any passwords they find to try to login to a user’s account on a different website, hoping they used the same password on both websites. Inform your users of this risk and recommend that they change their password on any website or service where they used a similar password. Force them to change their password for your service the next time they log in. Most users will try to “change” their password to the original password to get around the forced change quickly. Use the current password hash to ensure that they cannot do this.

It is likely, even with salted slow hashes, that an attacker will be able to crack some of the weak passwords very quickly. To reduce the attacker’s window of opportunity to use these passwords, you should require, in addition to the current password, an email loop for authentication until the user has changed their password. See the previous question, “How should I allow users to reset their password when they forget it?” for tips on implementing email loop authentication.

Also tell your users what kind of personal information was stored on the website. If your database includes credit card numbers, you should instruct your users to look over their recent and future bills closely and cancel their credit card.

What should my password policy be? Should I enforce strong passwords?

If your service doesn’t have strict security requirements, then don’t limit your users. I recommend showing users information about the strength of their password as they type it, letting them decide how secure they want their password to be. If you have special security needs, enforce a minimum length of 12 characters and require at least two letters, two digits, and two symbols.

Do not force your users to change their password more often than once every six months, as doing so creates “user fatigue” and makes users less likely to choose good passwords. Instead, train users to change their password whenever they feel it has been compromised, and to never tell their password to anyone. If it is a business setting, encourage employees to use paid time to memorize and practice their password.

If an attacker has access to my database, can’t they just replace the hash of my password with their own hash and login?

Yes, but if someone has accesss to your database, they probably already have access to everything on your server, so they wouldn’t need to login to your account to get what they want. The purpose of password hashing (in the context of a website) is not to protect the website from being breached, but to protect the passwords if a breach does occur.

You can prevent hashes from being replaced during a SQL injection attack by connecting to the database with two users with different permissions. One for the ‘create account’ code and one for the ‘login’ code. The ‘create account’ code should be able to read and write to the user table, but the ‘login’ code should only be able to read.

Why do I have to use a special algorithm like HMAC? Why can’t I just append the password to the secret key?

Hash functions like MD5, SHA1, and SHA2 use the Merkle–Damgård construction, which makes them vulnerable to what are known as length extension attacks. This means that given a hash H(X), an attacker can find the value of H(pad(X) + Y), for any other string Y, without knowing X. pad(X) is the padding function used by the hash.

This means that given a hash H(key + message), an attacker can compute H(pad(key + message) + extension), without knowing the key. If the hash was being used as a message authentication code, using the key to prevent an attacker from being able to modify the message and replace it with a different valid hash, the system has failed, since the attacker now has a valid hash of message + extension.

It is not clear how an attacker could use this attack to crack a password hash quicker. However, because of the attack, it is considered bad practice to use a plain hash function for keyed hashing. A clever cryptographer may one day come up with a clever way to use these attacks to make cracking faster, so use HMAC.

Should the salt come before or after the password?

It doesn’t matter, but pick one and stick with it for interoperability’s sake. Having the salt come before the password seems to be more common.

Why does the hashing code on this page compare the hashes in “length-constant” time?

Comparing the hashes in “length-constant” time ensures that an attacker cannot extract the hash of a password in an on-line system using a timing attack, then crack it off-line.

The standard way to check if two sequences of bytes (strings) are the same is to compare the first byte, then the second, then the third, and so on. As soon as you find a byte that isn’t the same for both strings, you know they are different and can return a negative response immediately. If you make it through both strings without finding any bytes that differ, you know the strings are the same and can return a positive result. This means that comparing two strings can take a different amount of time depending on how much of the strings match.

For example, a standard comparison of the strings “xyzabc” and “abcxyz” would immediately see that the first character is different and wouldn’t bother to check the rest of the string. On the other hand, when the strings “aaaaaaaaaaB” and “aaaaaaaaaaZ” are compared, the comparison algorithm scans through the block of “a” before it determines the strings are unequal.

Suppose an attacker wants to break into an on-line system that rate limits authentication attempts to one attempt per second. Also suppose the attacker knows all of the parameters to the password hash (salt, hash type, etc), except for the hash and (obviously) the password. If the attacker can get a precise measurement of how long it takes the on-line system to compare the hash of the real password with the hash of a password the attacker provides, he can use the timing attack to extract part of the hash and crack it using an offline attack, bypassing the system’s rate limiting.

First, the attacker finds 256 strings whose hashes begin with every possible byte. He sends each string to the on-line system, recording the amount of time it takes the system to respond. The string that takes the longest will be the one whose hash’s first byte matches the real hash’s first byte. The attacker now knows the first byte, and can continue the attack in a similar manner on the second byte, then the third, and so on. Once the attacker knows enough of the hash, he can use his own hardware to crack it, without being rate limited by the system.

It might seem like it would be impossible to run a timing attack over a network. However, it has been done, and has been shown to be practical. That’s why the code on this page compares strings in a way that takes the same amount of time no matter how much of the strings match.

How does the SlowEquals code work?

The previous question explains why SlowEquals is necessary, this one explains how the code actually works.

1.     private static boolean slowEquals(byte[] a, byte[] b)
2.     {
3.         int diff = a.length ^ b.length;
4.         for(int i = 0; i < a.length && i < b.length; i++)
5.             diff |= a[i] ^ b[i];
6.         return diff == 0;
7.     }

The code uses the XOR “^” operator to compare integers for equality, instead of the “==” operator. The reason why is explained below. The result of XORing two integers will be zero if and only if they are exactly the same. This is because 0 XOR 0 = 0, 1 XOR 1 = 0, 0 XOR 1 = 1, 1 XOR 0 = 1. If we apply that to all the bits in both integers, the result will be zero only if all the bits matched.

So, in the first line, if a.length is equal to b.length, the diff variable will get a zero value, but if not, it will get some non-zero value. Next, we compare the bytes using XOR, and OR the result into diff. This will set diff to a non-zero value if the bytes differ. Because ORing never un-sets bits, the only way diff will be zero at the end of the loop is if it was zero before the loop began (a.length == b.length) and all of the bytes in the two arrays match (none of the XORs resulted in a non-zero value).

The reason we need to use XOR instead of the “==” operator to compare integers is that “==” is usually translated/compiled/interpreted as a branch. For example, the C code “diff &= a == b” might compile to the following x86 assembly:

JZ equal
JMP done

The branching makes the code execute in a different amount of time depending on the equality of the integers and the CPU’s internal branch prediction state.

The C code “diff |= a ^ b” should compile to something like the following, whose execution time does not depend on the equality of the integers:


Why bother hashing?

Your users are entering their password into your website. They are trusting you with their security. If your database gets hacked, and your users’ passwords are unprotected, then malicious hackers can use those passwords to compromise your users’ accounts on other websites and services (most people use the same password everywhere). It’s not just your security that’s at risk, it’s your users’. You are responsible for your users’ security.

Source: Defuse Security

Hits: 382

Let’s Encrypt Wildcard SSL Certificate using CERTBOT

What is a Wildcard Certificate?

In computer networking, a wildcard certificate is a public key certificate which can be used with multiple subdomains of a domain. The principal use is for securing web sites with HTTPS, but there are also applications in many other fields. Compared with conventional certificates, a wildcard certificate can be cheaper and more convenient than a certificate for each subdomain.


A single wildcard certificate for https://* will secure all these subdomains on the domain:


Instead of getting separate certificates for subdomains, you can use a single certificate for all main domains and subdomains and reduce cost.

Because the wildcard only covers one level of subdomains (the asterisk doesn’t match full stops), these domains would not be valid for the certificate:


The “naked” domain is valid when added separately as a Subject Alternative Name (SubjectAltName):


Who is LetsEncrypt!

Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. It is a service provided by the Internet Security Research Group (ISRG).

Let’s Encrypt gives people the digital certificates they need in order to enable HTTPS (SSL/TLS) for websites, for free, in the most user-friendly way we can. We do this because we want to create a more secure and privacy-respecting Web.

The key principles behind Let’s Encrypt are:

  • Free: Anyone who owns a domain name can use Let’s Encrypt to obtain a trusted certificate at zero cost.
  • Automatic: Software running on a web server can interact with Let’s Encrypt to painlessly obtain a certificate, securely configure it for use, and automatically take care of renewal.
  • Secure: Let’s Encrypt will serve as a platform for advancing TLS security best practices, both on the CA side and by helping site operators properly secure their servers.
  • Transparent: All certificates issued or revoked will be publicly recorded and available for anyone to inspect.
  • Open: The automatic issuance and renewal protocol will be published as an open standard that others can adopt.
  • Cooperative: Much like the underlying Internet protocols themselves, Let’s Encrypt is a joint effort to benefit the community, beyond the control of any one organization.

More detailed information about how the Let’s Encrypt CA works.

What is Certbot?

Certbot is an easy-to-use automatic client that fetches and deploys SSL/TLS certificates for your webserver. Certbot was developed by EFF and others as a client for Let’s Encrypt and was previously known as “the official Let’s Encrypt client” or “the Let’s Encrypt Python client.” Certbot will also work with any other CAs that support the ACME protocol.

While there are many other clients that implement the ACME protocol to fetch certificates, Certbot is the most extensive client and can automatically configure your webserver to start serving over HTTPS immediately. For Apache, it can also optionally automate security tasks such as tuning ciphersuites and enabling important security features such as HTTP → HTTPS redirects, OCSP stapling, HSTS, and upgrade-insecure-requests.

Certbot is part of EFF’s larger effort to encrypt the entire Internet. Websites need to use HTTPS to secure the web. Along with HTTPS Everywhere, Certbot aims to build a network that is more structurally private, safe, and protected against censorship.

Certbot is the work of many authors, including a team of EFF staff and numerous open source contributors.

The Certbot privacy policy is described here.

Steps to generate Free Let’s Encrypt Wildcard SSL Certificate

Step#1: Install latest Certbot

$ wget
$ chmod a+x ./certbot-auto
$ sudo mv certbot-auto /usr/bin/certbot

Proceed to Step#2

Step#2: Generate the wildcard certificate with DNS Challenge (Eg. Domain: *

$ sudo certbot certonly \
--server \
--manual --preferred-challenges dns \
-d * -d

An important parameter to notice is --server, which will instruct the Certbot client to use v2 of the Let’s Encrypt API (we need that for wildcard certs). Also notice 2 domains, one is a wildcard and the second one is for TLD(Top Level Domain) as the wildcard does not cover TLD’s or records in subdomains (explained in the first section)

The Certbot client will walk you through the process of registering an account, and it will instruct you on what to do to complete the challenges.

Proceed to Step#3

Step#3: Create a DNS TXT Record as instructed in the “” DNS Zone File

Please deploy a DNS TXT record under the name with the following value:
Before continuing, verify the record is deployed.
Press Enter to Continue

Proceed to Step#4 for results

Step#4: Successful or Unsuccessful Messages

A Successful message would look like:

 - Congratulations! Your certificate and chain have been saved at:
 Your key file has been saved at:
 Your cert will expire on 2018-07-22. To obtain a new or tweaked
 version of this certificate in the future, simply run certbot
 again. To non-interactively renew *all* of your certificates, run
 "certbot renew"
 - If you like Certbot, please consider supporting our work by:

Donating to ISRG / Let's Encrypt:
 Donating to EFF:

Your certificate and chain have been saved at: /etc/letsencrypt/live/
Your key file has been saved at: /etc/letsencrypt/live/

An Unsuccessful message would look like:

Failed authorization procedure. (dns-01): urn:ietf:params:acme:error:unauthorized :: 
The client lacks sufficient authorization :: 
Incorrect TXT record "02HdxCLqTbjvjtO7mnxxxxXXxXxXHGZM2LlNXCSgOTQTzlp51ARngrBadcOnFigYvtv6SOg-BadcOnFigLts37Q0" 
found at


 - The following errors were reported by the server:
   Type:   unauthorized
   Detail: Incorrect TXT record
   found at
   To fix these errors, please make sure that your domain name was
   entered correctly and the DNS A/AAAA record(s) for that domain
   contain(s) the right IP address.

In such cases, please recheck your DNS TXT Record using DNS lookup tools like dig, nslookup etc. as shown below:

$ dig TXT

; <<>> DiG 9.10.6 <<>> TXT
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10301
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 4096



;; Query time: 33 msec
;; WHEN: Mon Apr 23 14:04:44 IST 2018
;; MSG SIZE  rcvd: 154

From the output above, it is clear that the TXT Record setup is not showing up the correct value as provided in Step#3. Hence re-run the tool and always ensure that the TXT Record should be setup as suggested for a Successful SSL Wildcard certificate generation.

Sources: Let’s Encrypt, Certbot

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Secure your API – Best Practices

Why API’s need special attention?

As an increasing number of organizations provide API access to make their information available to a wider audience, securing that access is likewise of increasing importance. With the growing adoption of cloud, mobile, and hybrid environments the risks are increasing. Cyber threats and DDoS attacks are targeting enterprise applications as back-end systems become more accessible. In such situations, the API can be a significant point of vulnerability with its ability to offer programmatic access to external developers.

The previous wave of security breaches occurred when organizations opened access to their web applications. Defences, such as web application firewalls (WAF), were introduced to mitigate those security breaches. However, these defences are not effective against all API attacks, and you’ll need to focus on security of your API interfaces.

API Security

The predominant API interface is the REST API, which is based on HTTP protocol, and generally JSON formatted responses. Securing your API interfaces has much in common with web access security, but present additional challenges due to:

  • Exposure to a wider range of data
  • Direct access to the back-end server
  • Ability to download large volumes of data
  • Different usage patterns

This topic has been covered in several sites such as OWASP REST Security, and we will summarise the main challenges and defences for API security.


The first defense is to identify the user, human or application, and determine that the user has permission to invoke your API. We are all familiar with entering a user ID and password, and possibly an additional identifier, to access a web interface. Corresponding for API, there is an API ID and an API key that are specified on the request to authenticate the user. The API key is a unique string generated for the application for each user of the API.

Access Control

Not all authenticated users will necessarily be authorized to access all provided APIs. For example, some users require access to retrieve (GET) information, but should not be able to change (PUT) any information. Using an access control framework, such as OAuth, you control the list of APIs that each specific API key can access.

To prevent a massive amount of API requests that can cause a DDoS attack or other misuse of the API service, apply a limit to the number of requests in a given time interval for each API. When the rate is exceeded, block access from the API key at least temporarily, and return the 429 (too many requests) HTTP error code.


To protect the request in transit, require HTTPS for all access so that the messages are secured and encoded with TLS.

To protect the JSON formatted response, consider employing the JSON Web Token (JWT) standard. As stated in that site,

JWT is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA.


As a best practice, we do not want to expose any more information than is required. For that reason, be careful that error messages do not reveal too much information.

Application Layer Attacks

Some actors can target your system after they circumvent your secured access and data interface. For example, they could obtain authorization credentials via phishing. You’ll need to validate all data to prevent application layer attacks, such as:

  • Cross-Site Scripting – Malicious scripts are injected into one of the request parameters
  • Code Injection – Inject valid code to services, such as SQL (SQL injection) or XQuery, to open the interface to user control
  • Business Logic Exploits –Allows the attacker to circumvent the business rules
  • Parameter Pollution Attacks – Exploit the data sent in the API request by modifying the parameters of the API request

Apply strict INPUT VALIDATION as you would on any interface, including:

  • Restrict, where possible, parameter values to a whitelist of expected values
  • To facilitate a whitelist, have strong typing of input value
  • Validate posted structures data against a formal schema language, to restrict the content and structure
  • Blacklist risky content, such as SQL schema manipulation statements

Log all API access. This is essential to assist resolving any issues. From these logs you can easily monitor activity and potentially discover any patterns or excessive usage activity.

Security Framework

Consider using an existing API framework that has many of the security features built in. If you must develop your own, separate out the security portion from the application portion. In this way, security is uniformly built in and developers can focus on the application logic.

After all that don’t neglect to allocate resources to test security of the APIs. Make sure to test all the defences mentioned in this article.


Keep Defending !

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Compile NGINX with NAXSI – Part#1

Why Re-Invent the Wheel ? ? ? ?

In this tutorial/walkthrough, I shall be providing you detailed instructions on how to compile and configure NAXSI on NGINX on Ubuntu 14.04 as the ubuntu standard repos have a very old version of NAXSI built NGINX  which I have personally found to be very buggy !


Part#1: Installation and basic configuration of NGINX-NAXSI

Part#2: Pumping the NGINX and NAXSI logs to ELASTICSEARCH

Part#3: Analyzing the logs and automated process of generating the false-positives and exclusions

Part#4: Conclusion


  • Ubuntu 14.04
  • GIT tools installed and setup on the server

Rest of the dependencies shall be provided below.

Step#1: Download Config Files

Ensure that you have the necessary tools handy that have been preconfigured to work with Ubuntu 14.04 from my GIT link:

cd /tmp
git clone

Step#2: Install Dependencies

As NAXSI and NGINX are being compiled from source, we will have to setup our server manually by installing all the dependencies below:

apt-get update
apt-get install automake gcc make pkg-config libtool g++ libfl-dev bison build-essential libbison-dev libyajl-dev liblmdb-dev libpcre3-dev libcurl4-openssl-dev libgeoip-dev libxml2-dev libyajl2 libxslt-dev openssl libssl-dev libperl-dev libgd2-xpm-dev

Step#3: Download and Setup

Run the following commands as given below. Ensure that the necessary permissions are given.

I have a habit of playing risky by running the commands as “sudo su” but it’s not really safe to play that risky on a production machine. Please follow your best standards to get things running on the server !

cd /usr/src 
tar -zxvf nginx-1.11.6.tar.gz 
git clone 
git clone 
cd /usr/src/nginx-1.11.6/ 
./configure --prefix=/etc/nginx \
 --add-module=/usr/src/naxsi/naxsi_src/ \
 --add-module=/usr/src/headers-more-nginx-module \
 --add-module=/usr/src/nginx-http-rdns/ \
 --with-cc-opt='-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2' \
 --with-ld-opt='-Wl,-Bsymbolic-functions -Wl,-z,relro' \
 --http-log-path=/var/log/nginx/access.log \
 --error-log-path=/var/log/nginx/error.log \
 --with-debug --with-pcre-jit --with-ipv6 \
 --with-http_ssl_module --with-http_stub_status_module \
 --with-http_realip_module \
 --with-http_addition_module \
 --with-http_dav_module \
 --with-http_geoip_module \
 --with-http_gzip_static_module \ --with-http_image_filter_module \
 --with-http_sub_module \
 --with-http_xslt_module \
 --with-mail \
 --with-mail_ssl_module \
 --http-client-body-temp-path=/tmp/client_body_temp \
 --http-proxy-temp-path=/tmp/proxy_temp \
 --http-fastcgi-temp-path=/tmp/fastcgi_temp \
 --http-uwsgi-temp-path=/tmp/uwsgi_temp \
make install

Step#4: Create/Copy Config files

Using the pre-configured steps provided in Step#1, we shall be now copying the files over to the correct locations as shown below:

cp /tmp/nginx_1.11.6-naxsi_latest/etc/init.d/nginx /etc/init.d/nginx
cp -r /tmp/nginx_1.11.6-naxsi_latest/nginx/conf/* /etc/nginx/conf/
ln -s /etc/nginx/sbin/nginx /usr/sbin/nginx
mkdir /etc/nginx/conf/sites-available
mkdir /etc/nginx/conf/sites-enabled
cp /usr/src/naxsi-master/naxsi_config/naxsi_core.rules /etc/nginx/conf/
mkdir /etc/nginx/conf/naxsi-whitelist/
touch /etc/nginx/conf/whitelist.conf

 Step#5: Configure your website on NGINX-NAXSI

You can use the sample configuration that can be found under “/tmp/nginx_1.11.6-naxsi_latest/nginx/sites-available” directory that has already been copied in the above step.

Edit the config file to match your requirements such as Site Name, Upstream IP/ Server etc. The sample has been provided below for quick reference:

server {
  listen 80 default_server;
  #listen [::]:80 default_server ipv6only=on;
  #root /var/www/nginx/html;
  #index index.html index.htm;
  # Make site accessible from http://localhost/
  server_name *; # Replace it with your website hostname. * is wildcard.
  set $naxsi_extensive_log 1;
  location / {
    # Uncomment to enable naxsi on this location
    include /etc/nginx/conf/naxsi.rules;
    include /etc/nginx/conf/naxsi-whitelist/*.rules;
    #try_files $uri $uri/ @rewrite;
    proxy_http_version 1.1;
    proxy_set_header Host $host;
    proxy_set_header Connection close;
    proxy_set_header X-Real-IP $remote_addr;
    # Comment the below line if there is already an upstream reverse proxy server that is setting the actual client IP
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;

Once the configuration is complete, run the command below to create a symlink of the config file in sites-enabled directory so that NGINX can be pick it up

cd /etc/nginx/conf/sites-enabled
ls -s ../sites-available/<virtual-host-config-file> .


With all the above steps, we are now ready to deploy our Web Application in a Alert-Only mode which start scanning our incoming web requests and starts generating events that trigger a lot of events.

In the upcoming second part, I shall be providing detailed steps on how to setup the logging for NGINX and NAXSI using Elasticsearch.

As always I say:

Keep Defending !

Hits: 251

The Fight with Fake Facebook Bots !


Daily Checked and Updated if Facebook modifies their list.

Am sure most of the web admins are tired with the FAKE Facebook Bots that set their User-Agents manually to facebook bots impersonating it and keep scraping the data

Hence here are the Lists for  IPv4 and IPv6 addresses in order to whitelist the real Facebook IP Addresses as per Facebook’s Documentation.

Please note that the list shall be checked and updated daily at 12:00PM IST (GMT + 5:30) if there is a change in the IP Lists, else you shall see how old the IP Addresses have not been changed from Facebook starting from the date of 1st check in on this Repo.

Link: GitHub Repo

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Secure SDLC


SDLC stands for Software Development LifeCycle.

WHY do we need to Secure it when it is already a global standard across any development cycle ? ? ?

Remember the quote?

A stitch in time, saves nine ! ! !

S-SDLC is a preventive process to avoid any security mishaps once a product is out there in the wild. The GOAL is to keep the application SAFE from breaches/attacks/hacks. Fixing a security vulnerability in a product which is already released is always a costly affair in terms of Brand, Revenue & Time!

Phase#1: SCOPING:

Gather as much information as possible in this phase regarding the feature in question. This helps us understand the business functionality much better so that no security measure in place shall affect the Business Requirements. Ultimately its our responsibility to deliver what Business wants. Do involve the key people in this phase like the Product Manager, Senior Management or even Application Developers

Phase#2: DESIGN:

In the Design phase, it is our responsibility to provide a flow which is secure by design. Think of the logical flow based attacks which purely attack the core logic of the feature. Ensure that sufficient checks can be placed at multiple places and the source of truth in core logics should not completely rely on the client side logic instead has some validation in place. For example, in a checkout flow for an e-commerce application, ensure that the client side cart value is not taken as the source of truth instead always keep that as a validation step but the source of truth should be from the merchant’s server for a payment gateway!

Phase#3: CODING:

Ensure that the best coding standards and practices are followed. As well perform static analysis of the code should be done at this phase and not at the next one so that the developers are not going through multiple cycles. For example, ensure that no passwords are being hardcoded in the code, instead define a strategy to pickup sensitive keys in runtime while building your application at the time of deployment and is always a part of the runtime memory. So even if a breach happens on the application server, the credentials/ keys are safe. Ensure that proper sanitizing wrappers are in place and there is always a service layer between your frontend application and critical infrastructure like Database which sanitizes/parametrizes the query inputs before executing them on the database server.

Please note that developer’s time is precious and this phase shall ensure that their time is not getting wasted in multiple redundant cycles!

Phase#4: TESTING:

The test cases for Security Testing should be pre-determined so that the RED Team once receives the build after QA Sanity, should start off with manual testing against the OWASP Top 10 vulnerabilities which ever relates to the feature. Ensure that there are proper regression tests also in place if it is an enhancement to an existing feature. Logical Flow testing is one of the most important tests in this phase and ensure that the flow developed is 100% complying to the flow proposed in Phase#2: Design. A proper VAPT should be done end-to-end and the results should be recorded in your bug/task tracking tools like JIRA for future regression testing references.

Always prioritize the Security Bugs w.r.t. the impact expected out of the issue identified the right way and ensure that the impact description is well understood by the stakeholders! Of course the bug bounty platforms have helped the bounty hunters and security researchers practice this step by providing a proper flow on their platforms. Try raising a Security Bug in BugCrowd!

If bugs are found, then get them fixed in a timely manner and DO ENSURE THAT YOU HAVE A SOLUTION for the bug raised beforehand because ultimately you are the who found the security bug and hence propose a secure solution regarding the same. For best practices and evasion techniques, please refer to the OWASP Top 10 Cheat Sheet and refer the same as a good standard while addressing vulnerabilities and requesting a fix.

Phase#5: DEPLOY:

This is the last phase in which we need to ensure that the environment in which the feature is getting deployed is secured. Ensure that the server has been hardened and the version being used is vulnerability free! As well do perform network assessment where and when required and try to avoid non-standard ports. Ensure that proper TLS Implementation is in place. A Security Architect with the right DevOps skills can help you in achieving the best results.

Once we are able to implement the above process, we can ensure that our applications that are getting deployed are well protected right from design itself. But remember:

Security is a Process, not a Product!

Ensure that you have sufficient monitoring and alerts in place. A proper Incident Response Team is well trained on the new feature and how to identify security incidents and attack attempts and how to identify them. Design a process on how to handle live attacks and ensure that there are sufficient tools and visibility provided to the monitoring teams like SOC

Keep Defending !

Hits: 53

Generate BKS Certificates for SSL Pinning in Android Apps


  1. Ensure that the Certificate against which we are generating the certificate is already loaded on the web server
  2. Generate the PKCS12 format Certificate from the PEM Certs using the command below:
    openssl pkcs12 -export -out cert-complete.pfx -inkey cert-key.pem -in cert-leaf.pem -certfile cert-chain.pem
    Enter Export Password:
    Verifying - Enter Export Password:
  3. Ensure that a strong password is set atleast of 12 Characters (alphanumeric + Special Chars)
  4. Download the tool “Portecle” from the Website
  5. Run the Tool
  6. Load the PKCS12 SSL certificate that was generated in the Step#2
  7. Enter the password that was set
  8. You shall see the certificate getting listed
  9. Highlight the entry, navigate to “Tools → Change Keystore Type → BKS”
  10. Enter the same password again
  11. If the Password is correct, you shall see the following message:
  12. Now Select the same entry, and Select the option “File → Save Keystore As”
  13. Save the file on the desired location after setting a password if prompted. ENSURE THAT THE EXTENSION OF THE FILE IS .bks

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Build your own DoS/DDoS/Bot Mitigation Gear & Fighting Fake Google/Yahoo/Bing/Apple Bots

HAPROXY(Rate Limiter + BadUser Detection)


Platforms used are completely Open Source. Please refer to their respective documentation from the respective links below:



In this article I shall be discussing on how can we build our own Service Protection Layer which shall include modules to protect our web application against DoS, DDoS, Bad Bots based on User-Agents and a Web Application Firewall.

Any additional layer in our web application architecture, when we introduce an extra service layer, the chances of increase in latency is high. Hence for best results always follow the below flow when you are building a Service Protection Layer in front of your firewall.

HAPROXY ———-StaticContent————> Web Application

HAPROXY —–DynamicContent —–> NGINX ——> Web Application

Using HAPROXY we shall perform:

Rate Limiting

User-Agent Detection (Bot Mitigation)

Using NGINX we shall perform:

Fake Google/Yahoo/Bing Bots


Please download the signatures from or feel free to write your own !

The art of detecting and blocking attacks is all about finding Signatures, Patterns and understanding the Attack Vector!

Rate Limiting: Rate Limiting is a technique used to limit a particular client from abusing our platform with over-whelming requests ensuring that the resources on the web application server are always available for the legitimate users. Rate Limiting for a web application should be gauged for 4 different parameters:

  1. Number of TCP Connections per client IP
  2. Rate of Incoming TCP Connections from a Client IP
  3. Rate of Incoming HTTP Request from a HTTP Client
  4. Rate of HTTP Errors getting generated by a HTTP Client

Note: Client at application/HTTP layer can be identified based on IP, Cookie, Parameter, IP+Cookie, IP+Cookie+User-Agent or any Other HTTP Header like Authorization etc.

When you are noticing a very huge traffic getting caught at Rate Controls, prepare yourself to start scaling your service protection later infra using techniques like AWS Auto-Scaling based on Network-In and Network-Out parameters at the instance level

Bad-bots: Most of the bots always leave a signature behind. User-Agent is one of the key-headers which detected the same. We shall be using the User-Agent comparison against a known bad bots list that can be downloaded from the git mentioned above

Fake Google/Yahoo/Bing/Applebots: Most of the web admins do not set any security controls on any of the friendly bots like mentioned above. Reason? Definitely if we block the above bots, our SEO, Visibility and Functionality also can take a hit as these are friendly bots and not bad-bots. How do they identify themselves? Connecting IP Address and User-Agent. All the above service providers clearly declare that they cannot publish their IP Address range as it keeps changing very dynamically and the only option left is to check the User-Agent which is consistent. But the challenge for a security administrator is that the User-Agent field is pretty much configurable and hence any client out there can start using the well known friendly bots User-Agent Signatures and cause harm to our application. Hence I adopted to the method of performing a reverse lookup approach for the connecting IP Address as suggested by Google! I am using the rDNS module to perform the connecting IP Address based on their User-Agents and when found they are fake are getting rejected automatically. Cool isn’t it?

GeoIP: ICANN distributes the IP Addresses all over the world and hence based on the same information, we can determine the country from which a client is getting connected and accessing our web application. What if we have been seeing a good amount of attacks from a specific country? As we cant keep mapping the IP Address, we can always opt for a Geo Blocking method by making NGINX Geo Aware using the GeoIP Module. Once done, we can choose which countries to be allowed explicitly and block others or vice-versa!

For the ease of administrators and not to keep this article very theoretical, I have provided config snippets below:

For any further queries please feel free to InMail me!

Happy Hunting folks!

HAPROXY Config Snippet:

In the Frontend Section:


# Capture the Rate Limit Headers and Actual Client IP in Logs

 capture request header X-Haproxy-ACL len 256

 capture request header X-Bad-User len 64

 capture request header X-Forwarded-For len 64

 capture request header User-Agent len 256

 capture request header Host len 32

 capture request header X-Geo len 8

 capture request header X-Google-Bot len 8

 capture request header Referer len 256

 # Do Not Rate Control Google based bots. Fake Google bot attacks shall be filtered at NGINX

 acl google-ua hdr(user-agent) -i -f <path-to-webclient-detection-directory>/google-ua.lst

 http-request add-header X-Google-Bot %[req.fhdr(X-Google-Bot,-1)]Goog-YES, if google-ua

 # Define a table that will store IPs associated with counter

 stick-table type ip size 500k expire 30s store conn_cur,conn_rate(3s),http_req_rate(10s),http_err_rate(10s)

 # Enable tracking of src IP in the sticktable - Secops

 tcp-request content track-sc0 src



acl sensitive-urls path -i /api/app/login path -i /api/app/otp path -i /api/app/forgotpass

 # Reject the new connection if the client already has 100 opened

 http-request add-header X-Haproxy-ACL %[req.fhdr(X-Haproxy-ACL,-1)]Rate-Limit-over-100-active-connections, if { src_conn_cur ge 100 }

 # Reject the new connection if the client has opened more than 65 TCP connections in 3 seconds

 http-request add-header X-Haproxy-ACL %[req.fhdr(X-Haproxy-ACL,-1)]Rate-Limit-over-65-connections-in-3seconds, if { src_conn_rate ge 65 }

 # Reject the connection if the client has passed the HTTP error rate (10 HTTP Errors in 10 Seconds)

 http-request add-header X-Haproxy-ACL %[req.fhdr(X-Haproxy-ACL,-1)]Rate-Limit-10-errors-in-10-seconds, if { sc0_http_err_rate() gt 10 }

 # Reject the connection if the client has passed the HTTP request rate (70 HTTP Requests in 10 Seconds)

 http-request add-header X-Haproxy-ACL %[req.fhdr(X-Haproxy-ACL,-1)]Rate-Limit-70-HTTPRequests-in-10-seconds, if { sc0_http_req_rate() gt 70 } !listing

 # Reject Requests that are hitting more than 20 requests in 10 seconds interval for restaurant listing - Sensitive URLs

 http-request add-header X-Haproxy-ACL %[req.fhdr(X-Haproxy-ACL,-1)]Rate-Limit-Listing, if { sc0_http_req_rate() gt 30 } sensitive-urls



acl badbots hdr_sub(user-agent) -f <path-to-webclient-detection-directory>/bad-bots.lst

 acl nullua hdr_len(user-agent) 0

 acl availua hdr(user-agent) -m found

 acl ua-regex hdr_reg(user-agent) -i .+?[/\s][\d.]+

 acl tornodes src -f <path-to-webclient-detection-directory>/tor-exit-nodes.lst

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]BadBot, if badbots

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]No-UA, if nullua !google-ua

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]No-UA, if !availua !google-ua

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]Invalid-UA, if !ua-regex !google-ua

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]Tor-Node, if tornodes

 http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]Trace-Method, if http_trace



acl scanner hdr_sub(user-agent) -f <path-to-webclient-detection-directory>/scanners.lst

http-request add-header X-Bad-User %[req.fhdr(X-Bad-User,-1)]Scanner, if scanner

 NGINX Config Snippet:

location / {

        #Enable Reverse DNS for all the Googlebots


 rdns_allow "(.*)google(.*)";

 rdns_allow "(.*)*)";

 rdns_allow "(.*)*)";

 rdns_allow "(.*)*)";

 rdns_deny "^(?!(.*)google(.*))|^(?!(.*)*))|^(?!(.*)*))|^(?!(.*)*))";

        if ($http_user_agent ~* "(.*)[Gg]oogle(.*)") {

            rdns on;


        if ($http_user_agent ~* "(.*)[Ss]lurp(.*)") {

            rdns on;


        if ($http_user_agent ~* "(.*)[Bb]ing(.*)") {

            rdns on;


        if ($http_user_agent ~* "(.*)[Aa]pplebot(.*)") {

            rdns on;


        # Set a Variable to take an action based on the result

        if ($rdns_hostname ~* ((.*)googlebot\.com)) {

            set $valid_bot 1;


        if ($rdns_hostname ~* ((.*)google\.com)) {

            set $valid_bot 1;


        # Return 403 is not Google

        if ($valid_bot = "0") {

            return 403;


 #Geo Block Section

        if ($allowed_country = yes) {

            set $exclusions 1;


        if ($exclusions = "0") {

            return 451;



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