Synchronizer Token Patterns

Cross-Site Request Forgery (CSRF) 


Cross-Site Request Forgery (CSRF) is a type of attack that occurs when a malicious web site, email, blog, instant message, or program causes a user’s web browser to perform an unwanted action on a trusted site for which the user is currently authenticated. The impact of a successful CSRF attack is limited to the capabilities exposed by the vulnerable application. For example, this attack could result in a transfer of funds, changing a password, or purchasing an item in the user's context. In effect, CSRF attacks are used by an attacker to make a target system perform a function via the target's browser without knowledge of the target user, at least until the unauthorized transaction has been committed.

Impacts of successful CSRF exploits vary greatly based on the privileges of each victim. When targeting a normal user, a successful CSRF attack can compromise end-user data and their associated functions. If the targeted end user is an administrator account, a CSRF attack can compromise the entire web application. Sites that are more likely to be attacked by CSRF are community websites (social networking, email) or sites that have high dollar value accounts associated with them (banks, stock brokerages, bill pay services). Utilizing social engineering, an attacker can embed malicious HTML or JavaScript code into an email or website to request a specific 'task URL'. The task then executes with or without the user's knowledge, either directly or by utilizing a Cross-Site Scripting flaw.

There are numerous ways you can specifically defend against CSRF. We recommend using one of the following (in ADDITION to the check recommended above):

1. Synchronizer (i.e.,CSRF) Tokens (requires session state)
2. Double Cookie Defense
3. Encrypted Token Pattern
4. Custom Header - e.g., X-Requested-With: XMLHttpRequest

These are listed in order of strength of the defense. So use the strongest defense that makes sense in your situation.

Synchronizer (CSRF) Tokens


  • Any state changing operation requires a secure random token to prevent CSRF attacks.

  • Characteristics of a CSRF Token

                  Unique per user session
                  Large random value
                  Generated by a cryptographically secure random number generator
  • The CSRF token is added as a hidden field for forms or within the URL if the state changing operation occurs via a GET

  • The server rejects the requested action if the CSRF token fails validation


The synchronizer token pattern requires the generating of random "challenge" tokens that are associated with the user's current session. These challenge tokens are then inserted within the HTML forms and links associated with sensitive server-side operations. When the user wishes to invoke these sensitive operations, the HTTP request should include this challenge token. It is then the responsibility of the server application to verify the existence and correctness of this token. By including a challenge token with each request, the developer has a strong control to verify that the user actually intended to submit the desired requests. The inclusion of a required security token in HTTP requests associated with sensitive business functions helps mitigate CSRF attacks as successful exploitation assumes the attacker knows the randomly generated token for the target victim's session. This is analogous to the attacker being able to guess the target victim's session identifier. The following synopsis describes a general approach to incorporate challenge tokens within the request.



In here I'm going to show you how to defend from the CSRF attacks using Synchronize Token Patterns.

I'm using PHP, OpenSSL to create the tokens. The source code is listed below

Login Page:


CSRF Token generate and validation.



The login page and after login user interfaces like this. I'm used bootstrap to create GUI. and all username and password are hardcoded. USername: sineth password: password



 After the logins to the application.

After updating the profile information it will state like below



So. now if an attacker tries to do anything it will give an error message CSRF token is invalid.



The ideal solution is to only include the CSRF token in POST requests and modify server-side actions that have a state changing effect to only respond to POST requests. This is in fact what the RFC 2616 requires for GET requests. If sensitive server-side actions are guaranteed to only ever respond to POST requests, then there is no need to include the token in GET requests.


You can download the source code from in my GitHub using this link below:



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