Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet

Introduction

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 that the user is currently authenticated to. For example, this attack could result in a transfer of funds, changing a password, or purchasing an item in the users’ context.

A successful CSRF exploit can compromise end user data and operation, when it targets a normal user. If the targeted end user is an administrator account, a CSRF attack can compromise the entire Web application. The sites that are more likely to be attacked are community Websites (social networking, email) or sites that have high dollar value accounts associated with them (banks, stock brokerages, bill pay services). This attack can happen even if the user is logged into a Web site using strong encryption (HTTPS).

Utilizing social engineering, an attacker will 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 users knowledge, either directly or by utilizing a Cross-site Scripting Flaw.

In effect, CSRF attacks are used by an attacker to make a target system perform a function (Funds Transfer, Form submission etc..) via the targets browser without knowledge of the target user, at least until the unauthorized function has been committed.

Prevention

Token Based

The most common defense of CSRF is to append challenge tokens to each sensitive request. These challenge tokens must be associated with the user's session. By including a challenge token with each request, the developer can ensure that the request is valid and not coming from another source other than the user. The following describes how to incorporate challenge tokens.

When a Web application formulates a request (by generating a link or form that causes a request when submitted or clicked by the user) the application should include as a query parameter as an “Input” tag of type “hidden” with a name like: CSRFToken. It is important to note that the CSRFToken should not be included in the URL or in URL parameters as they may be leaked. The CSRFToken should have a value that is a randomly generated such as a 128-bit hash that has been base 64 encoded. For each request this token need only be generated randomly once, after that the token value is stored in a session specific table mapping request names to validation tokens. When a request is performed by the user, before the request is executed the submitted CSRFToken has its value verified by comparing the provided token to the value stored in the mapping table for this request. If there is no value in the table for this request name, or if the value provided does not match the value in the table exactly then the Request Formulator is not the application, the request should be aborted and the event can be logged as a potential security incident.

The request name should be different for each request. The CSRFToken must be strictly protected, for example an application that does not use SSL or passes the token through a URL parameter, will result in the CSRFToken being leaked.