Understanding Cross-Site Request Forgery (CSRF)

What is Cross-Site Request Forgery (CSRF)?

Detailed Description

Cross-Site Request Forgery (CSRF), also known as XSRF, Sea Surf, or Session Riding, is a type of security vulnerability typically found in web applications. CSRF is an attack that forces an end user to execute unwanted actions on a web application in which they are currently authenticated.

Attackers exploit the trust that a web application has in the user's browser, essentially using the user's own credentials to perform malicious activities. CSRF attacks specifically target state-changing requests, not theft of data, since the attacker has no way to see the response to the forged request.

The goal is to trick the user’s browser into sending a request that will use the user’s credentials to perform an action inadvertently.

How Does CSRF Work?

A CSRF attack involves three parties: the victim, the web application, and the attacker. Consider the following steps that outline the typical flow of a CSRF attack:

1. The victim logs into a web application and receives a session cookie.
2. The attacker crafts a malicious link or script designed to perform an action on the web application using the victim's authenticated session.
3. The victim is tricked into clicking the link or loading the page with the malicious script.
4. The malicious request is sent from the victim’s browser, with the victim’s cookies, to the web application.
5. The web application executes the action without realizing the request was not intended by the user.

Examples

Example 1: Changing Email Address

Imagine a user is logged into their email account, which is vulnerable to CSRF. An attacker could craft a malicious link that points to the "change email address" form on the email service.

If the user clicks on this link, the request to change the email address is sent with the user's cookies, and the email address is changed without the user’s consent.

Example 2: Making a Transfer on a Banking Site

A user is logged into their online banking account. An attacker sends an email with a link containing a hidden request to transfer money to the attacker’s bank account. If the user clicks the link, the banking site processes the request as legitimate because it comes with the user's authenticated session.

Security Recommendations

To protect against CSRF attacks, developers and administrators should implement the following security measures:

• Use Anti-CSRF Tokens: A unique token for each user session should be embedded in web forms. This token must be verified with every request to ensure its validity.
• SameSite Cookie Attribute: Use the SameSite attribute in cookies to restrict them to first-party contexts, preventing browsers from sending these cookies along with cross-site requests.
• Double Submit Cookie: This technique involves sending a cookie and a request parameter with the same value in each state-changing request, and then verifying if they match on the server side.
• Custom Headers: Use custom request headers as they cannot be added by browsers to cross-origin requests without CORS preflight.

References

• OWASP - CSRF Attacks
  
• Mozilla Developer Network - Same-origin Policy
  
• PortSwigger - CSRF Overview
  

By understanding and implementing these security measures, developers can significantly mitigate the risk of CSRF attacks, protecting both the application and its users from potential harm.

Frequently Asked Questions