What is XSRF in Web Security?

Cross-Site Request Forgery (XSRF), also commonly abbreviated as CSRF, is a type of malicious web security exploit where unauthorized commands are transmitted from a user that a web application trusts. Unlike Cross-Site Scripting (XSS), which exploits the trust a user has for a particular site, XSRF exploits the trust that a site has in a user's browser.

In an XSRF attack, a malicious actor tricks an authenticated user into unknowingly sending a request to a web application. This can result in actions being performed without the user's knowledge or consent, such as changing account settings, transferring funds, or making purchases.

Why is XSRF a Serious Vulnerability?

XSRF is considered a serious vulnerability for several reasons:

• Exploits trusted sessions: The attack leverages legitimate user sessions, making malicious requests appear authentic to the server.
• Difficult to detect: Since requests originate from the victim's browser with valid credentials, distinguishing them from legitimate actions is challenging.
• Wide-ranging impact: Successful attacks can lead to financial loss, data breaches, unauthorized account changes, and reputation damage.
• User unawareness: Victims typically have no idea an attack has occurred until they notice unauthorized changes.

How Does XSRF Work?

An XSRF attack typically follows this pattern:

1. The victim authenticates with a trusted website and receives a session cookie.
2. The attacker crafts a malicious link, image, or hidden form containing a request to the trusted site.
3. The attacker tricks the victim into clicking the link or visiting a page containing the malicious code.
4. The victim's browser automatically includes session cookies with the forged request.
5. The trusted website processes the request as if it were legitimate, executing the attacker's intended action.

Real-World Examples

Bank Transfer Forgery: An attacker sends a user a link disguised as an innocent image. If the user is logged into their online banking, clicking the link could trigger a request to transfer money to the attacker's account. The browser automatically includes the user's session cookies, making the request appear legitimate.

Password Change Attack: An attacker creates a hidden form on a malicious website that automatically submits a POST request to a social media site's password change endpoint. If the victim visits this page while logged into the social media platform, their password could be changed without their knowledge.

When Was XSRF First Identified?

XSRF vulnerabilities have been known since the early 2000s, with the term becoming widely recognized in the web security community around 2001-2002. The vulnerability has been documented extensively by organizations like OWASP (Open Web Application Security Project) and continues to be listed among critical web application security risks.

Which Security Measures Protect Against XSRF?

Several effective countermeasures can protect web applications against XSRF attacks:

• CSRF Tokens: Implementing unique, unpredictable tokens for each user session that must be included with state-changing requests. The server validates these tokens before processing requests.
• SameSite Cookies: Setting the SameSite attribute on cookies to Strict or Lax prevents browsers from sending cookies with cross-site requests. See MDN Web Docs for implementation details.
• Custom Request Headers: Requiring custom headers (like X-Requested-With) for sensitive operations, as these cannot be set by simple cross-origin requests.
• Double Submit Cookies: Sending a random value both as a cookie and as a request parameter, then verifying they match on the server.
• Re-authentication: Requiring users to re-enter credentials or complete additional verification for sensitive actions.

For comprehensive guidance on XSRF prevention, consult the OWASP CSRF Prevention Cheat Sheet and PortSwigger's Web Security Academy.