Cross-Site Request Forgery (CSRF) is a type of security vulnerability that tricks users into performing unintended actions on web applications where they are authenticated. It exploits the trust that a web application has in a user's browser, allowing attackers to execute malicious commands on behalf of a victim. In this article, we will explore what CSRF is, how it works, and the methods to protect users from this dangerous type of attack.

What is CSRF?​

CSRF (also known as XSRF or "sea-surf") is an attack where a malicious actor tricks a user into performing an action on a website where they are already authenticated, without the user’s knowledge or consent. The attacker uses the victim’s session to make requests on their behalf, potentially causing actions like transferring funds, changing account settings, or deleting data.

Unlike other attacks like Cross-Site Scripting (XSS), which involve injecting malicious scripts into a website, CSRF exploits the trust between the website and the user's browser. When the user is logged into a site, their browser automatically sends cookies or session tokens with each request. This means that if the user visits a malicious website, an attacker can craft a request that is sent to a target site with the victim’s credentials attached.

How Does CSRF Work?​

In a CSRF attack, the attacker typically sets up a page or email that includes a request to a target website. The request could be a form submission, a URL with parameters, or an AJAX request. If the victim is logged into the target website, their session cookie or authentication token will be sent along with the malicious request, allowing the attacker to perform actions on behalf of the victim.

Here’s a simplified example of how a CSRF attack works:

  1. Victim's Login: The user logs into a website (e.g., a banking site) and their browser stores a session cookie.
  2. Malicious Request: The attacker crafts a malicious URL or form submission that would trigger an action (e.g., transferring money) on the banking site.
  3. Victim Visits Malicious Site: The victim unknowingly visits a malicious website or opens an email with a link.
  4. Request Sent with Victim’s Cookies: The malicious page sends a request to the banking site, automatically including the victim's session cookie.
  5. Action Performed: The banking site believes the request is legitimate because the victim’s session is valid, and the action (e.g., money transfer) is carried out.

The Risks of CSRF Attacks​

  • Unauthorized Actions: Attackers can perform actions on behalf of the victim, such as transferring money, changing account settings, or making purchases.
  • Data Loss: Attackers can delete or modify sensitive data stored in the victim’s account.
  • Reputation Damage: If CSRF attacks are not mitigated, they can damage the reputation of businesses by compromising user data and financial security.
  • Loss of User Trust: If users realize that their accounts can be exploited by such attacks, they may lose trust in the platform.

How to Protect Users from CSRF Attacks​

To protect users from CSRF attacks, developers need to implement certain security measures in web applications. Here are the best practices to defend against CSRF vulnerabilities:

  1. Use Anti-CSRF TokensOne of the most effective methods to prevent CSRF is the use of anti-CSRF tokens (also called CSRF tokens or synchronizer tokens). These are unique, unpredictable strings generated by the server for each user session. The server sends this token to the client as part of the page response (usually in a hidden form field or in HTTP headers) and expects it to be sent back with every state-changing request (e.g., form submissions).
    • When a user submits a form or sends a request to the server, the application checks that the token in the request matches the one stored on the server.
    • If the token is missing or invalid, the request is rejected.
      Anti-CSRF tokens ensure that malicious requests cannot be forged because the attacker cannot predict or obtain the valid token associated with the victim’s session.
  2. SameSite CookiesThe SameSite cookie attribute provides an additional layer of protection by preventing browsers from sending cookies along with cross-site requests. There are three options for SameSite cookies:
    • SameSite=Strict: The cookie is only sent if the request is coming from the same origin as the website.
    • SameSite=Lax: The cookie is sent for same-origin requests and some top-level navigations, but not for most cross-origin requests.
    • SameSite=None; Secure: The cookie is sent with cross-origin requests, but only if the request is sent over HTTPS.
      By setting the SameSite attribute to Strict or Lax for session cookies, you can prevent attackers from including the session cookie in cross-site requests.
  3. Double Submit CookiesThe double-submit cookie approach involves sending the CSRF token both as a cookie and as a request parameter (e.g., in a hidden form field or HTTP header). When the server receives the request, it compares the token in the cookie with the token in the request body. If they don’t match, the request is rejected.
    • This method ensures that the attacker cannot forge a valid CSRF token because they cannot set the cookies in the victim's browser.
  4. Check the Referrer HeaderAnother defense is to check the Referer HTTP header, which indicates the origin of the request. If the referer is not from the trusted domain, the server can reject the request.
    • Although this method is not foolproof (because the referer header can be manipulated), it can be used as an additional layer of protection, particularly when combined with other methods.
  5. Use Same Origin Policy for Sensitive ActionsEnsure that sensitive actions such as fund transfers, password changes, and email address updates are only accessible through POST requests and are protected by anti-CSRF mechanisms. Avoid using GET requests for any action that causes state changes, as GET requests can be easily exploited.
  6. Limit HTTP Methods and Enforce HTTPSEncourage the use of secure HTTP methods (like POST, PUT, DELETE) for operations that change data. This reduces the likelihood that state-changing actions can be triggered accidentally through URL-based GET requests.Additionally, ensure that all pages are served over HTTPS, which encrypts the data in transit and protects users from man-in-the-middle attacks.
  7. Educate UsersAlthough developers are responsible for securing applications, educating users about the risks of CSRF attacks and safe browsing practices can help further reduce the risk. Advise users to be cautious of suspicious links and to avoid visiting untrusted websites while logged into sensitive accounts.

Conclusion​

CSRF attacks are a significant security threat that exploit the trust between users and websites, allowing malicious actors to carry out unauthorized actions on behalf of users. By implementing security mechanisms like anti-CSRF tokens, SameSite cookies, and using secure HTTP methods, developers can protect users from these attacks. Ensuring the security of web applications and educating users about the potential risks will significantly reduce the likelihood of CSRF vulnerabilities and protect user data from malicious exploitation.