· Alex · security  · 8 min read

Tackling the Invalid CSRF Token Error

What is CSRF, protection mechanisms, and how to deal with the invalid CSRF token error

What is CSRF, protection mechanisms, and how to deal with the invalid CSRF token error

Invalid CSRF Token Error

CSRF stands for Cross-Site Request Forgery, which is a type of attack that tricks the user’s browser into executing unwanted actions on a web application they’re logged into. The “Invalid CSRF token” error typically pops up when there’s an issue with the mechanism used to protect your web application from these CSRF attacks. It means your application is trying to protect itself from a potential security threat. You just need to figure out what’s causing the error and how to fix it.

Hint: if you’re a user, most times, the solution is as easy as refreshing the page. If you’re a developer, keep reading.

Understanding Cross-Site Request Forgery (CSRF) Attacks

Definition of CSRF attacks

Alright, let’s get down to the nitty-gritty of CSRF attacks. Cross-Site Request Forgery, or CSRF for short, is a cyber attack that tricks users’ browsers into performing unauthorized actions on a website or web application without the user’s knowledge.

Here’s a simple example to help you wrap your head around it: imagine you’re logged into your favorite online shopping site, and you receive an email with a link to click. Unbeknownst to you, that link actually sends a request to the shopping site to purchase a super expensive item. Since you’re already logged in and the site trusts your browser, the transaction goes through, and you’re left with an unwanted purchase and an empty wallet.

The main goal of CSRF attacks is to exploit the user’s authenticated session, taking advantage of the fact that the user’s browser automatically sends cookies and other authentication information to the site. This allows attackers to forge requests as if they were the user, leading to potentially disastrous consequences.

CSRF Protection Mechanisms

Synchronizer Token Pattern

The synchronizer token pattern is one of the most common CSRF protection mechanisms. It works by adding a unique, hard-to-guess token to each form or request that the user submits. This token is tied to the user’s session and must be validated by the server before the request is processed. When a user submits a form, the token is included with the request, and the server checks if it matches the one stored in the user’s session. If the tokens match, the request is considered legitimate and is processed. If not, the server rejects the request. Since attackers don’t have access to the user’s session, they can’t forge a valid token, and the CSRF attack is thwarted.

The double submit cookie pattern is another CSRF protection mechanism. Instead of storing the CSRF token in the user’s session, it’s stored in a separate cookie that’s sent with each request. The server then compares the token in the cookie to the one in the request to determine if it’s legitimate. While this method doesn’t require server-side storage, it can be less secure than the synchronizer token pattern, since attackers might be able to steal the token from the cookie. However, it’s still a decent option for some applications, especially those that want to avoid server-side state management.

The SameSite cookie attribute works by restricting when cookies are sent in cross-site requests. By setting the SameSite attribute on your authentication cookies, you can prevent them from being sent along with cross-site requests, effectively blocking CSRF attacks. There are two values you can use for the SameSite attribute: “Lax” and “Strict”. “Lax” allows cookies to be sent with top-level navigations, while “Strict” completely blocks them from being sent in cross-site requests. The right choice depends on your specific needs and the trade-offs you’re willing to make between security and user experience. Since 2021 Chrome sets the attribute to “Lax” by default.

Custom Request Headers

Custom request headers are yet another way to protect your web application from CSRF attacks. This method involves adding a custom header to requests sent via JavaScript (e.g., XMLHttpRequest or Fetch API). Since these headers can’t be added by simple HTML forms or links, it’s harder for attackers to forge requests. To implement this protection mechanism, you’ll need to modify your client-side code to include the custom header and update your server-side code to validate its presence. While this can be a bit more involved than other methods, it’s a highly effective way to prevent CSRF attacks.

Causes of the ‘Invalid CSRF token’ Error

Missing or Expired CSRF Token

One possible cause of the “Invalid CSRF token” error is a missing or expired CSRF token. This can happen if the token isn’t included in the request, or if it has expired by the time the request reaches the server. To fix this issue, you’ll need to make sure that your CSRF token is being properly generated, stored, and included with each request. Additionally, you might need to adjust the token’s expiration time to prevent it from expiring too quickly.

Token Mismatch

Another potential cause of the “Invalid CSRF token” error is a token mismatch. This occurs when the token sent with the request doesn’t match the one stored on the server. This could be due to an issue with your token generation or storage, or it could be a sign that an attacker is trying to forge a request. To resolve this problem, double-check your token handling code to ensure it’s correctly generating and storing tokens, and make sure you’re properly validating tokens on the server-side.

Incorrect Token Validation

Sometimes, the “Invalid CSRF token” error can be the result of incorrect token validation on the server-side. This can happen if there’s an issue with your server-side code, such as a bug or a logic error. To fix this, you’ll need to carefully review your server-side token validation code and make any necessary corrections.

Malfunctioning CSRF Protection Mechanisms

If your CSRF protection mechanism isn’t working as intended, it could be causing the “Invalid CSRF token” error. This could be due to a bug or misconfiguration in your code or a problem with the protection mechanism itself. To resolve this issue, double-check your implementation of the CSRF protection mechanism to ensure it’s properly set up and functioning correctly.

Diagnosing and Resolving the ‘Invalid CSRF token’ Error

  1. Inspecting the CSRF token

The first step in diagnosing the “Invalid CSRF token” error is to take a close look at the CSRF token itself. Make sure it’s being properly generated, stored, and included in your requests. If you spot any issues with the token (e.g., it’s missing or malformed), you’ll need to address them to resolve the error.

  1. Examining Server Logs

Another valuable tool in diagnosing the “Invalid CSRF token” error is your server logs. These logs can provide valuable insights into what’s happening on the server-side when the error occurs. Look for any error messages, warnings, or other indicators that might point to the root cause of the issue. This could include problems with token validation, issues with your CSRF protection mechanism, or even server configuration issues.

  1. Analyzing Application Code

Finally, a thorough analysis of your application code is crucial in identifying the cause of the “Invalid CSRF token” error. Review your code carefully, paying particular attention to the parts that deal with CSRF token generation, storage, and validation.

Best Practices for Preventing CSRF Attacks

Implementing a Robust CSRF Protection Mechanism

  1. Choose the Right Mechanism

First up, you’ll need to choose the right CSRF protection mechanism for your specific needs. We’ve discussed several options earlier in this post, including the synchronizer token pattern, double submit cookie pattern, SameSite cookie attribute, and custom request headers. Each of these mechanisms has its pros and cons, so consider factors like your application’s architecture, security requirements, and user experience when making your choice.

  1. Generate and Store CSRF Tokens Securely

Once you’ve chosen your protection mechanism, you’ll need to ensure that your CSRF tokens are generated and stored securely. Use a strong random number generator to create unique tokens, and store them in a safe and accessible location, like a session or a secure cookie. Also, make sure to set appropriate expiration times for your tokens to strike a balance between security and usability.

  1. Implement Token Validation

Next, you’ll need to implement token validation on the server-side. This involves checking each incoming request for the presence of a CSRF token and verifying that it matches the stored token.

  1. Integrate the Mechanism with Your Application

Now that you’ve set up your CSRF protection mechanism, you’ll need to integrate it with your web application. This might involve updating your forms, AJAX requests, or API endpoints to include the CSRF token.


As we’ve discussed, addressing this error is crucial to protecting your web application and its users from potential harm. By understanding the error, its causes, and how to resolve it, you’re taking a major step towards building a more secure online environment for everyone.


About the Author:


Application Security Engineer and Red-Teamer. Over 15 years of experience in Application Security, Software Engineering and Offensive Security. OSCE3 & OSCP Certified. CTF nerd.

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