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Lab: Exploiting HTTP request smuggling to capture other users' requests


This lab involves a front-end and back-end server, and the front-end server doesn't support chunked encoding.

To solve the lab, smuggle a request to the back-end server that causes the next user's request to be stored in the application. Then retrieve the next user's request and use the victim user's cookies to access their account.


  • Although the lab supports HTTP/2, the intended solution requires techniques that are only possible in HTTP/1. You can manually switch protocols in Burp Repeater from the Request attributes section of the Inspector panel.
  • The lab simulates the activity of a victim user. Every few POST requests that you make to the lab, the victim user will make their own request. You might need to repeat your attack a few times to ensure that the victim user's request occurs as required.


Manually fixing the length fields in request smuggling attacks can be tricky. Our HTTP Request Smuggler Burp extension was designed to help. You can install it via the BApp Store.


If you encounter a timeout, this may indicate that the number of bytes you're trying to capture is greater than the total number of bytes in the subsequent request. Try reducing the Content-Length specified in the smuggled request prefix.


  1. Visit a blog post and post a comment.
  2. Send the comment-post request to Burp Repeater, shuffle the body parameters so the comment parameter occurs last, and make sure it still works.
  3. Increase the comment-post request's Content-Length to 400, then smuggle it to the back-end server:

    POST / HTTP/1.1 Host: YOUR-LAB-ID.web-security-academy.net Content-Type: application/x-www-form-urlencoded Content-Length: 256 Transfer-Encoding: chunked 0 POST /post/comment HTTP/1.1 Content-Type: application/x-www-form-urlencoded Content-Length: 400 Cookie: session=your-session-token csrf=your-csrf-token&postId=5&name=Carlos+Montoya&email=carlos%40normal-user.net&website=&comment=test
  4. View the blog post to see if there's a comment containing a user's request. Note that the target user only browses the website intermittently so you may need to repeat this attack a few times before it's successful.
  5. Copy the user's Cookie header from the comment, and use it to access their account.


If the stored request is incomplete and doesn't include the Cookie header, you will need to slowly increase the value of the Content-Length header in the smuggled request, until the whole cookie is captured.

Community solutions

Michael Sommer