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.

Reproduction and proof of concept

  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:

Host: lab-id.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 256
Transfer-Encoding: chunked


POST /post/comment HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 400
Cookie: session=your-session-token

  1. 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.

A few times, like around 5:

HTTP header smuggling

Then headers as a comment, but not enough yet. So, repeating with 700, then 800 (nearly there), and finally, with a content-length of 810:

HTTP header smuggling

  1. Intercept, carlos’s login, change the user’s Cookie header from the comment, and the csrf token from the one used in the comments requests, and use it to access his account.

HTTP header smuggling


An attacker will need to 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.