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.
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.
Visit a blog post and post a comment.
Send the comment-post request to Burp Repeater, shuffle the body parameters so the comment parameter occurs last, and make sure it still works.
Increase the comment-post request's Content-Length to 400, then smuggle it to the back-end server:
POST / HTTP/1.1
POST /post/comment HTTP/1.1
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.
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.
Copy the user's Cookie header from the comment, and use it to access their account.