Response queue poisoning via H2.TE request smuggling


This lab is vulnerable to request smuggling because the front-end server downgrades HTTP/2 requests even if they have an ambiguous length.

Reproduction and proof of concept

  1. Intercept home page and send the request to Repeater. Set Repeater (top most menu) to Allow HTTP/2 ALPN override.

  2. Expand the Inspector’s Request Attributes section and change the protocol to HTTP/2.

  3. Using Burp Repeater, try smuggling an arbitrary prefix in the body of an HTTP/2 request using chunked encoding.

Transfer-Encoding: chunked


  1. Every second request sent, receives a 404 response, confirming that the back-end appends the subsequent request to the smuggled prefix.

  2. Create a request, which smuggles a complete request to the back-end server. The path in both requests points to a non-existent endpoint. This means that the request will always get a 404 response. Once the response queue is poisoned, this will make it easier to recognise any other users’ responses that have successfully been captured.

Transfer-Encoding: chunked


GET /x HTTP/1.1

Note: Terminate the smuggled request properly by including the sequence \r\n\r\n after the Host header.

  1. Send the request to poison the response queue.

  2. Wait for around 5 seconds, then send the request again to fetch an arbitrary response. Most of the time, you will receive your own 404 response. Response codes other than 404 indicate a response intended for the admin user has successfully been captured. Repeat the process until a 302 response is captured containing the admin’s new post-login session cookie.

HTTP header smuggling

Note: If you receive some 200 responses but can’t capture a 302 response even after a lot of attempts, send 10 ordinary requests to reset the connection and try again. This whole process can take some time. It took me ten minutes.

  1. Copy the stolen session cookie and use it to send a request to gain access to the admin panel:

GET /admin HTTP/1.1
Cookie: session=hVyiIwVQdrDveY8dV6Il427xj1q8G17f
  1. Send the request repeatedly until you receive a 200 response containing the admin panel.

  2. In the response, find the URL for deleting Carlos (/admin/delete?username=carlos), then update the path in the request accordingly. Send the request to delete Carlos.

HTTP header smuggling


An attacker will need to delete the user carlos by using response queue poisoning to break into the admin panel at /admin. An admin user will log in approximately every 15 seconds. The connection to the back-end is reset every 10 requests. If the connection gets into a bad state, send a few normal requests to get a fresh connection.