Self Notes: Web Security Academy HTTP Request Smuggling Lab 11

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Link to Lab 11

Lab 11 is tagged expert, but it wasn't so difficult, the Lab involves a Front-end server which doesn't support chunked encoding and a Back-end which does, also the Front-end server is serving as web cache for static resources. The task for this Lab is to perform a request smuggling attack to steal other users' API key via web cache deception. Since we already know that we are dealing with a CL-TE server pair and because this Lab is very similar to the previous Lab, I will skip to exploitation.


The Endpoint which serves the API key is /my-account so the idea is that when the server processes the normal request, the smuggled request will be left on the back-end socket. When any normal user makes a request for a static resource, their request will get appended to the smuggled request, the Back-end server will then respond with the appropriate response for /my-account endpoint, the Front-end server will cache the response and we can retrieve the user's API key by simply requesting the original static resource which they requested. The only caveat is that we don't know the static resource the user request so we need to check all the static resources as fast as possible. To accomplish that, we create a wordlist of the cached static resources URLs and pass the list to Burp Intruder.

 
The above resources are cached by the Front-end server, so we use the list as payload to Intruder. As already indicated in the Lab task we have to issues a lot of requests to successfully achieve web cache deception.


After multiple requests, we successfully retrieve the admin API key.


After submitting the API key:


Lab Solved.

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