T-Reqs: HTTP Request Smuggling with Differential Fuzzing

HTTP Request Smuggling (HRS) is an attack that exploits the HTTP processing discrepancies between two servers deployed in a proxy-origin configuration, allowing attackers to smuggle hidden requests through the proxy. While this idea is not new, HRS is soaring in popularity due to recently revealed novel exploitation techniques and real-life abuse scenarios. In this work, we step back from the highly-specific exploits hogging the spotlight, and present the first work that systematically explores HRS within a scientific framework. We design an experiment infrastructure powered by a novel grammar-based differential fuzzer, test 10 popular server/proxy/CDN technologies in combinations, identify pairs that result in processing discrepancies, and discover exploits that lead to HRS. Our experiment reveals previously unknown ways to manipulate HTTP requests for exploitation, and for the first time documents the server pairs prone to HRS.