A proxy chain is a path of forwarding components between a client and an origin server, such as load balancers, reverse proxies, API gateways, and caches. Each hop may normalize, buffer, split, or reassemble HTTP in slightly different ways, so the same request can be interpreted differently at each layer.
This matters because attackers often target those differences. In HTTP request smuggling and desynchronization attacks, a small parsing mismatch can make one proxy believe a request has ended while the next component still treats extra bytes as part of the same message. That can lead to queue poisoning, cache confusion, or unauthorized requests riding along with a valid session. Defenders should test every front-end and back-end combination, reject control characters in headers, and verify that request framing is consistent across the entire proxy chain.