Mitigating HTTP Request Smuggling

HTTP Request Smuggling exploits inconsistencies between how front-end and back-end servers parse HTTP requests, allowing attackers to smuggle a request through unnoticed. This occurs when malformed or abnormal HTTP requests are interpreted differently by devices in the data flow between a user and a web server.

The problem arises when a request contains both Content-Length and Transfer-Encoding headers, causing different servers to prioritize their parsing differently. If a front-end server uses Content-Length and a back-end server uses Transfer-Encoding, the back-end might stop reading early, treating the remaining bytes as the start of a new request—often crafted by the attacker.

For example, a request with Content-Length: 13 Transfer-Encoding: chunked 0\r\n \r\n SMUGGLED could be interpreted as two separate requests—one by the front-end and another by the back-end—resulting in the attacker-controlled bytes being executed.

Mitigations include upgrading to HTTP/2, rejecting ambiguous requests at the edge, and using consistent parsing configurations. Rejecting both headers immediately prevents smuggling by ensuring a single parsing method. Testing with tools like Burp Suite's HTTP Request Smuggler extension can validate protection measures.