CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

HTTP requests or responses ("messages") can be malformed or unexpected in ways that cause web servers or clients to interpret the messages in different ways than intermediary HTTP agents such as load balancers, reverse proxies, web caching proxies, application firewalls, etc. For example, an adversary may be able to add duplicate or different header fields that a client or server might interpret as one set of messages, whereas the intermediary might interpret the same sequence of bytes as a different set of messages. For example, discrepancies can arise in how to handle duplicate headers like two Transfer-encoding (TE) or two Content-length (CL), or the malicious HTTP message will have different headers for TE and CL.

The inconsistent parsing and interpretation of messages can allow the adversary to "smuggle" a message to the client/server without the intermediary being aware of it.

This weakness is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents.