This entails the adversary injecting malicious user input into various standard and/or user defined HTTP headers within a HTTP Request through user input of Carriage Return (CR), Line Feed (LF), Horizontal Tab (HT), Space (SP) characters as well as other valid/RFC compliant special characters and unique character encoding. This malicious user input allows for web script to be injected in HTTP headers as well as into browser cookies or Ajax web/browser object parameters like XMLHttpRequest during implementation of asynchronous requests.

This attack is usually the result of the usage of outdated or incompatible HTTP protocol versions as well as lack of syntax checking and filtering of user input in the HTTP agents receiving HTTP messages in the path.

This differs from CAPEC-34 HTTP Response Splitting, which is usually an attempt to compromise a client agent (e.g., web browser) by sending malicious content in HTTP responses from back-end HTTP infrastructure. HTTP Request Splitting is an attempt to compromise aback-end HTTP agentvia HTTP Request messages.

HTTP Smuggling (CAPEC-33 and CAPEC-273) is different from HTTP Splitting due to the fact it relies upon discrepancies in the interpretation of various HTTP Headers and message sizes and not solely user input of special characters and character encoding. HTTP Smuggling was established to circumvent mitigations against HTTP Request Splitting techniques.