The system uses one or more entities, such as a proxy or a firewall, to process requests between the client and the server. These entities do not process HTTP requests consistently, thus it is possible to post malformed requests to make one of the entities process a request without the other ones noticing it. The reason the entities do not process the requests consistently is that the web server software each one uses delimits requests differently.
Start with Fluid Attacks
We are a proud corporate member of the OWASP Foundation