HTTP Request Smuggling is a flaw in web security that is derived from variations in the way different web servers or intermediaries, such as load balancers and proxies handle HTTP request sequences. By creating malicious HTTP requests that exploit these inconsistencies, an attacker can control the order in which requests are processed, possibly resulting in unauthorized access, circumvention of security controls, session hijacking, or injection of malicious content into responses meant for other users.

Source: GBHackers