X-Forwarded-For Header: Trust Issues and Security Strategies

The X-Forwarded-For (XFF) HTTP header is crucial for tracking the origin IP address of client requests, especially when traversing multiple intermediaries like proxies and load balancers. However, XFF is not foolproof; malicious actors can forge it. This article delves into XFF's mechanics, uses (user authentication, load balancing, data localization, etc.), security risks (spoofing, invalid IP addresses, injection attacks, etc.), and how to use XFF safely. It covers identifying the true client IP using trusted proxy lists or counts, and recommends the more secure Forwarded header as an alternative.