HeaderManipulation

HeaderManipulation is the practice of altering HTTP headers in requests and responses to influence server behavior, client behavior, or to bypass controls. It can occur anywhere along the web request path, including browsers, application servers, proxies, and content delivery networks. Because headers carry essential information about routing, authentication, and preferences, manipulating them can have wide-ranging effects.

Common areas include headers that the client can influence, such as Host, Referer, User-Agent, Accept, and X-Forwarded-For when forwarded by intermediaries. Proxies and load balancers often rewrite or append headers, which can create inconsistencies if applications rely on header values for security checks or routing decisions. Improper handling can lead to header injection or misrouting.

Risks associated with header manipulation include cache poisoning, open redirects, cross-site request forgery, session fixation, and authorization bypass where decisions rely on untrusted header data. Logging, analytics, and rate limiting can also be distorted by manipulated headers. Security controls that depend on headers must assume headers may be forged or altered by clients or intermediaries.

Mitigation involves not trusting client-supplied headers for security decisions, validating and canonicalizing header values, and implementing allowlists or strict schemas. Infrastructure should be configured to limit header manipulation, with clear policies for Host and TLS-related headers, and to normalize headers before use. Consider signing sensitive header data or using server-side state to reduce reliance on client-provided information.

HeaderManipulation is a dual-use concept in web security and software design. It should be studied in context, with ethical considerations and appropriate authorization when testing or deploying controls.