Validation bypass is a weakness where an application fails to properly check input, request data, or user state before acting on it. The checks may be missing, applied to the wrong field, only enforced on one code path, or skipped after an internal redirect or wrapper function. In security-sensitive systems, validation is the first line of defense that keeps malformed requests, unsafe methods, and unauthorized actions from reaching privileged logic.
This matters because attackers often look for paths that accept a request at the edge but forget to re-check it deeper in the stack. In web APIs, that can mean a batch handler, proxy, or internal subrequest inherits trust from the outer request and executes actions it should not. Defenders reduce this risk by validating every entry point independently, enforcing authorization on each subrequest, rejecting malformed or unexpected parameters, and failing closed when checks are incomplete. A validation bypass can turn a simple input bug into account abuse, configuration tampering, or remote code execution.