Fail open is a security design flaw where a system treats an error, timeout, or malformed input as if the check passed. Instead of blocking access when validation cannot be completed, the code returns success. That can happen in authentication, authorization, TLS certificate checks, or header parsing, and it turns a simple bug into a security bypass.
In cyber security, fail open matters because defenders usually rely on checks to be strict by default. If a plugin, proxy, or API gateway fails open, an attacker may be able to skip login, impersonate a user, or reach protected functions after triggering a parsing edge case. Good defenses do the opposite: fail closed, log the error, and deny the request until validation succeeds. Careful testing of error paths, null values, and unexpected headers is essential because many real attacks target the branch developers least expect to be reachable.