An authentication flaw is a weakness in the checks that confirm a user, device, or session is allowed to log in. Instead of properly verifying credentials, tokens, cookies, certificates, or access state, the system may accept requests that should be denied. This can let an attacker bypass normal login controls, impersonate a user, or reach protected functions without valid authorization.
In cyber security, authentication flaws matter because they often sit at the front door of a service. If a VPN, web portal, or API can be accessed without correct identity checks, the attacker may gain an initial foothold and then escalate, move laterally, or steal data. Defenders look for missing verification, broken session handling, and logic errors in access control. Strong patching, MFA, session review, and monitoring for unusual login patterns help reduce the risk.