Valid-account reuse is the use of stolen, guessed, or reused credentials to sign in as a real user. Unlike password cracking or malware-based intrusion, the attacker often logs in through normal authentication and looks like a legitimate user at first. The account may belong to a current employee, a contractor, or a service identity.
This matters because many defenses are tuned to stop broken logins, not successful ones. When credentials are reused across services or exposed in phishing, credential stuffing, or password dumps, attackers can bypass perimeter controls, MFA gaps, and trust built around known accounts. In real attacks, valid-account reuse is often the first step to email access, cloud access, data theft, or privilege escalation. Defenders reduce the risk with phishing-resistant MFA, unique passwords, least privilege, conditional access, and monitoring for unusual login location, device, timing, and session behavior.