A primary sign-in method is the first authentication step a user uses to begin access to an account or service. It may be a password, SMS code, passkey, FIDO2 key, or another factor that proves identity before any secondary checks are applied. In identity systems like Microsoft Entra ID, this is the default path a tenant expects users to use at login.
This matters because the primary method is often the easiest target for attackers and the first point where defenders can raise the security baseline. Weak or reusable methods such as SMS codes are easier to phish, intercept, or socially engineer than phishing-resistant options like passkeys. In attacks, criminals try to steal or bypass the primary login step to gain an initial foothold. In defense, administrators replace legacy methods, enforce Conditional Access, and require stronger authentication so users begin with a method that is harder to copy or replay.