External Authentication Methods are a cloud identity feature that lets approved third-party MFA providers participate in a sign-in flow. Instead of relying only on built-in factors, a tenant can allow an external method to supply the second factor or other authentication step under policy control. This is useful for flexibility, but it also expands the trust boundary of the identity system.
In cyber security, EAMs matter because attackers who gain tenant administration rights may try to change authentication policy, register an untrusted provider, or abuse a legitimate sign-in ceremony to collect credentials. Defenders should treat EAM configuration as sensitive control-plane state: monitor method registrations, review admin permissions, and alert on unexpected changes to authentication policies. A successful login is not proof that the workflow was safe; the method used to reach it must also be trusted.