Authentication-as-a-Service is a hosted identity platform that manages user sign-in, account lifecycle, session issuance, and token handling for applications. Instead of building login flows in-house, teams delegate those functions to a provider that supports standards such as OAuth 2.0, OpenID Connect, and sometimes SAML. AaaS often also provides multi-factor authentication, federation, and passkey or WebAuthn support.
AaaS matters because authentication is a security control plane, not just a convenience feature. The provider and its configuration affect how credentials are protected, how sessions are validated, and how quickly defenses can respond to abuse. In real attacks, adversaries target this layer with credential stuffing, phishing, automated sign-up abuse, and account takeover. Defenders use AaaS to centralize policy, enforce MFA, reduce password exposure, and add phishing-resistant sign-in. The tradeoff is that the application team still must validate sessions, scope service credentials, and configure claims and permissions carefully, because outsourcing authentication does not outsource responsibility.