An authentication stack is the set of identity tools, policies, and services that verify users before they reach systems and data. It often includes directories, single sign-on, multi-factor authentication, federation services, password policies, and privileged access controls. Together, these components decide who can log in, how identity is proven, and what level of access is granted.
This layer matters because attackers do not always need to break a workstation if they can subvert the trust system itself. Common attacks target the authentication flow by stealing credentials, abusing federation, tampering with MFA, or changing login policies to keep access active. Defenders should treat changes to identity infrastructure as high-risk events, monitor privileged authentication activity, and review logs for unusual policy edits or successful logins that do not match normal behavior. A compromised authentication stack can become a persistence layer across the environment.