SAML vs OAuth/OIDC — When to Use What in Enterprise — Auth in Depth: From Passwords to Zero Trust

I. Core Purpose: Authentication vs. Authorization

SAML (Security Assertion Markup Language): Primarily an authentication protocol (which can also pass authorization attributes) that lets a trusted Identity Provider vouch for a user's identity to an application.
OAuth 2.0: An authorization framework, not an authentication protocol. It is used for access delegation, verifying what an application is allowed to do on behalf of a user, rather than verifying who the user is.
OIDC (OpenID Connect): An authentication layer built directly on top of OAuth 2.0. It provides the identity verification that raw OAuth lacks, allowing applications to securely verify a user's identity.

Data Format: SAML relies on verbose XML-based messages. OAuth and OIDC utilize lightweight JSON and REST APIs.
Tokens vs. Assertions: SAML communicates via digitally signed XML assertions. OAuth 2.0 relies on Access Tokens and Refresh Tokens to grant access to protected resources. OIDC introduces an ID Token (formatted as a JSON Web Token, or JWT) that explicitly carries identity claims (like name and email) about the authenticated user.
Transport Mechanism: SAML relies almost entirely on browser redirects (such as HTTP-POST or HTTP-Redirect bindings). OAuth and OIDC use a combination of browser redirects and back-channel HTTP calls, making them much more adaptable for API communications.

SAML Advantages: It is a mature, reliable, and well-established standard widely supported by legacy enterprise Identity Providers (like Active Directory) and offers strong support for features like single logout.
SAML Disadvantages: It is complex to implement, relies on heavy XML parsing, requires manual X.509 certificate management, and lacks compatibility with modern Single Page Applications (SPAs) and mobile apps.
OAuth/OIDC Advantages: They are modern, lightweight, and developer-friendly with broad SDK support. They are built specifically to handle mobile apps, SPAs, and distributed microservice architectures, and support dynamic key management.
OAuth/OIDC Disadvantages: They require strict adherence to security best practices (such as secure token storage and using PKCE for SPAs) to prevent token leakage and misconfiguration vulnerabilities.

When to use SAML: Best suited for Enterprise Single Sign-On (B2B), legacy web applications, and internal workforce identity systems where strict corporate compliance and centralized auditing are required.
When to use OAuth 2.0: Ideal for API access delegation, machine-to-machine communication, and allowing third-party applications to securely access specific user data on another service (e.g., giving an app permission to read a user's Google Calendar).
When to use OIDC: The preferred choice for consumer-facing applications (B2C), social logins (like "Sign in with Google" or "Sign in with Apple"), and modern cloud-native, mobile, or SPA architectures.