SAML vs OAuth: A Deep Dive for Azure AD

In the landscape of identity and access management, particularly within Microsoft Azure Active Directory (Azure AD), two prominent protocols stand out: Security Assertion Markup Language (SAML) and OAuth 2.0. While both are crucial for enabling secure authentication and authorization, they serve distinct purposes and operate on different principles. This post will demystify these protocols, compare their core functionalities, and highlight their roles within the Azure AD ecosystem.

What is SAML?

SAML is an open standard that allows identity providers (IdPs) to pass authorization credentials to service providers (SPs). It's primarily used for web-based single sign-on (SSO). When a user tries to access a service provider, SAML redirects the user to an identity provider. The IdP authenticates the user and then sends a SAML assertion (an XML-formatted document containing user attributes and authentication information) back to the SP. The SP then grants access based on this assertion.

Key Characteristics of SAML:

XML-based: SAML assertions are formatted in XML.
Federation: Primarily used for federated identity, enabling SSO across different security domains.
Authentication focused: Its main strength is authenticating users and asserting their identity.
Browser redirects: Relies heavily on browser redirects (HTTP POST or Redirect bindings).
Stateless: Each request/response pair is largely independent.

What is OAuth 2.0?

OAuth 2.0 is an authorization framework that enables a user to grant a third-party application limited access to their resources without exposing their credentials. Unlike SAML, OAuth 2.0 is not about authentication itself but about delegated authorization. It allows users to grant applications permission to access their data or perform actions on their behalf, using access tokens.

Key Characteristics of OAuth 2.0:

Token-based: Uses access tokens (often JSON Web Tokens - JWTs) to grant access.
Authorization focused: Designed for granting specific permissions to applications.
APIs and Mobile Apps: Widely used for securing APIs and enabling third-party application access, especially in mobile and single-page applications (SPAs).
Multiple Flows: Supports various authorization flows (e.g., Authorization Code, Implicit, Client Credentials) tailored to different client types.
RESTful: Typically operates over HTTP using RESTful principles.

SAML vs. OAuth 2.0: A Comparison

While both protocols are vital for modern application security, their fundamental goals and mechanisms differ:

Feature	SAML	OAuth 2.0
Primary Purpose	Authentication & SSO	Authorization (Delegated Access)
Data Format	XML (SAML Assertions)	JSON (Access Tokens, often JWTs)
Use Cases	Enterprise SSO, Web application access	API access, Mobile app access, Third-party app integration
Focus	User identity assertion	Granting permission to resources
Actors	Identity Provider (IdP), Service Provider (SP), User	Resource Owner, Client, Authorization Server, Resource Server
Credential Management	User logs into IdP, IdP asserts identity to SP	User grants permission to Client via Authorization Server; Client gets token
Protocol Type	Federation protocol	Authorization framework

How Azure AD Integrates Them

Azure AD plays a pivotal role in managing both SAML and OAuth 2.0 for applications. It acts as a versatile Identity Provider (IdP) for SAML-based SSO and as an Authorization Server for OAuth 2.0.

Azure AD and SAML:

Azure AD seamlessly integrates with thousands of pre-configured enterprise applications that use SAML for SSO. Administrators can configure applications in Azure AD to use SAML by specifying the SAML signing certificate, SSO URL, and other metadata. Azure AD then handles the generation and signing of SAML assertions, allowing users to log in once and access multiple applications without re-authenticating.

Example SAML Flow (Simplified):


User requests access to App SP
-> App SP redirects User to Azure AD IdP
-> Azure AD authenticates User (if not already logged in)
-> Azure AD generates a SAML assertion for User
-> Azure AD sends SAML assertion back to User's browser (POST or Redirect)
-> User's browser posts SAML assertion to App SP
-> App SP validates assertion and grants User access

Azure AD and OAuth 2.0:

Azure AD is a robust OAuth 2.0 authorization server. Developers register their applications (web apps, mobile apps, APIs) in Azure AD, defining their redirect URIs and requested permissions (scopes). Azure AD then issues access tokens (often JWTs) to these applications, which can be used to securely access protected resources (like Microsoft Graph API or custom APIs). Azure AD also supports OpenID Connect (OIDC), a layer built on OAuth 2.0 that adds an identity layer, allowing applications to verify the identity of the end-user based on the authentication performed by Azure AD and to obtain basic profile information about the end-user.

Example OAuth 2.0 Authorization Code Flow (Simplified):


Client App requests authorization from User
-> User is redirected to Azure AD Authorization Server
-> User logs into Azure AD and grants consent to Client App
-> Azure AD Authorization Server redirects User back to Client App with an Authorization Code
-> Client App exchanges Authorization Code for an Access Token (and Refresh Token) at Azure AD Token Endpoint
-> Client App uses Access Token to access protected Resource Server API

Key Takeaways for Azure AD Environments:

SAML for SSO: Ideal for federating access to SaaS applications and internal web apps, providing a seamless single sign-on experience.
OAuth 2.0/OIDC for APIs & Modern Apps: The standard for authorizing access to APIs, securing mobile applications, and enabling delegated permissions.
Azure AD as the Central Hub: Azure AD acts as the single pane of glass for managing identities, authentication, and authorization policies for both SAML and OAuth 2.0 flows.
Interoperability: Understanding both protocols is essential for building resilient and secure applications that integrate with the Azure AD ecosystem.

Conclusion

While SAML and OAuth 2.0 have different primary objectives—SAML focusing on authentication and SSO, and OAuth 2.0 on delegated authorization—they are often complementary. Azure AD's comprehensive support for both protocols makes it a powerful platform for managing identity and access across a wide spectrum of applications and services. By grasping the core principles and use cases of each, developers and administrators can design more secure, efficient, and user-friendly authentication and authorization strategies.

SAML vs OAuth 2.0 for Azure AD