What Is SAML vs OAuth vs OIDC: The Differences Explained

The alphabet soup of authentication protocols confuses even experienced engineers. SAML, OAuth, OIDC - they all seem to do similar things, they all involve tokens and redirects, and vendors often use the terms interchangeably.

But these protocols serve fundamentally different purposes, and using the wrong one creates security vulnerabilities. Understanding the distinctions is essential for making correct architectural decisions.

Protocol	Purpose	Primary Use Case
SAML 2.0	Authentication + basic authorization	Enterprise SSO
OAuth 2.0	Authorization only	API access delegation
OpenID Connect	Authentication + authorization	Modern web/mobile apps

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The Core Distinction: Authentication vs Authorization

Before diving into protocols, understand the fundamental concepts they address.

Concern	Question	Protocol
Authentication	Who are you?	SAML, OIDC
Authorization	What can you access?	OAuth, (SAML partially)
Both	Who are you and what can you do?	OIDC + OAuth

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SAML 2.0: The Enterprise Standard

Security Assertion Markup Language 2.0, released in 2005, remains the dominant protocol for enterprise SSO. Despite its age, SAML powers the vast majority of business-to-business authentication integrations.

How SAML Works

SAML uses XML-based messages called assertions that contain authentication statements. The flow involves browser redirects between three parties.

Component	Description
Identity Provider (IdP)	Authenticates users, issues assertions
Service Provider (SP)	Application accepting assertions
Assertion	XML document proving authentication
Bindings	How messages travel (POST, Redirect)

SAML Assertion Structure

A SAML assertion contains:

<saml:Assertion>
  <saml:Issuer>https://idp.company.com</saml:Issuer>
  <saml:Subject>
    <saml:NameID>user@company.com</saml:NameID>
  </saml:Subject>
  <saml:Conditions NotBefore="..." NotOnOrAfter="..." />
  <saml:AuthnStatement AuthnInstant="..." />
  <saml:AttributeStatement>
    <saml:Attribute Name="groups">
      <saml:AttributeValue>admins</saml:AttributeValue>
    </saml:Attribute>
  </saml:AttributeStatement>
</saml:Assertion>

SAML Strengths and Weaknesses

Strength	Weakness
Mature, well-understood	XML is verbose and complex
Wide enterprise support	Not designed for mobile/API
Rich attribute passing	Browser-dependent flow
Works with existing IdPs	No native refresh mechanism

OAuth 2.0: Delegated Authorization

OAuth 2.0, finalized in 2012, solves a different problem than SAML. It enables applications to access user resources on other services without receiving the user's password.

The OAuth Problem

Consider this scenario: You want a third-party app to post to your Twitter account. Before OAuth:

• Option 1: Give the app your Twitter password (terrible idea)
• Option 2: Build custom integration per service (does not scale)

OAuth provides a standardized way to grant limited access without sharing credentials.

OAuth Actors and Tokens

Actor	Role
Resource Owner	User who owns the data
Client	Application requesting access
Authorization Server	Issues tokens
Resource Server	API hosting the data

Token Type	Purpose	Lifetime
Access Token	Grants API access	Minutes to hours
Refresh Token	Obtains new access tokens	Days to months

OAuth Grant Types

Grant	Use Case	Security Level
Authorization Code	Web apps with backend	Highest
Authorization Code + PKCE	Mobile/SPA apps	High
Client Credentials	Machine-to-machine	High (no user)
Implicit	Legacy SPAs	Deprecated - do not use
Password	Trusted first-party apps	Deprecated - do not use

OpenID Connect: Authentication Meets Modern Architecture

OpenID Connect, released in 2014, builds an authentication layer on top of OAuth 2.0. It answers the question OAuth deliberately avoided: "Who is this user?"

OIDC = OAuth 2.0 + Identity Layer

OAuth 2.0 Provides	OIDC Adds
Authorization framework	Authentication semantics
Access tokens	ID tokens
Scope-based permissions	Standard identity claims
Token endpoints	UserInfo endpoint

The ID Token

OIDC introduces the ID token - a JWT containing identity claims:

{
  "iss": "https://idp.company.com",
  "sub": "user123",
  "aud": "client_app_id",
  "exp": 1699999999,
  "iat": 1699999000,
  "email": "user@company.com",
  "name": "John Doe"
}

Claim	Purpose
iss	Token issuer (IdP)
sub	Subject identifier (user ID)
aud	Intended audience (client)
exp/iat	Expiration and issued-at times
Standard claims	email, name, picture, etc.

OIDC Flows

Flow	Use Case	Tokens Returned
Authorization Code	Server-side apps	ID + Access (+ Refresh)
Implicit	Legacy SPAs	ID + Access (deprecated)
Hybrid	Specific security requirements	Combination

Protocol Comparison: When to Use What

The choice between protocols depends on your specific requirements. There is no universal "best" option.

Feature Comparison

Feature	SAML 2.0	OAuth 2.0	OIDC
Authentication	Yes	No	Yes
Authorization	Basic	Yes	Yes
Format	XML	JSON	JSON
Token type	Assertion	Access token	ID token + Access token
Mobile support	Poor	Good	Good
API protection	Poor	Good	Good
Enterprise IdP support	Excellent	Moderate	Good
Complexity	High	Medium	Medium

Decision Matrix

Scenario	Recommended Protocol
Enterprise SSO with existing IdP	SAML 2.0
SaaS integration with enterprise	SAML 2.0
Modern web application auth	OIDC
Mobile application auth	OIDC
API-to-API authorization	OAuth 2.0 (Client Credentials)
Social login	OIDC
Consumer-facing application	OIDC
Third-party API access	OAuth 2.0

Migration Considerations

From	To	Difficulty	Approach
SAML	OIDC	Medium	Run in parallel, migrate apps gradually
Custom auth	OIDC	Medium-High	Requires application changes
OAuth (misused for auth)	OIDC	Low	Add ID token validation

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Security Considerations

Each protocol has unique security concerns that implementations must address.

SAML Security

Attack	Mitigation
XML signature wrapping	Use validated libraries only
Replay attacks	Check NotOnOrAfter, use one-time assertion IDs
Man-in-the-middle	Always use HTTPS, validate IdP certificate
Assertion theft	Short validity windows, audience validation

OAuth Security

Attack	Mitigation
Authorization code interception	Use PKCE for all clients
Token leakage via referrer	Use POST, avoid tokens in URLs
Client impersonation	Use client authentication, PKCE
Scope creep	Request minimal scopes, validate on resource server

OIDC Security

Attack	Mitigation
ID token injection	Validate signature, issuer, audience
Nonce replay	Generate unique nonce per request, verify
State parameter manipulation	Generate cryptographically random state
Token substitution	Verify at_hash claim matches access token

Common Mistakes Across Protocols

Mistake	Consequence
Skipping signature validation	Authentication bypass
Long-lived tokens without refresh	Prolonged compromise window
Overly broad scopes	Excessive access on compromise
Missing audience validation	Token confusion attacks
Hardcoded secrets in code	Credential theft

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Implementation Patterns

Practical implementation differs by application architecture.

Server-Side Web Applications

Best choice: OIDC Authorization Code Flow

User → App → IdP login → Auth code → App backend → Token exchange → Session

Step	Security Consideration
Redirect to IdP	Include state parameter
Auth code return	Validate state, exchange immediately
Token exchange	Happens server-side, secrets protected
Session creation	Use secure, HttpOnly cookies

Single-Page Applications

Best choice: OIDC Authorization Code with PKCE

Mobile Applications

Best choice: OIDC Authorization Code with PKCE

Platform	Implementation
iOS	Use ASWebAuthenticationSession
Android	Use Chrome Custom Tabs
Both	Avoid embedded WebViews (security risks)

API-to-API (Machine-to-Machine)

Best choice: OAuth 2.0 Client Credentials

Service A → Auth server (client_id + secret) → Access token → Service B API

No user involved. Service authenticates itself to obtain tokens.

Enterprise SSO Integration

Best choice: SAML 2.0 (most enterprise IdPs prefer it)

Configuration	Purpose
SP Metadata	Your app's endpoints, certificate
IdP Metadata	IdP's endpoints, certificate
Attribute mapping	Map IdP attributes to app fields
Just-in-time provisioning	Create users on first login

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Real-World Implementation Scenarios

Understanding how protocols combine in real architectures clarifies when to use each.

Scenario 1: Enterprise SaaS Product

A B2B SaaS product needs to support customer IdPs for SSO.

Requirement	Solution
Customer SSO	SAML 2.0 (customers expect it)
Internal users	OIDC with your IdP
Mobile app	OIDC with PKCE
API access	OAuth 2.0 tokens

Scenario 2: Consumer Application with Social Login

A consumer-facing app allowing social and email login.

Provider	Protocol	Implementation
Google	OIDC	Google Sign-In SDK
Facebook	OAuth 2.0 + proprietary	Facebook Login SDK
Apple	OIDC	Sign in with Apple
Email/password	Custom	Your auth system

Scenario 3: Microservices Architecture

Internal services need to communicate securely.

Communication	Protocol	Token Type
User-initiated requests	OIDC	ID token → access token
Service-to-service	OAuth 2.0 Client Credentials	Access token
Background jobs	OAuth 2.0 Client Credentials	Access token

Scenario 4: Hybrid Enterprise

Organization with on-premises AD and cloud applications.

System	Protocol	Notes
On-prem apps	Kerberos/NTLM	Existing infrastructure
Cloud apps	SAML 2.0	Via Azure AD / ADFS federation
Modern apps	OIDC	Native Azure AD integration

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Common Mistakes to Avoid

These mistakes appear repeatedly in real-world implementations.

Mistake 1: Using OAuth for Authentication

Wrong	Right
Validate access token → "user is authenticated"	Validate ID token → user is authenticated
Use /userinfo to get identity	ID token contains identity claims

Mistake 2: Storing Tokens Insecurely

Storage Location	Risk Level	Use For
localStorage	High (XSS vulnerable)	Never for sensitive tokens
sessionStorage	Medium (XSS vulnerable)	Short-lived, low-risk tokens only
HttpOnly cookies	Low	Recommended for web apps
Secure mobile storage	Low	Keychain (iOS), Keystore (Android)

Mistake 3: Not Validating All Token Claims

Claim	Must Validate	Why
Signature	Always	Proves token integrity
Issuer (iss)	Always	Prevents cross-IdP attacks
Audience (aud)	Always	Prevents token confusion
Expiration (exp)	Always	Prevents replay attacks
Nonce	When present	Prevents injection

Mistake 4: Overly Broad Scopes

Request minimal scopes. If your app only needs email, don't request full profile access.

Bad	Good
scope=openid profile email phone address	scope=openid email
scope=https://graph.microsoft.com/.default	Specific Graph scopes needed

Mistake 5: Long-Lived Access Tokens

Token Lifetime	Appropriate Use
5-15 minutes	Standard access tokens
1-24 hours	Low-risk, internal APIs
7-90 days	Refresh tokens with rotation
Never expires	Avoid - use refresh tokens

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Identity Provider Implementations

Major IdPs implement these protocols with platform-specific features.

Microsoft Entra ID (Azure AD)

Protocol	Support	Notes
SAML 2.0	Full	Enterprise app gallery
OAuth 2.0	Full	Microsoft Graph API
OIDC	Full	Microsoft Identity Platform v2.0

Okta

Protocol	Support	Notes
SAML 2.0	Full	7000+ pre-built integrations
OAuth 2.0	Full	Authorization servers
OIDC	Full	Native support

Auth0

Protocol	Support	Notes
SAML 2.0	Full	As both IdP and SP
OAuth 2.0	Full	Multiple grant types
OIDC	Full	Primary focus

Google Workspace

Protocol	Support	Notes
SAML 2.0	Full	Google as IdP only
OAuth 2.0	Full	Google APIs
OIDC	Full	Google Sign-In

Implementation Libraries

Platform	SAML	OAuth/OIDC
.NET	ITfoxtec, Sustainsys	Microsoft.Identity
Java	Spring Security SAML	Spring Security OAuth
Node.js	passport-saml	passport-openidconnect
Python	python3-saml	Authlib, OAuthLib

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Troubleshooting Common Issues

Protocol debugging requires understanding the flow and knowing where to look.

SAML Troubleshooting

Issue	Diagnostic Steps
Signature validation fails	Compare IdP certificate, check expiry
Audience mismatch	Verify SP Entity ID matches assertion
Time-based errors	Check server time sync (NTP)
Attribute not passed	Check IdP attribute release configuration
Redirect loop	Verify ACS URL, check session handling

OAuth/OIDC Troubleshooting

Error	Likely Cause
invalid_client	Wrong client ID or secret
invalid_grant	Expired auth code, already used code
invalid_scope	Requested scope not configured
access_denied	User denied consent
invalid_request	Missing required parameter

Token Validation Issues

Problem	Check
Token rejected	Signature algorithm matches IdP config
Wrong user	Validate sub claim, not just access
Permissions wrong	Check scopes in token vs requested
Token expired	Client clock sync, token lifetime config

Common Integration Issues

Symptom	Investigation
Works in test, fails in production	Environment-specific URLs, certificates
Intermittent failures	Clock skew, network timeouts
Works for some users	Group/role membership, conditional access
Mobile only issues	Custom tab/WebView configuration

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Conclusion: Choosing the Right Protocol

The choice between SAML, OAuth, and OIDC is not about which is "best" but which fits your requirements.

Quick Decision Guide

Key Takeaways

Takeaway	Detail
SAML is not dead	Still dominant for enterprise B2B SSO
OAuth is not authentication	Use OIDC when you need identity
OIDC is the modern choice	For new apps, especially mobile
Multiple protocols coexist	Most organizations use several
Security requires attention	Each protocol has specific risks

Implementation Priority

Priority	Action
1	Use established libraries - never roll your own crypto
2	Validate all token claims - signatures, issuer, audience, expiry
3	Secure token storage - server-side or secure device storage
4	Minimal scopes - request only what you need
5	Short lifetimes - use refresh tokens for longevity

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For deeper protocol exploration, see our dedicated guides on SAML 2.0 Configuration, OAuth 2.0 Best Practices, and OpenID Connect Implementation.