One does authorization, the other does authentication. They overlap more than you'd think.
| OAuth 2.0 | SAML 2.0 | |
|---|---|---|
| Primary purpose | Authorization (delegated access) | Authentication (single sign-on) |
| Token format | JSON (JWT common) | XML assertions |
| Transport | HTTP redirects + REST APIs | HTTP redirects + POST bindings |
| Best for | APIs, mobile apps, SPAs | Enterprise SSO, web apps |
| Complexity | Simpler, more developer-friendly | Complex XML, signature validation |
| Mobile support | Native — designed for it | Awkward — XML in mobile is painful |
| Attack surface | Token theft, redirect URI manipulation | XML signature wrapping, assertion replay |
SAML is an XML-based standard for federated authentication, built for enterprise single sign-on. OAuth 2.0 is a delegated authorization framework — it exists so an application can act on a user's behalf — and OpenID Connect is the identity layer built on top of it that makes OAuth usable for login. Comparing SAML to bare OAuth is comparing a login protocol to a permission-granting protocol; the fair comparison is SAML against OIDC.
Are you building enterprise SSO or consumer login? This decides it most of the time. Enterprise identity providers, corporate directories and B2B customers' IT departments still run on SAML, and a B2B product that wants to sell upmarket will be asked for SAML support regardless of technical preference. Consumer sign-in, mobile applications and anything with a public API want OIDC.
Is a browser always involved? SAML assumes one — the protocol works by POSTing a signed XML assertion through the user's browser. That makes it awkward for native mobile applications and useless for machine-to-machine flows. OAuth has grant types designed for exactly those cases, including client credentials for service-to-service and device flow for input-constrained hardware.
Do you need to call APIs on the user's behalf? If yes, OAuth is the answer and SAML isn't in the running — issuing scoped access tokens to third-party applications is the problem OAuth was created for, and SAML has no equivalent.
What will you actually have to implement? A practical consideration people discover late: SAML requires XML signature verification, which is a genuinely treacherous area with a long history of canonicalization and signature-wrapping vulnerabilities. If you implement SAML, use a maintained library and never hand-roll the verification. OIDC's JWTs are simpler to validate correctly, though they have their own trap in the alg header.
An employee at a customer company signs in to your SaaS product.
With SAML: they hit your login page, you redirect them to their company's identity provider with an AuthnRequest. They authenticate there. The IdP returns a signed XML assertion — via a browser POST to your Assertion Consumer Service URL — containing their identity and group memberships. You verify the signature against the IdP's certificate, check the audience and the timestamps, confirm the assertion hasn't been replayed, and create a session. Everything you learn about the user arrived in that one document.
With OIDC: you redirect to the provider's authorization endpoint with a state, a PKCE challenge and a scope list. They authenticate. You get an authorization code back on your redirect URI, exchange it server-side for an ID token and an access token, validate the ID token's signature, issuer, audience and nonce, and create a session. The access token then lets you call the provider's userinfo endpoint or other APIs as that user — which is the part SAML has no answer for.
Both took one redirect round trip. The difference is what you hold afterwards: an assertion that has already been consumed, versus tokens you can keep using.
SAML failures cluster in signature handling. XML signature wrapping — where an attacker adds a second, unsigned assertion that the parser reads while the verifier checks the original — has recurred across implementations for over a decade. Canonicalization differences let attackers alter content without invalidating a signature. Beyond signatures: assertions accepted without checking the audience, so a valid assertion for one service provider works at another; missing replay protection; and unsigned metadata allowing certificate substitution.
OAuth and OIDC failures cluster in the redirect. Loose redirect_uri matching — prefix matching, wildcard subdomains, open redirects on registered hosts — leaks authorization codes, and is still the most productive area in bug bounty work on these flows. Missing state allows CSRF on the callback. Implicit flow leftovers put tokens in URL fragments where they reach logs and referrers. And on the token side, accepting a JWT without verifying issuer and audience means a token minted for one service is honoured by another.
Support both if you sell to enterprises — that's the common end state, with OIDC as the primary path and SAML available for customers whose IdP requires it. Use the authorization code flow with PKCE for everything on the OAuth side, including confidential clients. Validate exhaustively rather than trusting the library default: issuer, audience, expiry, nonce, and the signing algorithm pinned server-side. More at Authentication and JWT.
Not on its own — it's an authorization framework, and using bare OAuth for login is a well-known anti-pattern that leads to token-substitution attacks. OpenID Connect is the identity layer built on OAuth that does authentication properly.
Neither inherently. SAML's risk concentrates in XML signature verification, which is difficult to get right; OIDC's concentrates in redirect URI validation and token validation. Both are safe with a maintained library and careful configuration.
No. It remains entrenched in enterprise identity infrastructure, and B2B products are routinely required to support it. New greenfield work generally chooses OIDC, but SAML support is a commercial requirement more often than a technical choice.
Proof Key for Code Exchange binds the authorization code to the client that requested it, so an intercepted code is useless to anyone else. It was introduced for mobile applications and is now recommended for all clients, including server-side ones.
Poorly. SAML assumes a browser round trip with a form POST, which makes native integration awkward. OIDC with the authorization code flow and PKCE is the intended answer for mobile.