OAuth alternatives: the protocols people use instead, and when each one fits

"Alternatives to OAuth" usually means one of two things: a different way to sign users in, or a different way to let one service call another. Those are different jobs, and OAuth only does the second one well.

Short answer: if you need login, the alternative to raw OAuth is OpenID Connect, which adds authentication on top of OAuth 2.0. If you need enterprise browser SSO, it is SAML 2.0; on campus and in older Java systems, CAS; for internal directories and network gear, LDAP; in Windows estates, Kerberos; for simple server-to-server calls, API keys; and for high-assurance service identity, mutual TLS.

Why "alternative to OAuth" is often the wrong question

OAuth 2.0 (RFC 6749) is an authorization framework. It lets a user grant an application limited access to a resource without sharing the password. It does not define how the user authenticates, and it does not tell the application who the user is. That is why OAuth alone is not a login protocol: the access token is for an API, not a statement of identity.

If your real question is "how do I log users in", the direct answer is OpenID Connect, which is OAuth 2.0 plus an ID token and a discovery document. If your question is "how do I let a service call an API", OAuth client credentials, API keys and mTLS are the actual choices.

The six alternatives at a glance

OpenID Connect

OpenID Connect is the mainstream alternative when OAuth is being used for login. It adds a signed ID token with claims such as subject, email and name, a UserInfo endpoint, and a discovery document at .well-known/openid-configuration. The client sends the user to the authorization endpoint with scope openid, receives an authorization code, exchanges it for tokens, and validates the ID token.

Use it for web and mobile login, social login, and any application that needs a standards-based session. For the protocol details, see what OpenID Connect is and OAuth vs OIDC.

SAML 2.0

SAML 2.0 is the enterprise SSO protocol. The identity provider authenticates the user and posts a signed XML assertion to the service provider. It is older than OIDC and more verbose, but it is still the only SSO option many enterprise applications support.

Use SAML when a SaaS product or an internal application only speaks SAML, or when a customer's identity team expects SAML metadata and certificate rotation. See SAML vs OIDC and what SAML is.

CAS

The Central Authentication Service is a browser SSO protocol built around service tickets. It is common in universities and in Java applications that predate SAML. A CAS client redirects to the CAS server, the user logs in, and the server redirects back with a ticket that the application validates.

Use CAS when an application already has a CAS client and replacing it is not worth the work. It is not the right choice for a new consumer product. See what CAS is.

LDAP

LDAP is a directory protocol, not an SSO protocol. Applications bind to the directory with a username and password, and search for users and groups. It is how many VPNs, Wi-Fi controllers, printers and older internal applications authenticate.

Use LDAP when the client only speaks LDAP, or when you need a directory that other systems can query. Do not expose LDAP directly to the internet without a gateway. See LDAP vs Active Directory.

Kerberos

Kerberos uses a key distribution center to issue tickets. In practice, it is the authentication layer behind Active Directory: a Windows user signs in once and receives a ticket-granting ticket, and services accept service tickets.

Use Kerberos inside a Windows or AD estate. For web applications that need browser SSO, put SAML or OIDC in front of it instead of integrating Kerberos directly. See Kerberos vs LDAP.

API keys and mutual TLS

API keys are shared secrets. They are simple and fine for internal services and low-risk public APIs, but they identify an application, not a user, and they are hard to scope, rotate and audit.

Mutual TLS goes further: the client presents an X.509 certificate, and the server verifies it. It is common in banking, service mesh and high-assurance machine-to-machine APIs. For OAuth-based APIs, RFC 8705 defines mTLS client authentication and certificate-bound access tokens.

Which one should you use?

• Users signing in to an app: OpenID Connect.
• Enterprise customer SSO: SAML 2.0 or OIDC, depending on what the customer's IdP and the app support.
• University or legacy Java SSO: CAS.
• Internal apps, VPN and network gear: LDAP, usually behind a gateway.
• Windows desktops and AD-integrated services: Kerberos.
• Service-to-service calls: OAuth client credentials, API keys or mTLS, in that order of preference as assurance requirements rise.

Most organizations end up with several of these at once. That is normal: the identity provider is the one place where the protocols meet, and the application keeps the protocol it already speaks.

Casdoor is an open-source identity provider that speaks OpenID Connect, OAuth 2.0, SAML 2.0, CAS, LDAP and RADIUS from one deployment, so one user directory can serve a modern SPA, an enterprise SaaS customer, a legacy CAS application and a VPN. It is self-hostable for free; Casdoor Cloud starts at $9 per month for a dedicated instance with no per-user fees.

Related reading: what OAuth 2.0 is, OAuth vs SAML and the protocols compared.