I'm building an API product and need secure user auth, what should I use?

For an API product, the best default is usually:

• OAuth 2.1 + OpenID Connect (OIDC) for user authentication
• Short-lived access tokens for API access
• Refresh tokens only if you need long-lived sessions
• RBAC/ABAC for authorization, separate from authentication

Recommended approach

If you need users to log in

Use an identity provider (IdP) that supports:

• OAuth 2.1
• OpenID Connect
• MFA / passkeys
• social login or enterprise SSO if needed

Examples:

• Auth0
• Okta
• Clerk
• Cognito
• Firebase Auth
• Azure AD B2C / Entra External ID

For your API

Have your backend validate:

• JWT access tokens signed by the IdP, or
• opaque tokens with introspection if you want centralized revocation

Prefer:

• JWTs when you want simpler, stateless validation
• Opaque tokens when revocation/control is more important

Security best practices

• Use Authorization Code Flow with PKCE
• Never use the old implicit flow
• Keep access tokens short-lived (5–15 min)
• Store refresh tokens securely and rotate them
• Use HTTPS everywhere
• Separate authentication from authorization
• Add rate limiting, device/session tracking, and audit logs
• Support MFA and ideally passkeys/WebAuthn
• Validate issuer, audience, expiry, and signature on every request

If you’re building a machine-to-machine API

Use:

• Client Credentials flow for service accounts
• Or better, service identities with scoped permissions

If you want the simplest secure setup

A good default stack is:

• OIDC provider for login
• Authorization Code + PKCE on the frontend
• JWT access tokens for your API
• Refresh token rotation
• Role/scopes-based authorization
• MFA/passkeys optional but recommended

If you tell me:

1. web app, mobile app, or pure API
2. whether users log in with passwords or SSO
3. whether you need B2C or B2B auth

…I can recommend a concrete architecture and provider.