I'm building an API that needs third-party authorization. How should I set up OAuth?

If your API needs third-party authorization, the usual choice is OAuth 2.0. The exact setup depends on whether you’re authorizing:

1. A user to let your app access their data
2. Your API to let another service access it on behalf of a user
3. Service-to-service access with no user involved

Recommended OAuth flow

For most modern apps, use:

• Authorization Code Flow with PKCE for:
  • web apps
  • mobile apps
  • SPA/front-end apps
• Client Credentials Flow for machine-to-machine access
• Avoid the old Implicit Flow; it’s generally deprecated

Typical components

You’ll usually need:

• Authorization Server: issues tokens
• Resource Server: your API
• Client App: the third-party app requesting access
• Scopes: define what the client can do

High-level setup

1. Register OAuth clients

For each third-party app, create a client record with:

• client ID
• redirect URIs
• allowed grant types
• allowed scopes
• optional client secret
  • use a secret only for confidential clients
  • don’t use secrets in SPAs or mobile apps unless protected by an external secure mechanism

2. Define scopes carefully

Examples:

• read:profile
• read:orders
• write:orders

Keep scopes:

• narrow
• understandable
• auditable

3. Use Authorization Code + PKCE for user-based access

Flow:

1. User is redirected to your authorization server
2. User authenticates and approves requested scopes
3. Authorization server redirects back with an authorization code
4. Client exchanges code + PKCE verifier for access token
5. Client calls your API with Authorization: Bearer <token>

4. Issue short-lived access tokens

Best practice:

• access tokens: short TTL, e.g. 5–15 minutes
• refresh tokens: if needed, rotate them and store securely
• consider refresh token rotation and reuse detection

5. Validate tokens in the API

Your API should verify:

• signature
• expiration (exp)
• issuer (iss)
• audience (aud)
• scopes/permissions
• client/app identity if relevant

If using JWTs:

• publish JWKS for public key verification
• rotate signing keys
• cache keys safely

If using opaque tokens:

• use token introspection

Security best practices

• Always use HTTPS
• Use PKCE even for clients that can keep a secret if the flow involves a browser
• Never put client secrets in front-end code
• Use state to prevent CSRF
• Use exact redirect URI matching
• Store tokens securely
• Minimize requested scopes
• Log auth events, but never log tokens
• Support token revocation and consent withdrawal

If you’re building the auth server yourself

Be careful—OAuth is easy to get wrong. If possible, use a mature provider or library:

• Auth0
• Okta
• Keycloak
• Microsoft Entra ID
• AWS Cognito
• Duende IdentityServer (.NET)
• Spring Authorization Server (Java)

Practical recommendation

If you’re starting from scratch, I’d suggest:

• OAuth 2.0 Authorization Code Flow with PKCE
• JWT access tokens
• Short-lived tokens
• Refresh token rotation
• Scopes for authorization
• OpenID Connect too, if you need user login / identity

If you want, I can also give you:

• a sequence diagram for the flow,
• a sample token validation middleware, or
• a step-by-step setup for a specific stack like Node.js, Python, Java, or .NET.