OAuth Explained Simply: Authorization Code, PKCE, Access Tokens, and Refresh Tokens
OAuth lets an application receive limited access to another service without receiving the user's password.
It is primarily an authorization framework, not a login protocol by itself. Login/identity is commonly layered on top through OpenID Connect.
The Main Roles
- Resource owner — the user/entity whose resources are protected.
- Client — the application requesting access.
- Authorization server — authenticates/authorizes and issues tokens.
- Resource server — the API that accepts access tokens.
Authorization Code + PKCE
For browser/mobile/user-facing authorization, the modern baseline is the Authorization Code flow with PKCE.
PKCE binds the authorization request to the later token exchange, reducing authorization-code interception/injection risk. RFC 9700 requires PKCE for public clients and recommends it for confidential clients as well.
Use the S256 challenge method.
Access Tokens
The access token is presented to the resource API and should carry only the authority needed for that client/use case.
Keep access tokens short-lived enough for your risk model and validate issuer/audience/scope according to the token format and authorization-server contract.
Refresh Tokens
A refresh token can obtain new access tokens without making the user repeat the entire authorization flow.
Treat refresh tokens as high-value credentials. Their use, rotation, expiration, and revocation should follow the authorization server's security model.
Important 2026 Status
OAuth 2.1 is still an active IETF Internet-Draft as of September 2026, so do not describe it as a finalized RFC yet.
For production security today, follow OAuth 2.0 specifications together with RFC 9700, OAuth 2.0 Security Best Current Practice. RFC 9700 also discourages legacy implicit-token flows in favor of code-based flows and requires exact redirect-URI matching in normal cases.
Final Takeaway
OAuth is a protocol for delegated authorization. For modern apps, understand Authorization Code + PKCE first, keep redirect URIs tightly registered, scope tokens narrowly, and use OpenID Connect when you need standardized user authentication/identity.
References
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