OAuth 2.0 Authorization Sequence Diagram Template

This sequence diagram template maps out the OAuth 2.0 Authorization Code Grant flow, illustrating every interaction between the four core participants: the Resource Owner (end user), the Client application, the Authorization Server, and the Resource Server. Step by step, it traces the redirect to the authorization endpoint, the issuance of an authorization code, the back-channel token exchange, and the final authenticated API call. By visualizing these exchanges in chronological order, the diagram makes it immediately clear which party initiates each request, what data is passed, and where security boundaries exist.

## When to Use This Template

This template is most valuable when onboarding engineers to an OAuth 2.0 integration, conducting security reviews, or writing technical documentation for a product that delegates authentication to an identity provider such as Google, Okta, or Auth0. It is equally useful during threat modeling sessions, where teams need to identify where tokens are exposed, how the state parameter prevents CSRF attacks, and why the authorization code is short-lived. Product managers and technical writers can also use it to explain the login flow to non-engineering stakeholders without diving into raw HTTP requests.

## Common Mistakes to Avoid

One of the most frequent errors when diagramming this flow is collapsing the front-channel and back-channel exchanges into a single arrow, which obscures the critical security distinction between them. The authorization code travels through the browser (front channel), while the token exchange happens server-to-server (back channel) — these must be shown as separate, clearly labeled lifelines. Another common mistake is omitting the `state` parameter round-trip, which is essential for demonstrating CSRF protection and is often scrutinized in security audits. Developers also frequently forget to show the PKCE (Proof Key for Code Exchange) extension when the client is a public app such as a single-page application or mobile client; leaving it out can give a misleading picture of the security posture. Finally, avoid merging the Authorization Server and Resource Server into one box unless your architecture genuinely combines them — keeping them separate reflects real-world deployments and helps readers understand token validation responsibilities.