Authorization Code with PKCE is the default flow for modern browser and mobile applications because it keeps tokens out of the front channel and gives the authorization server control over client verification.

Why PKCE Matters

Public clients cannot safely store a client secret. PKCE solves that gap by binding the authorization request to the token exchange with a one-time verifier. If an authorization code is intercepted, the attacker still cannot redeem it without the original verifier.

Flow Overview

The app redirects the user to the identity platform, the user authenticates, consent is evaluated, and an authorization code is returned to the client's redirect URI. The app then exchanges that code for tokens through a back-channel request.

Backend APIs should validate issuer, audience, expiration, and signature before trusting any access token. The browser never becomes the authority for permission checks.

Operational Considerations

Keep redirect URIs exact, rotate refresh tokens, and log failed exchange attempts. Those details usually matter more than the diagram when teams move from local demos into production environments.