CORS

CORS errors are the most frustrating errors in OAuth development. The browser blocks your request, the error message is generic, and the actual cause could be any of 8+ different scenarios. This guide covers every CORS error you’ll encounter in OAuth 2.0 and OIDC flows, with exact browser error messages and provider-specific fixes. Quick Diagnostic: Which Error Are You Seeing? Browser Console Error Jump To No 'Access-Control-Allow-Origin' header on /authorize Scenario 1: Calling /authorize via fetch No 'Access-Control-Allow-Origin' header on /token Scenario 2: Token endpoint CORS AADSTS9002327: Cross-origin token redemption Scenario 3: Azure AD SPA registration CORS error only after session timeout Scenario 4: Keycloak error response bug wildcard '*' when credentials mode is 'include' Scenario 5: Wildcard with credentials Response to preflight request doesn't pass Scenario 6: Preflight failures CORS error on /revoke endpoint Scenario 7: Token revocation Everything works except in production Scenario 8: Proxy/CDN stripping headers Which OAuth Endpoints Support CORS? Before debugging, know which endpoints are designed to accept cross-origin requests: ...