------------------------------------------------------------------ == PKCE parameters (RFC 7636) ------------------------------------------------------------------ code_verifier s3e6dmr3wSf4_lvze8F93m7moAckkHVcu6hKAerx3Ug77jqz38_iNhnOtio2dNj6 (64 chars) code_challenge fIGXtoxshuoWNmv8gk1b7wWtLzxOzB0fSenfe02UwAs code_challenge_method S256 The verifier never leaves the client until the token request. The challenge is all the authorization request carries, and it is a one-way hash of the verifier. ------------------------------------------------------------------ == 1. Log in to the authorization server (browser session) ------------------------------------------------------------------ $ curl -c jar -d username=alice -d password=password -d _csrf= http://localhost:9000/login HTTP/1.1 302 Location: http://localhost:9000/ ------------------------------------------------------------------ == 2. GET /oauth2/authorize (client=demo-spa) ------------------------------------------------------------------ $ curl -b jar 'http://localhost:9000/oauth2/authorize?response_type=code&client_id=demo-spa&redirect_uri=http%3A%2F%2F127.0.0.1%3A8080%2Fauthorized&scope=openid%20orders.read&state=xyz123&code_challenge=fIGXtoxshuoWNmv8gk1b7wWtLzxOzB0fSenfe02UwAs&code_challenge_method=S256' -> 302 http://localhost:9000/oauth2/consent?scope=openid%20orders.read&client_id=demo-spa&state=zjgza5hwc4CbJeJepOgTFZXTDk1IBgjWjsGo_mupymU%3D ------------------------------------------------------------------ == 3. The consent page ------------------------------------------------------------------ The authorization endpoint redirected to OUR page, at the path given to .consentPage("/oauth2/consent"). Note the query string it hands over: http://localhost:9000/oauth2/consent?scope=openid%20orders.read client_id=demo-spa state=zjgza5hwc4CbJeJepOgTFZXTDk1IBgjWjsGo_mupymU%3D Scopes rendered as checkboxes (openid deliberately not among them): orders.read The hidden state the form must echo back: zjgza5hwc4CbJeJepOgTFZXTDk1IBgjWjsGo_mupymU= (this is NOT the client's state=xyz123 - it is the server's own correlation handle for the pending authorization request, and sending the client's value instead is what produces the consent redirect loop) ------------------------------------------------------------------ == 4. POST the approval to /oauth2/authorize ------------------------------------------------------------------ $ curl -b jar -X POST -d client_id=demo-spa -d state=zjgza5hwc4CbJeJepOgTFZXTDk1IBgjWjsGo_mupymU= -d _csrf=f7zUD7ezBE6G0H5DQuPikT9CkEEKR9JuA2Y8njWwhSkoewGDG4ziN46BZXerthpycc7Wp1x6vSBrJetDOwMLr1SI4EpOGDG1 -d scope=orders.read http://localhost:9000/oauth2/authorize -> 302 http://127.0.0.1:8080/authorized?code=whzdCa9Z_53d6sCkZYkNNswxQLQ46kDsMy9ynR0rOvy6KqP741fpHOAC5nF4PsN6O0Sqvi3R9kksVer6L6V3Zk9Ii_LcHOYdHqWGEiE9O8jDDRJjqr-LiTnpEDI12St1&state=xyz123 ------------------------------------------------------------------ == 5. The authorization code ------------------------------------------------------------------ code = whzdCa9Z_53d6sCkZYkNNswxQLQ46kDsMy9ynR0rOvy6KqP741fpHOAC5nF4PsN6O0Sqvi3R9kksVer6L6V3Zk9Ii_LcHOYdHqWGEiE9O8jDDRJjqr-LiTnpEDI12St1 state = xyz123 (the client's own value, returned untouched - compare it) ------------------------------------------------------------------ == 6a. Exchange the code WITHOUT the verifier ------------------------------------------------------------------ This is the request an attacker who stole the code can make. HTTP 401 (empty response body) >>> Rejected. invalid_grant is deliberately vague: the server will not tell >>> a caller whether the code was wrong, expired, already used, or missing a >>> verifier, because each of those is information an attacker can use. Note: this consumed the code. Authorization codes are single-use, so the successful exchange below needs a fresh one. ------------------------------------------------------------------ == 6b. A fresh code, exchanged properly ------------------------------------------------------------------ fresh code = B3GUYBE-2tE9C3eSx8_m8ejjXhPwFYEHuTrMwvmNpj-xZuDsPhutSgDt3pY88aE4bSmclgB8mDWFb_SiCODkeheeyCkogZkPwSLFFY4MqbZ1WlUipMynXR1jn9L0UIav $ curl -d grant_type=authorization_code -d code=... -d code_verifier=... http://localhost:9000/oauth2/token HTTP 200 { "access_token": "eyJraWQiOiIyYjNjZGFhMy1kMDE1LTQ3NTItOWZiYS0xNmM3YjI0NTJiNTciLCJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJhbGljZSIsImF1ZCI6Im9yZGVycy1hcGkiLCJuYmYiOjE3ODc1Mzg3NDYsInNjb3BlIjpbIm9wZW5pZCIsIm9yZGVycy5yZWFkIl0sInJvbGVzIjpbIkFETUlOIiwiVVNFUiJdLCJpc3MiOiJodHRwOi8vbG9jYWxob3N0OjkwMDAiLCJleHAiOjE3ODc1MzkwNDYsImlhdCI6MTc4NzUzODc0NiwianRpIjoiZmI3ZTUyOTEtMTc2OC00NjI2LWJhODUtNWQ3NWI2OGZmNjcyIiwidGVuYW50IjoiYWNtZSJ9.e048ANvC61A6JPe7rE5k_aQDFcQTTCWNRg11j6Nj3MoUsqPvRIPFtYWvK0_l20HnUWZn0X84hy-ifgi4f_nBYDXDXFI-Ya2bRjRnnhtincjZsLNB8RUUDK5tqAPMSdfcceM48cTLLDrvwrxIp0ASG697aBuRVmndcUWyMVfZPzSidR3h0ydeWUVaY7NmL8d6pOzgLLNlDTIjyipURUboda7Mcw7KTHxodM_saz1xwQTzozRtcybmreUw44O6b07wjATcIzAwAJzASaiX2ElUyYs9lWAtxQ-JSiGX4htmfLJiONVnuEZNSy3uvLQnTfnl4G4OY2mjK1zJzIT-wBfX6w", "scope": "openid orders.read", "id_token": "eyJraWQiOiIyYjNjZGFhMy1kMDE1LTQ3NTItOWZiYS0xNmM3YjI0NTJiNTciLCJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJhbGljZSIsImF1ZCI6ImRlbW8tc3BhIiwiYXpwIjoiZGVtby1zcGEiLCJhdXRoX3RpbWUiOjE3ODc1Mzg3NDYsImlzcyI6Imh0dHA6Ly9sb2NhbGhvc3Q6OTAwMCIsInByZWZlcnJlZF91c2VybmFtZSI6ImFsaWNlIiwiZXhwIjoxNzg3NTQwNTQ2LCJpYXQiOjE3ODc1Mzg3NDYsImp0aSI6IjBjY2IzZjI4LTk1MWYtNGU5ZS1iNzkzLTNjNWExNjdmNDg1MSIsInNpZCI6InQ1dUpXUDM2V1dYWTNXX2hkaURueTR1OGlEZlVyZHlLd1EyRE9XWUdoSzgifQ.B6kGKui5mOyakllBv7xmmlVcsy6BYUN-JNALZLKMPz-O0WZO6WigSYfHKXdHtYVGsNdxVpFgeyWYPyi6JOTX9ALaDeJcEAiUwZFEN4PJa-lSzZvdT6m5atD1OCQqXOd7c_4bqD596I94Gu22arBij6UYaNBagLdBmtbAhJYMhBT5Q7Ygzo5m0w7Ru-oxU_cK06R4ZggNGo9B0NWJfqXroTOdFzfJmptl3CN9Ddh14pj5pz4w0hAMkxSMsJ-GAgm85ldD9aGVzFfjZ4-lzI7gveUVn_IzWZQbCY3_SYUuoGQFe60I0L6cDyot88qfuXn3m1udEaQVvrJlE7SBxMIFmQ", "token_type": "Bearer", "expires_in": 299 } ------------------------------------------------------------------ == 7. The access token ------------------------------------------------------------------ { "alg": "RS256", "kid": "2b3cdaa3-d015-4752-9fba-16c7b2452b57" } { "aud": "orders-api", "exp": 1787539046, "iat": 1787538746, "iss": "http://localhost:9000", "jti": "fb7e5291-1768-4626-ba85-5d75b68ff672", "nbf": 1787538746, "roles": [ "ADMIN", "USER" ], "scope": [ "openid", "orders.read" ], "sub": "alice", "tenant": "acme" } ------------------------------------------------------------------ == 8. The id_token - a different token, for a different audience ------------------------------------------------------------------ { "aud": "demo-spa", "auth_time": 1787538746, "azp": "demo-spa", "exp": 1787540546, "iat": 1787538746, "iss": "http://localhost:9000", "jti": "0ccb3f28-951f-4e9e-b793-3c5a167f4851", "preferred_username": "alice", "sid": "t5uJWP36WWXY3W_hdiDny4u8iDfUrdyKwQ2DOWYGhK8", "sub": "alice" } aud is the CLIENT here, not the API. Sending this to a resource server is the classic mix-up: it verifies (same issuer, same key) and then fails the audience check, or worse, passes it if nobody checks audience. ------------------------------------------------------------------ == 9. Calling the resource server ------------------------------------------------------------------ GET /api/orders -> 200 {"orders":[{"total":"42.00","id":1}],"subject":"alice","clientId":null,"scopes":["openid","orders.read"],"roles":["ADMIN","USER"],"tenant":"acme","audience":["orders-api"]} GET /api/admin -> 200 {"authorities":["SCOPE_openid","ROLE_USER","SCOPE_orders.read","ROLE_ADMIN","FactorGrantedAuthority [authority=FACTOR_BEARER, issuedAt=2026-08-24T02:32:26.714311634Z]"],"message":"admin only"} ------------------------------------------------------------------ == 10. Sending the id_token instead ------------------------------------------------------------------ HTTP/1.1 401 WWW-Authenticate: Bearer error="invalid_token", error_description="An error occurred while attempting to decode the Jwt: the required audience orders-api is missing", error_uri="https://tools.ietf.org/html/rfc6750#section-3.1", resource_metadata="http://localhost:8090/.well-known/oauth-protected-resource"