Add HTTP status check before parsing token responses

- Prevents misleading 'Missing access_token parameter' errors when
  server returns HTTP errors.
- Properly raises HTTPError with the actual error message received
  from the server for 4xx/5xx responses.

Author: orgads

requests_oauthlib/oauth2_session.py

@@ -4,6 +4,7 @@
 from oauthlib.oauth2 import WebApplicationClient, InsecureTransportError
 from oauthlib.oauth2 import LegacyApplicationClient
 from oauthlib.oauth2 import TokenExpiredError, is_secure_transport
+from oauthlib.oauth2.rfc6749.errors import CustomOAuth2Error
 import requests
 
 log = logging.getLogger(__name__)
@@ -199,6 +200,17 @@ def authorization_url(self, url, state=None, **kwargs):
             state,
         )
 
+    def validate_token_response(self, r):
+        message = ""
+        try:
+            r.raise_for_status()
+        except requests.HTTPError as e:
+            message = str(e)
+            if r.text:
+                message += f"\nBody: {r.text}"
+        if message:
+            raise CustomOAuth2Error('Response error', message, uri=r.request.url, status_code=r.status_code)
+
     def fetch_token(
         self,
         token_url,
@@ -403,6 +415,7 @@ def fetch_token(
             log.debug("Invoking hook %s.", hook)
             r = hook(r)
 
+        self.validate_token_response(r)
         self._client.parse_request_body_response(r.text, scope=self.scope)
         self.token = self._client.token
         log.debug("Obtained token %s.", self.token)
@@ -493,6 +506,7 @@ def refresh_token(
             log.debug("Invoking hook %s.", hook)
             r = hook(r)
 
+        self.validate_token_response(r)
         self.token = self._client.parse_request_body_response(r.text, scope=self.scope)
         if "refresh_token" not in self.token:
             log.debug("No new refresh token given. Re-using old.")

tests/test_oauth2_session.py

@@ -12,6 +12,7 @@
 from oauthlib.common import urlencode
 from oauthlib.oauth2 import TokenExpiredError, OAuth2Error
 from oauthlib.oauth2 import MismatchingStateError
+from oauthlib.oauth2.rfc6749.errors import CustomOAuth2Error
 from oauthlib.oauth2 import WebApplicationClient, MobileApplicationClient
 from oauthlib.oauth2 import LegacyApplicationClient, BackendApplicationClient
 from requests_oauthlib import OAuth2Session, TokenUpdated
@@ -524,6 +525,63 @@ def fake_send(r, **kwargs):
                 sess.fetch_token(url)
             self.assertTrue(sess.authorized)
 
+    def test_fetch_token_http_error_handling(self):
+        """Test that HTTP errors are properly raised instead of parsing error responses as tokens."""
+        url = "https://example.com/token"
+
+        # Test 400 error response (like the original issue)
+        error_400 = {"messages": [{"logLevel": "Error", "text": "User not Found"}], "status": "Forbidden"}
+
+        def fake_400_error(r, **kwargs):
+            resp = mock.MagicMock()
+            resp.text = json.dumps(error_400)
+            resp.status_code = 400
+            resp.request = mock.MagicMock()
+            resp.request.url = url
+            resp.raise_for_status.side_effect = requests.exceptions.HTTPError(
+                "400 Client Error", response=resp
+            )
+            return resp
+
+        for client in self.clients:
+            sess = OAuth2Session(client=client)
+            sess.send = fake_400_error
+
+            if isinstance(client, LegacyApplicationClient):
+                self.assertRaises(
+                    CustomOAuth2Error,
+                    sess.fetch_token,
+                    url,
+                    username="username1",
+                    password="password1",
+                )
+            else:
+                self.assertRaises(CustomOAuth2Error, sess.fetch_token, url)
+
+    def test_refresh_token_http_error_handling(self):
+        """Test that HTTP errors are properly raised instead of parsing error responses as tokens."""
+        url = "https://example.com/refresh"
+
+        # Test 400 error response
+        error_400 = {"error": "invalid_grant",
+                     "error_description": "Refresh token is invalid"}
+
+        def fake_400_error(r, **kwargs):
+            resp = mock.MagicMock()
+            resp.text = json.dumps(error_400)
+            resp.status_code = 400
+            resp.request = mock.MagicMock()
+            resp.request.url = url
+            resp.raise_for_status.side_effect = requests.exceptions.HTTPError(
+                "400 Client Error", response=resp
+            )
+            return resp
+
+        for client in self.clients:
+            sess = OAuth2Session(client=client, token=self.token)
+            sess.send = fake_400_error
+            self.assertRaises(CustomOAuth2Error, sess.refresh_token, url)
+
 
 class OAuth2SessionNetrcTest(OAuth2SessionTest):
     """Ensure that there is no magic auth handling.