From 5a3422e829abcf795038567daaec9501ccc5e3f5 Mon Sep 17 00:00:00 2001 From: manjunathbhaskar Date: Sun, 26 Jul 2026 09:40:55 +0200 Subject: [PATCH] fix(client): don't send client_id in token body under client_secret_basic RFC 6749 section 2.3 requires that with HTTP Basic auth, client credentials must not also appear in the request body. prepare_token_auth stripped client_secret from the body for the client_secret_basic branch but left client_id in, so strict token endpoints (Keycloak, Okta in strict mode, and the RFC 6749 compliance test suite) reject the request as presenting two authentication methods at once. Fixes #3138 Two existing tests asserted the old behavior explicitly, updated both to assert the corrected one, and added the same check to the refresh token test for symmetry. Signed-off-by: manjunathbhaskar --- src/mcp/client/auth/oauth2.py | 7 +++++-- tests/client/test_auth.py | 8 +++++++- 2 files changed, 12 insertions(+), 3 deletions(-) diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py index 073e6a8039..c4e2965f16 100644 --- a/src/mcp/client/auth/oauth2.py +++ b/src/mcp/client/auth/oauth2.py @@ -207,8 +207,11 @@ def prepare_token_auth( credentials = f"{encoded_id}:{encoded_secret}" encoded_credentials = base64.b64encode(credentials.encode()).decode() headers["Authorization"] = f"Basic {encoded_credentials}" - # Don't include client_secret in body for basic auth - data = {k: v for k, v in data.items() if k != "client_secret"} + # RFC 6749 section 2.3: with HTTP Basic auth, client credentials must not also + # appear in the request body. Strict token endpoints reject a request that + # presents both the Authorization header and client_id in the body as two + # authentication methods at once. + data = {k: v for k, v in data.items() if k not in ("client_secret", "client_id")} elif auth_method == "client_secret_post" and self.client_info.client_id and self.client_info.client_secret: # Include client_id and client_secret in request body (RFC 6749 ยง2.3.1) data["client_id"] = self.client_info.client_id diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py index 9e9599f86c..24259eacf5 100644 --- a/tests/client/test_auth.py +++ b/tests/client/test_auth.py @@ -680,7 +680,10 @@ async def test_basic_auth_token_exchange(self, oauth_provider: OAuthClientProvid # client_secret should NOT be in body for basic auth content = request.content.decode() assert "client_secret=" not in content - assert "client_id=test%40client" in content # client_id still in body + # RFC 6749 section 2.3: with Basic auth, client_id must not appear in the body + # either, it is already presented via the Authorization header, and strict token + # endpoints reject a request that presents credentials both ways. + assert "client_id=" not in content @pytest.mark.anyio async def test_basic_auth_refresh_token(self, oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken): @@ -716,6 +719,9 @@ async def test_basic_auth_refresh_token(self, oauth_provider: OAuthClientProvide # client_secret should NOT be in body content = request.content.decode() assert "client_secret=" not in content + # ...and neither should client_id, for the same RFC 6749 section 2.3 reason as + # token exchange. + assert "client_id=" not in content @pytest.mark.anyio async def test_none_auth_method(self, oauth_provider: OAuthClientProvider):