Block HS* algorithms when JWTVerifier is configured with JWKS (#3419)

* Block HS* algorithms with JWKS in JWT verifier

🤖 Generated with GPT-5.2-Codex

* Fix ruff format: remove extra blank line in test_supabase.py

🤖 Generated with Claude Code

Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>

---------

Co-authored-by: Marvin Context Protocol <41898282+Marvin Context Protocol@users.noreply.github.com>
Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>

src/fastmcp/server/auth/providers/jwt.py

@@ -225,6 +225,12 @@ class JWTVerifier(TokenVerifier):
         }:
             raise ValueError(f"Unsupported algorithm: {algorithm}.")
 
+        if jwks_uri and algorithm.startswith("HS"):
+            raise ValueError(
+                "HMAC algorithms (HS256/HS384/HS512) require a shared secret via "
+                "public_key and cannot be used with jwks_uri"
+            )
+
         # Parse scopes if provided as string
         parsed_required_scopes = (
             parse_scopes(required_scopes) if required_scopes is not None else None

tests/server/auth/providers/test_supabase.py

@@ -93,7 +93,7 @@ class TestSupabaseProvider:
 
     @pytest.mark.parametrize(
         "algorithm",
-        ["HS256", "RS256", "ES256"],
+        ["RS256", "ES256"],
     )
     def test_algorithm_configuration(self, algorithm):
         """Test that algorithm can be configured for different JWT signing methods."""
@@ -106,6 +106,15 @@ class TestSupabaseProvider:
         assert isinstance(provider.token_verifier, JWTVerifier)
         assert provider.token_verifier.algorithm == algorithm
 
+    def test_algorithm_hs256_rejected(self):
+        """Test that HS256 is rejected with Supabase JWKS verification."""
+        with pytest.raises(ValueError, match="cannot be used with jwks_uri"):
+            SupabaseProvider(
+                project_url="https://abc123.supabase.co",
+                base_url="https://myserver.com",
+                algorithm="HS256",
+            )
+
     def test_algorithm_default_es256(self):
         """Test that algorithm defaults to ES256 when not specified."""
         provider = SupabaseProvider(

tests/server/auth/test_jwt_provider.py

@@ -201,6 +201,15 @@ class TestSymmetricKeyJWT:
         assert provider.algorithm == "HS256"
         assert provider.jwks_uri is None
 
+    def test_initialization_rejects_hs_algorithm_with_jwks_uri(self):
+        """Test that HMAC algorithms cannot be used with JWKS URI."""
+        with pytest.raises(ValueError, match="cannot be used with jwks_uri"):
+            JWTVerifier(
+                jwks_uri="https://test.example.com/.well-known/jwks.json",
+                issuer="https://test.example.com",
+                algorithm="HS256",
+            )
+
     def test_initialization_with_different_symmetric_algorithms(
         self, symmetric_key_helper: SymmetricKeyHelper
     ):