fix(mcp): reject YAML aliases in OpenAPI specs

litellm/proxy/_experimental/mcp_server/openapi_to_mcp_generator.py

@@ -183,6 +183,11 @@ def _parse_openapi_spec(text: str) -> Mapping[str, Any]:
         import yaml
 
         class _NoMergeSafeLoader(yaml.SafeLoader):
+            def compose_node(self, parent: Any, index: Any) -> Any:
+                if self.check_event(yaml.events.AliasEvent):
+                    raise yaml.YAMLError("YAML aliases are not supported")
+                return super().compose_node(parent, index)
+
             def flatten_mapping(self, node: Any) -> None:
                 if any(key_node.tag == "tag:yaml.org,2002:merge" for key_node, _ in node.value):
                     raise yaml.YAMLError("YAML merge keys are not supported")

tests/unit/proxy/_experimental/mcp_server/test_openapi_to_mcp_generator.py

@@ -315,7 +315,7 @@ def test_parse_openapi_spec_rejects_non_object_document() -> None:
 def test_parse_openapi_spec_rejects_yaml_merge_aliases() -> None:
     import yaml
 
-    with pytest.raises(yaml.YAMLError, match="YAML merge keys are not supported"):
+    with pytest.raises(yaml.YAMLError, match="YAML aliases are not supported"):
         gen._parse_openapi_spec(
             """
 base: &base
@@ -330,6 +330,23 @@ merged:
         )
 
 
+def test_parse_openapi_spec_rejects_yaml_aliases() -> None:
+    import yaml
+
+    with pytest.raises(yaml.YAMLError, match="YAML aliases are not supported"):
+        gen._parse_openapi_spec(
+            """
+shared: &shared
+  type: string
+components:
+  schemas:
+    Request:
+      properties:
+        first: *shared
+"""
+        )
+
+
 def _create_mock_client(method: str, response_text: str, status_code: int = 200) -> AsyncMock:
     """Utility to create a mocked async httpx client for the given method.