Summary

Hardcoded Wildcard CORS (Access-Control-Allow-Origin: )

- https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289 - https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525

Attack Scenario An attacker-controlled web page instructs the victim's browser to open GET https://internal-mcp-server/sse. Because Access-Control-Allow-Origin: allows cross-origin SSE reads, the attacker's page receives the endpoint event — which contains the session ID. The attacker can then POST to that endpoint from their page using the victim's browser as a relay.

Comparison with python-sdk No Access-Control-Allow-Origin header is emitted by either Python transport. The browser's default same-origin policy remains in full effect. https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/sse.py https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamablehttp.py

Recommendation In the SDK, the transport layer should not own CORS policy. Server implementors who need cross-origin access can add a CORS filter at the servlet filter or Spring Security layer.

Resources

- https://cheatsheetseries.owasp.org/cheatsheets/HTTPHeadersCheatSheet.html#access-control-allow-origin