Summary

The java-sdk contains a DNS rebinding vulnerability. This vulnerability allows an attacker to access a locally or network-private java-sdk MCP server via a victims browser that is either local, or network adjacent.

This allows an attacker to make any tool call to the server as if they were a locally running MCP connected AI agent.

Details

Prior to 1.0.0 no Origin header validation was occurring, in violation of the MCP specification. Base Protocol > Transports: 2.0.1 Security Warning:

1: Servers MUST validate the Origin header on all incoming connections to prevent DNS rebinding attacks.

When the web server serving HTTP traffic to the MCP server does not perform standard CORS checks, a DNS rebinding attack is possible.

Some default server configurations and frameworks come with embedded Origin header validation. MCP servers built using those are not vulnerable to this issue. For example, the following are NOT vulnerable: - Spring AI

Impact

Any developer connecting to a malicious website can inadvertently allow an attacker to make tool calls to local or private-network MCP servers.

Workarounds

Users can mitigate this risk by: 1. Running the MCP server behind a reverse proxy (like Nginx or HAProxy) configured to strictly validate the Host and Origin headers. 2. Using a framework that inherently enforces strict CORS and Origin validation (such as Spring AI).

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