Gearboy advisory / Gearsystem body: Unauthenticated network exposure of MCP HTTP tools

Gearboy is a Game Boy, Game Boy Color, and Super Game Boy emulator and debugger with an embedded MCP server. Gearsystem, the affected component, has an optional --mcp-http mode that accepts unauthenticated MCP requests from any reachable network peer despite localhost-only documentation and logging. In the described platforms/shared/desktop/mcp/mcp_transport.h transport, the socket binds INADDR_ANY, normally on port 7777, and POST requests to /mcp reach dispatch without Authorization, Host, or Origin checks. Responses and OPTIONS advertise Access-Control-Allow-Origin: *. A peer can initialize an MCP session and invoke exposed emulator/debugging operations; even a garbage bearer token does not block dispatch, while a loopback-bind negative control prevents network access. The reported impact is bounded to the emulator process, debugging state, and local files accepted as media, rather than demonstrated host code execution. Gearboy 3.8.7 contains a patch.