Gearboy, a Game Boy emulator, includes an MCP server that can be enabled with a --mcp-http flag. The server binds to all network interfaces and accepts HTTP POST requests to /mcp without any authentication or header checks. An attacker who reaches the MCP HTTP port can start a session and invoke debugging or emulator functions that are normally protected, effectively allowing remote manipulation of the emulator’s state. Although the flaw does not provide direct host code execution, the exposed operations could potentially access local files or affect the debugging environment, creating a high‑impact vector within the confines of the running process.

The vulnerability affects all installations of drhelius Gearboy versions prior to 3.8.7 that are run with the optional --mcp-http mode enabled. Version 3.8.7 and later include a fix that removes or restricts the accidentally exposed network access.

The CVSS score of 8.6 indicates high severity. EPSS data is not available, so the current exploitation probability is unknown, but the flaw is publicly disclosed and listed in a GitHub advisory, making it potentially exploitable. The CISA KEV catalog does not list this vulnerability, so it is not confirmed as a known exploit, yet the easy network reachability and lack of authentication represent an attractive attack vector for adversaries who can discover the port on a host running Gearboy. The likely attack path is a remote host sending unauthenticated HTTP POST requests to the MCP endpoint, without needing credentials or CORS restrictions, leading to unauthorized command execution inside the emulator process.