CVE-2026-61542: Gearboy advisory / Gearsystem body: Unauthenticated network exposure of MCP HTTP tools

Q: Which deployments are exposed to remote attackers?

Deployments running Gearsystem with the optional --mcp-http mode are exposed if port 7777, or the configured MCP HTTP port, is reachable by an untrusted network peer. The affected transport binds to INADDR_ANY rather than loopback; a loopback-only bind prevents network access.

Q: Does an attacker need credentials or user interaction?

No. POST requests to /mcp are dispatched without Authorization, Host, or Origin validation, and even a garbage bearer token does not prevent dispatch. An attacker only needs network reachability to the MCP HTTP listener.

Q: What should be done if upgrading cannot happen immediately?

Do not run the optional --mcp-http mode on networks reachable by untrusted peers, or ensure the listener is bound only to loopback. Restricting access to the MCP HTTP port also reduces exposure.

Published Oct 28, 2026

Updated

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/mcptransport.h transport, the socket binds INADDRANY, 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.

Affected Software

1 affected component

Gearboy Gearsystem<3.8.7

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade Gearboy to a version that resolves this vulnerability.
Fixed in 3.8.7
Compensating control
Bind the MCP HTTP server to the loopback interface instead of INADDR_ANY to prevent network access from reachable peers.

Event History

Oct 28, 2026

CVE Published

via MITRE·07:48 PM

Data Sourced

via MITRE·07:48 PM

DescriptionSeverityWeakness

Frequently Asked Questions

Which deployments are exposed to remote attackers?

Deployments running Gearsystem with the optional --mcp-http mode are exposed if port 7777, or the configured MCP HTTP port, is reachable by an untrusted network peer. The affected transport binds to INADDR_ANY rather than loopback; a loopback-only bind prevents network access.

Does an attacker need credentials or user interaction?

No. POST requests to /mcp are dispatched without Authorization, Host, or Origin validation, and even a garbage bearer token does not prevent dispatch. An attacker only needs network reachability to the MCP HTTP listener.

What can an unauthenticated peer do?

A peer can initialize an MCP session and invoke exposed emulator and debugging operations. The reported impact is limited to the emulator process, debugging state, and local files accepted as media; host code execution was not demonstrated.

What should be done if upgrading cannot happen immediately?

Do not run the optional --mcp-http mode on networks reachable by untrusted peers, or ensure the listener is bound only to loopback. Restricting access to the MCP HTTP port also reduces exposure.

Which version includes a patch?

Gearboy 3.8.7 contains a patch.