MCP Bridge API Command Injection via Servers Endpoint

Technical Analysis

The vulnerability resides in an MCP bridge component that exposes a network-facing endpoint accepting command and args parameters intended to invoke server-side tool processes. Because these inputs are passed to a shell or process execution function without adequate validation or escaping, an attacker can inject additional shell metacharacters or arbitrary commands. The entry point is the Servers Endpoint of mcp-bridge.js, reachable remotely without requiring complex prerequisites, giving the attacker code execution in the context of the bridge process. Since MCP bridges act as intermediaries granting agents access to external tools/servers, compromising this component can allow an attacker to pivot into any backend systems or credentials the bridge can reach, effectively crossing the boundary from an agent-facing API into full host compromise.

Affected Systems

mcp-bridge-api (INQUIRELAB); protocols: MCP

Detection Signatures

• Unexpected shell metacharacters (;, |, &&, $(), backticks) in command/args parameters sent to mcp-bridge Servers Endpoint
• Anomalous outbound connections or child processes spawned by the mcp-bridge.js process
• Requests to Servers Endpoint containing unusual binary names or shell built-ins not part of expected tool command whitelist
• Repeated failed or malformed command/args payloads from a single remote source indicating fuzzing/exploitation attempts

Remediation Steps

1. 1

   Apply patch or upgrade

   Monitor the INQUIRELAB mcp-bridge-api repository for the pending pull request fix and upgrade as soon as it is merged and released.

2. 2

   Input validation and sanitization

   Implement strict allow-listing of permitted commands and arguments, and avoid passing user-controlled input directly to shell execution functions (use parameterized process spawning instead of shell interpretation).

3. 3

   Network restriction

   Restrict access to the Servers Endpoint to trusted internal networks or authenticated callers only, and place it behind a reverse proxy or firewall with rate limiting.

4. 4

   Run with least privilege

   Run the mcp-bridge process under a low-privilege service account and sandbox/container it to limit blast radius if command injection succeeds.

5. 5

   Monitoring and logging

   Enable detailed logging of command/args requests to the Servers Endpoint and alert on anomalous or shell-metacharacter-containing input.