mcp-ssh-tool Path Traversal and Timing Attack Vulnerabilities

Versions 2.1.0 and earlier of mcp-ssh-tool, a tool often used in CI/CD environments, contain security vulnerabilities related to file transfer path authorization and HTTP bearer authentication. Specifically, the tool suffers from insufficient local path policy enforcement in transfer-related filesystem handling, leading to potential bypasses of configured path restrictions. Additionally, a non-constant-time HTTP bearer token comparison exposes a timing side channel. The vulnerability allows attackers with sufficient access to potentially read or write files outside of their intended scope. Upgrade to version 2.1.1 or implement provided workarounds to mitigate risks.

Attack Chain

1. An attacker gains access to an MCP client, either through compromised credentials or a vulnerable service.
2. The attacker crafts a file transfer request with a manipulated path, exploiting insufficient canonicalization in the mcp-ssh-tool’s path policy checks.
3. The crafted path bypasses the configured deny-prefix path policy.
4. The attacker initiates a file transfer operation targeting a restricted file system location.
5. The mcp-ssh-tool incorrectly authorizes the transfer due to the policy bypass.
6. The attacker successfully reads or writes files outside of the intended scope.
7. In HTTP deployments, an attacker attempts to authenticate using a brute-force approach, leveraging timing differences in bearer token comparison.
8. By analyzing response times, the attacker identifies valid token characters and reconstructs the valid bearer token over time.

Impact

Successful exploitation of the path traversal vulnerability could allow unauthorized access to sensitive files and directories on the server. The timing side channel vulnerability in bearer token comparison could lead to unauthorized access to the system via HTTP. The severity of the impact is dependent on the file system permissions and sensitivity of the data exposed.

Recommendation

• Upgrade to mcp-ssh-tool >= 2.1.1 to remediate both the path traversal and timing attack vulnerabilities.
• For deployments that cannot immediately upgrade, avoid exposing HTTP transport beyond loopback to mitigate the timing attack.
• Implement strict filesystem policy configuration as described in the mcp-ssh-tool documentation to minimize the risk of path traversal.
• Monitor audit logs for unexpected transfer operations to identify and respond to potential exploit attempts.