CVE-2026-55580: CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection') in sonirico mcp-shell

The mcp-shell product, a server for running shell commands securely and auditably, had a security flaw before version 0.6.0 where the Security.Enabled flag was initialized to false by default. When the environment variable MCP_SHELL_SEC_CONFIG_FILE is unset, the application starts without a security policy, causing the SecurityValidator.validateCommand function to allow all commands without restriction. This enables an attacker, such as an LLM connected over stdio, to execute arbitrary OS commands as the mcp-shell process user. The insecure default configuration is documented in the README and client setup instructions. The vulnerability is addressed in version 0.6.0.

An attacker with access to the mcp-shell service can execute arbitrary OS commands with the privileges of the mcp-shell process user due to the lack of command validation in the default insecure configuration. This can lead to full system compromise depending on the process privileges. There are no known exploits in the wild reported.

Upgrade to mcp-shell version 0.6.0 or later, where the security policy enforcement is enabled by default and the vulnerability is fixed. Until upgraded, explicitly configure MCP_SHELL_SEC_CONFIG_FILE to a valid security policy file to avoid running in the insecure default state.