CVE-2026-55581

Summary mcp-shell ships a default Docker configuration (security.yaml) that includes /bin/bash in the allowed_executables allowlist. The command validator (security.go) only checks whether the first token of the supplied command matches an allowed executable; it does not inspect or reject shell command-mode flags such as -c. As a result, any MCP tool caller can send command=/bin/bash -c <arbitrary-command> to the shell_exec tool and execute commands that are not in the allowlist — including id, env, curl, wget, and any other binary present in the container. The bypass works with the default Docker image, requires no authentication, and requires no modifications to server configuration. Successful exploitation gives the attacker arbitrary OS command execution inside the container as mcpuser. Details mcp-shell implements a *secure mode* in which command execution is restricted to an explicit allowlist of executables defined in security.yaml. The Docker image ships this file with the following entry: security.yaml (line 29) allowed_executables: - "ls" - ... - "/bin/bash" # Only allow if you trust the arguments The comment itself acknowledges the risk, but the shipped default does not enforce any argument-level restriction. The validation logic in security.go is responsible for enforcing secure mode: // security.go:84-96 for _, allowed := range v.config.AllowedExecutables { if v.matchesExecutable(executable, allowed) { if err := v.checkBlockedPatternsAndCommands(command); err != nil { return err } return nil } } executable is derived solely from parts[0] after splitting the input on whitespace (security.go:67). When the command is /bin/bash -c id, executable evaluates to /bin/bash, which matches the allowlist entry. The -c flag and subsequent arguments are passed to checkBlockedPatternsAndCommands, which only checks for shell metacharacters (|, &, ;, <, >, (, ), {, }, [, ], , $, \, ", ') and a configurable list of blocked_commands/blocked_patterns — both of which

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