CVE-2026-39987: Marimo terminal WebSocket endpoint missing authentication allows pre-auth RCE

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 16 September 2026. Confidence: high.

critical priorityUnauthenticated network-reachable remote code execution with a CVSS 4.0 score of 9.3, CISA KEV listing and near-certain EPSS probability makes this an urgent patch-first issue.

What it is

Marimo, a reactive Python notebook, exposes a terminal WebSocket endpoint (/terminal/ws) that skips the validate_auth() check applied to other WebSocket endpoints, accepting connections based only on running mode and platform support. An unauthenticated attacker can therefore obtain a full PTY shell and run arbitrary system commands. The flaw is fixed in 0.23.0 and is listed in CISA KEV with a very high EPSS score, so it warrants urgent action.

Impact

An attacker gains a full interactive PTY shell on the host running Marimo and can execute arbitrary system commands with the privileges of the Marimo process. This is complete loss of confidentiality, integrity and availability for the affected instance.

Attack surface

Reachable over the network via the /terminal/ws WebSocket endpoint; no authentication is required and no user interaction is needed, as the endpoint omits the validate_auth() call used elsewhere. Any network-exposed Marimo instance prior to 0.23.0 is a candidate target.

Exploitation

CVE-2026-39987 is in CISA KEV (added 2026-04-23, due 2026-05-07) and has an EPSS 30-day probability of 0.98945 (99.9th percentile). References are tagged Exploit, including a vendor advisory and a third-party write-up describing exploitation within 10 hours of disclosure, so active exploitation should be assumed.

What to do

• Upgrade Marimo to 0.23.0 or later, which fixes the missing authentication check (patch commit c24d4806398f30be6b12acd6c60d1d7c68cfd12a).
• If immediate patching is not possible, remove network exposure of the Marimo instance (bind to localhost, restrict access via firewall or reverse proxy) and follow CISA KEV required action per BOD 22-01.
• Disable or block the /terminal/ws endpoint at the reverse proxy or load balancer until the upgrade is applied.
• Audit any internet-facing Marimo deployments for unauthorized access and rotate credentials or secrets reachable from the host.
• Track the CISA KEV due date of 2026-05-07 and confirm remediation before it lapses.

Detection

• Monitor WebSocket upgrade requests to /terminal/ws in reverse proxy or application logs, especially from unauthenticated or unexpected source IPs.
• Alert on shell or PTY process creation spawned by the Marimo process (e.g., unexpected /bin/sh, bash or python child processes).
• Hunt for outbound connections or command execution originating from Marimo hosts that do not match normal notebook activity.
• Review Marimo access logs for connections to /terminal/ws that bypass normal authentication flows.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.