⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Critical · CVSS 9.3 ⚠ ACTIVELY EXPLOITED — CISA KEV

How to Fix CVE-2026-39987: Missing Authentication in marimo

By Sai Kiran Pandrala

⚡ At a glance
SeverityCVSS 9.3 - Critical
Actively exploited?Yes, listed in CISA KEV (added 2026-04-23)
Affectedmarimo < 0.23.0
Fixed inSee vendor advisory
Type (CWE)CWE-306: Missing Authentication for Critical Function

Patch immediately. CISA added CVE-2026-39987 to the Known Exploited Vulnerabilities catalog on 2026-04-23. Federal civilian agencies must remediate by 2026-05-07. Treat every internet-reachable instance as a priority patch.

What is CVE-2026-39987?

CVE-2026-39987 is a Missing Authentication flaw in marimo-team marimo. It carries a CVSS base score of 9.3 (critical). CISA confirmed real-world exploitation by adding it to the Known Exploited Vulnerabilities catalog on 2026-04-23.

From the source record: marimo is a reactive Python notebook. Prior to 0.23.0, Marimo 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. This vulnerability is fixed in 0.23.0.

Why it matters in practice: KEV-listed CVEs draw continuous internet-wide scanning. Any unpatched, internet-reachable installation is on borrowed time. The blast radius depends on how the affected service is exposed. An internet-facing instance with no compensating controls is the highest-risk configuration.

Am I affected?

You are affected if your installation of marimo matches a version listed in the Affected row above.

Check the installed version of marimo against the Affected row above. If the version sits at or below the affected range and the vendor patch has not been applied, you are vulnerable.

How to fix CVE-2026-39987

Apply the vendor patch. Target the patched build listed on the vendor advisory. The runnable command set below covers the most common deployment patterns for marimo.

Generic upgrade path


# Debian / Ubuntu
sudo apt-get update && sudo apt-get install --only-upgrade marimo
# RHEL / Rocky / Alma
sudo dnf upgrade --security -y
# Windows
# PowerShell:
#   winget upgrade --all --silent --accept-package-agreements --accept-source-agreements
# macOS
sudo softwareupdate -i -a -R

After applying the patch

  1. Restart the service or device so the patched binary loads.
  2. Confirm the running version matches the Fixed in row using the verification command below.
  3. Rotate credentials and API keys that the affected service could access if the asset was exposed during the disclosure window.

If you can't patch immediately

Until the patch lands, narrow the attack surface with these runnable controls.

Reduce the attack surface

Restrict network reach to the affected service to the smallest set of hosts that must access it. On Linux:


# Vendor advisory: https://github.com/marimo-team/marimo/security/advisories/GHSA-2679-6mx9-h9xc
sudo iptables -A INPUT -p tcp --dport <service-port> -s 10.10.10.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport <service-port> -j DROP

On Windows:


# Vendor advisory: https://github.com/marimo-team/marimo/security/advisories/GHSA-2679-6mx9-h9xc
New-NetFirewallRule -DisplayName "Block CVE-2026-39987 inbound" -Direction Inbound -Action Block -Protocol TCP -LocalPort <service-port>

Mitigations are temporary. Apply the vendor patch as soon as a maintenance window opens.

How to verify the fix worked

Confirm the patched build is the one actually running.

Run the product's --version or about command and compare against the Fixed in row (See vendor advisory). Re-run an authenticated vulnerability scan and confirm the scanner no longer flags CVE-2026-39987.

Also worth doing: pull recent log windows for any indicators of compromise listed in the vendor advisory, and re-run an authenticated vulnerability scan with up-to-date signatures.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-39987 being exploited in the wild?

Yes. CISA added CVE-2026-39987 to the Known Exploited Vulnerabilities catalog on 2026-04-23. KEV listing means at least one confirmed real-world exploitation report exists.

Do I have to take downtime to patch?

For most marimo-team marimo deployments, the patched build needs a service restart or device reboot. HA pairs and clusters can roll the upgrade by patching the standby first, failing over, then patching the former primary.

Will a WAF or IDS rule alone close CVE-2026-39987?

No. Network filters cut down opportunistic scans but they do not remove the flaw. The vendor patch is the only durable fix.

Why is CVE-2026-39987 rated critical?

The CVSS base score of 9.3 reflects network reach, low attack complexity, and high impact on confidentiality, integrity, and availability. That combination is what the rating model maps to critical.

References

*Assembled from the official vendor advisory, the NVD record, and the CISA KEV listing on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*