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CVE-2026-39987

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Marimo. For every flaw being used in attacks, see Exploited flaws.

CVE-2026-39987 is a security flaw in Marimo. Attackers can run their own code on Marimo from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 23 Apr 2026.[1]

Marimo is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Marimo has published a fix or advice.

Description

CISA calls it a remote code execution in Marimo Marimo. In its words: “Marimo contains an pre-authorization remote code execution vulnerability, allowing an unauthenticated attacked to shell access and execute arbitrary system commands.”[1]

The CVE record adds: “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.”[2]

The underlying weakness is missing authentication (CWE-306), in MITRE’s list of common software weaknesses.

It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]

Affected versions

NVD’s record doesn’t list version ranges for it; Marimo’s advisory has them[3]. CISA names the product as Marimo Marimo.

Most families won’t have Marimo; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 23 Apr 2026, 14 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 7 May 2026 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]

CISA assesses it as automatable: attackers can run it against many targets at once.

CISA doesn’t say who was attacked, or how.

Fix

Marimo has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[1]

  1. Check whether you use it.Marimo is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 7 May 2026 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4]

See also

Other missing authentication flaws

  • CVE-2026-111357, Ivanti Connect Secure: missing authentication for critical function ·
  • CVE-2026-67277, MikroTik RouterOS: missing authentication for critical function ·
  • CVE-2026-81578, PaperCut NG/MF: missing authentication for critical function ·
  • CVE-2026-72529, TrueConf Server: missing authentication for critical function ·
  • CVE-2026-56164, Microsoft SharePoint Server: missing authentication for critical function ·

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Marimo Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 23 Apr 2026.
  2. ^ National Institute of Standards and Technology. “CVE-2026-39987 Detail”. National Vulnerability Database, published 9 Apr 2026.
  3. ^ Marimo. Advisories and fixes: github.com.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.996, percentile 99.9, 31 Oct 2026.
  5. CVE Program. “CVE-2026-39987”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.