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Public proof of concept

CVE-2026-121437

Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters

A public proof of concept exists; no exploitation attested.

  • Published2026-11-03
  • WeaknessCWE-407

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.

Timeline

dates in UTC
  1. 2026-11-0314:28CVE publishedCNA

Versions per branch

Products declared by the CNA

As VulnCheck, the CNA that assigned this CVE, declared them in the CVE record; versions as written. NVD lists no product for this CVE yet: this is the only product data so far.

  • lepture mistuneother versions: not affected
    Affected
    • ≥ 3.3.0, < 3.3.4fixed in 3.3.4semver

    Not affected = 3.3.4

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-03

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • CVE-2026-121437 Red Hat Product Security25 known affected

    mistune: mistune: Denial of Service via quadratic-time emphasis delimiter processing

    rhoai/odh-feature-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI) · rhoai/odh-pipeline-runtime-datascience-cpu-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI) · rhoai/odh-pipeline-runtime-minimal-cpu-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI) · +22 more

Full record on NVD

Fix

  1. Workaround from the vendor Red Hat Product Security CVE-2026-121437 · 25 products--- CME Mitigation Guidance (2026-11-04) --- Disclaimer: The following mitigation solutions are provided as a courtesy, pending the application of an official patch. This content was generated utilizing AI tools in conjunction with data from https://cmetaxonomy.org. Customers are advised to indepen…
  2. No fix available, says the vendor Red Hat Product Security CVE-2026-121437 · 25 productsAffected

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • CISA-ADPSSVC decisionpoc2026-11-03
  • OpenCVESSVC decisionpocknown since 2026-11-03
  • CNACVSS 4.08.72026-11-03
  • CNACVSS 3.17.52026-11-03
  • NVDCVSS 3.17.5read 2026-11-04
  • CNAweakness (CWE)CWE-4072026-11-03
  • NVDweakness (CWE)CWE-407read 2026-11-04
  • FIRST EPSSEPSS score0.6 %2026-11-04
  • GitHub Advisoriesadvisory—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 4.0 vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published
2026-11-03
Last Modified
2026-11-03
NVD status
Deferred
GitHub Advisory
GHSA-xr4w-hw52-gjv9

Frequently asked questions

What is CVE-2026-121437?

Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.

Is CVE-2026-121437 actively exploited?

A proof-of-concept exploit exists for CVE-2026-121437, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-121437?

CVE-2026-121437 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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Sources for this record