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CVE-2026-121437: Mistune Security Vulnerability

CVE-2026-121437 explained: high Security Vulnerability in Mistune. CVSS 8.7. Affected versions, detection, fixes and CTEM priority.

Last updated: Nov 3, 2026

CVSS
8.7 HIGH
EPSS
n/a
CISA KEV
Not listed
CWE
CWE-407

Sources: National Vulnerability Database, CVE.org, CISA KEV, FIRST EPSS

CVE-2026-121437 Overview

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.

Critical Impact

Remote, unauthenticated attackers can exploit this Security Vulnerability in Mistune. The CVSS base score is 8.7 (High).

Affected Products

  • Mistune: see the vendor advisory for the exact affected versions

Discovery Timeline

  • 2026-11-03: CVE-2026-121437 published to the National Vulnerability Database by disclosure@vulncheck.com
  • 2026-11-03: last updated in the NVD database

Technical Details for CVE-2026-121437

Vulnerability Analysis

CVE-2026-121437 is a Security Vulnerability weakness (CWE-407) in Mistune. Based on its CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction.

With a CVSS base score of 8.7, apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Root Cause

The vendor description states: 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.

Attack Vector

Based on the CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. Attack complexity is low complexity.

No exploitation in the wild is confirmed by CISA at the time of the last update. Check the references below for any vendor or researcher proof of concept.

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

Detection Methods for CVE-2026-121437

Detection Strategies

  • Inventory every instance of Mistune and compare the running version with the affected range above.
  • Check external attack surface scans for internet facing instances of the component.
  • Run an authenticated vulnerability scan once your scanner ships a check for this CVE.

Monitoring Recommendations

  • Forward Mistune application, access and audit logs to your SIEM for retrospective hunting.
  • Alert on new processes, configuration changes or outbound connections from the affected service.
  • Track the vendor advisory and the CISA KEV catalog for changes in exploitation status.

How to Mitigate CVE-2026-121437

Immediate Actions Required

  • Apply the fixed version of Mistune named in the vendor advisory.
  • Prioritize internet facing and business critical instances first.
  • Rescan after the change to confirm the exposure is closed.

Patch Information

Use the fixed version listed in the vendor advisory for Mistune.

Workarounds

  • If patching must wait, restrict network access to the affected service and place it behind authentication or a web application firewall rule.

CWE References

Technical References

This entry is built directly from NVD, CISA KEV and EPSS data. Verify fixed versions in the vendor advisory before you deploy.

Frequently asked questions

CVE-2026-121437 is a high severity Security Vulnerability in Mistune with a CVSS score of 8.7. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

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