---
id: CVE-2026-121437
title: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
summary: >-
  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 patt…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-407
vendor: lepture
product: mistune
affected:
  - mistune >= 3.3.0 < 3.3.4
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-03T16:58:25.525426Z'
exploitAvailable: true
published: '2026-11-03'
updated: '2026-11-03'
sourceUpdated: '2026-11-03T16:59:17.405Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-121437'
references:
  - url: 'https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9'
    label: GitHub Security Advisory (GHSA-qp42-r9j6-83g9)
  - url: >-
      https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90
    label: Patch Commit
  - url: >-
      https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters
    label: >-
      VulnCheck Advisory: Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via
      _process_emphasis_delimiters
tags:
  - cve.org
  - exploit-available
epss: 0.00619
epssPercentile: 0.49909
ingestedAt: '2026-11-05T20:33:06.294Z'
---

## 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.

## Affected

- `mistune >= 3.3.0 < 3.3.4`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.