---
id: CVE-2026-121438
title: Mistune before 3.3.3 Denial of Service via Balanced Bracket Link Parsing
summary: >-
  Mistune before 3.3.3 contains a denial of service vulnerability in
  InlineParser.parse_link that allows attackers to cause quadratic CPU growth by
  supplying balanced nested brackets. Attackers can submit crafted Markdown with
  thousands of…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cvssSource: cna
cwe:
  - CWE-400
vendor: lepture
product: mistune
affected:
  - mistune < 3.3.3
ssvc:
  exploitation: poc
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-11-07T03:01:54.408797Z'
exploitAvailable: true
published: '2026-11-03'
updated: '2026-11-07'
sourceUpdated: '2026-11-07T03:02:22.098Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-121438'
references:
  - url: 'https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7'
    label: GitHub Security Advisory (GHSA-47p6-w2rc-5qf7)
  - url: >-
      https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7
    label: Patch Commit
  - url: >-
      https://www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balanced-bracket-link-parsing
    label: >-
      VulnCheck Advisory: Mistune before 3.3.3 Denial of Service via Balanced
      Bracket Link Parsing
tags:
  - cve.org
  - exploit-available
epss: 0.00291
epssPercentile: 0.21842
ingestedAt: '2026-11-05T19:30:42.970Z'
---

## Overview

Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.

## Affected

- `mistune < 3.3.3`

## Remediation

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