---
id: CVE-2026-86434
title: >-
  league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a
  denial of service vulnerability in UniqueSlugNormalizer::normalize(), which
  restarts its numeric-suffix search from 1 on every slug collision, resulting
  in O(K^…
summary: >-
  league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a
  denial of service vulnerability in UniqueSlugNormalizer::normalize(), which
  restarts its numeric-suffix search from 1 on every slug collision, resulting
  in O(K^…
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'
cwe:
  - CWE-407
vendor: thephpleague
product: commonmark
affected:
  - commonmark >= 2.0.0 < 2.8.4
published: '2026-09-07'
updated: '2026-09-19'
sourceUpdated: '2026-09-19T15:17:06.503'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-86434'
references:
  - url: >-
      https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/commonmark-2.0.0-through-2.8.3-denial-of-service-via-slug-collision
    label: disclosure@vulncheck.com
tags:
  - nvd
  - cve.org
epss: 0.00509
epssPercentile: 0.41081
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-18T17:42:05.970571Z'
ingestedAt: '2026-09-08T20:10:03.179Z'
---

## Overview

league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying service.

## Remediation

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