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Back to active CVEs
High · CVSS 8.7CVE-2026-86434GHSA-MH25-X5HQ-WRQP

commonmark 2.0.0 through 2.8.3 Denial of Service via Slug CollisionCVE-2026-86434

Answer in brief

CVE-2026-86434 records a High severity (CVSS 8.7) vulnerability in commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision. The current sources do not mark it as known exploited. The current feed maps league/commonmark (composer), thephpleague/commonmark (generic), league/commonmark (packagist). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 7, 2026Updated Oct 20, 2026Source checked Oct 25, 2026First seen by HOL Aug 6, 2026Material review Oct 20, 2026
Upstream Advisory

Key facts

Risk
High · CVSS 8.7
Exploitation
Not marked as known exploited
Affected software
3 mapped packages or products
Fix availability
Available

Why this deserves its current priority

CVSS is 8.7. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps league/commonmark (composer), thephpleague/commonmark (generic), league/commonmark (packagist). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
league/commonmarkcomposer>=2.0.0,<2.9.02.9.0
thephpleague/commonmarkgeneric>=2.0.0 <2.8.42.8.4
league/commonmarkpackagist>=2.0.0 <2.9.02.9.0

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for league/commonmark, thephpleague/commonmark, league/commonmark.
  2. 2Review the reported fix. Update league/commonmark to 2.9.0; thephpleague/commonmark to 2.8.4; league/commonmark to 2.9.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Sep 7, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Oct 20, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Aug 6, 2026

Sources and claim methodology

  • GitHub security advisorygithub.com
  • Source referencevulncheck.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.

Record context

Vulnerability class
Missing Auth
EPSS
Not reported
CWE IDs
CWE-407
Source
CVE List V5
Source checked
Oct 25, 2026
References
5 linked sources
Open source record