commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision (CVE-2026-86434) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-86434

commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision

Answer in brief

CVE-2026-86434 records a Unknown severity 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 thephpleague/commonmark (generic). 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 Sep 7, 2026Source checked Sep 7, 2026First seen by HOL Sep 7, 2026Material review Sep 7, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. 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 thephpleague/commonmark (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
thephpleague/commonmarkgeneric	>=2.0.0 <2.8.4	2.8.4

Recommended response

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

Evidence timeline and material changes

Published upstream
Sep 7, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 7, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 7, 2026

Sources and claim methodology

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.