---
id: GHSA-mh25-x5hq-wrqp
title: 'league/commonmark: Denial of service via colliding heading slugs'
summary: 'league/commonmark: Denial of service via colliding heading slugs'
severity: high
cvss: 7.5
cwe:
  - CWE-407
vendor: league
product: league/commonmark
ecosystem: composer
affected:
  - 'league/commonmark >= 2.0.0, < 2.9.0'
patched:
  - league/commonmark 2.9.0
published: '2026-08-06'
updated: '2026-08-06'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-mh25-x5hq-wrqp'
references:
  - url: >-
      https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp
  - url: 'https://github.com/thephpleague/commonmark/releases/tag/2.9.0'
  - url: 'https://github.com/advisories/GHSA-mh25-x5hq-wrqp'
tags:
  - ghsa
  - composer
ingestedAt: '2026-08-06T21:04:21.976Z'
---

## Overview

### Impact

`UniqueSlugNormalizer::normalize()` makes each slug document-unique by searching for an unused numeric suffix, but **restarts that search from `1` on every collision**. The k-th heading that collapses to the same base slug performs k−1 array lookups, so K colliding slugs cost Σ(k−1) = **O(K²)**. An attacker can force every heading onto a single base slug trivially — many empty ATX headings, identical heading text, or punctuation-only headings that normalize to the empty string.

The path is reached whenever the shared slug normalizer runs over attacker-controlled text. That happens when `HeadingPermalinkExtension` is registered (its `HeadingPermalinkProcessor` normalizes every heading), independently through `FootnoteExtension` (its `AnonymousFootnoteRefParser` normalizes every `^[label]` reference), and on any `TableOfContentsExtension` site (which requires `HeadingPermalinkExtension` to be co-registered). The default `slug_normalizer/unique` setting (`UniqueSlugNormalizerInterface::PER_DOCUMENT`) accumulates collisions across the whole document. No authentication is required — a small document body turns into seconds of CPU and denies service. Availability impact only. **`UniqueSlugNormalizer` was introduced in 2.0.0 (first shipped in 2.0.0-beta1, May 2021); the 1.x heading-permalink slug generator performed no de-duplication and is not affected. All 2.x releases (including 2.8.x) are affected.**

### Workarounds

Integrators who cannot upgrade immediately can:

- **Set `slug_normalizer/unique` to `false` / `UniqueSlugNormalizerInterface::DISABLED`**, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).
- **Disable `HeadingPermalinkExtension`** (and `TableOfContentsExtension`, which depends on it), and `FootnoteExtension` where anonymous footnotes reach the same normalizer, for untrusted Markdown.
- **Cap the accepted document size / heading count upstream** so K cannot reach the quadratic danger zone.

Each of these trades off functionality or correctness; upgrading to the patched release (which removes the quadratic behavior while keeping unique ids and identical output) is the recommended remediation.

## Affected packages

- `league/commonmark >= 2.0.0, < 2.9.0`

## Remediation

Upgrade to a patched release:

- `league/commonmark 2.9.0`