league/commonmark: Denial of service via colliding heading slugs

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-86434: 0.51% estimated exploitation activity in the next 30 days; percentile 43.53 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 slugnormalizer/unique setting (UniqueSlugNormalizerInterface::PERDOCUMENT)…

Published by the source

6 Aug 2026, 20:41 UTC

Last modified by the source

20 Oct 2026, 12:11 UTC

Source last checked

26 Oct 2026, 14:50 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mh25-x5hq-wrqp

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mh25-x5hq-wrqp

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# Patch Next advisory brief: CVE-2026-86434

Advisory: league/commonmark: Denial of service via colliding heading slugs
IDs: CVE-2026-86434, GHSA-mh25-x5hq-wrqp
Record: https://patchnext.com/advisories/cve-2026-86434 (revision 2, source checked 26 Oct 2026, 14:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: league/commonmark 2.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist league/commonmark: affected >= 2.0.0, < 2.9.0; fixed in 2.9.0

## Suggested inspection (read-only)
- Packagist league/commonmark: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on league/commonmark.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mh25-x5hq-wrqp (OSV record GHSA-mh25-x5hq-wrqp lists affected ranges: Packagist league/commonmark >=2.0.0 <2.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-mh25-x5hq-wrqp (GitHub-reviewed advisory GHSA-mh25-x5hq-wrqp lists affected ranges: Packagist league/commonmark >=2.0.0 <2.9.0)

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