CVE-2026-86434 commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision
An attacker can cause excessive CPU consumption and denial of service in the commonmark library by exploiting a slug collision algorithm flaw when specific extensions are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-86434?
CVE-2026-86434 is a high-severity denial of service vulnerability in thephpleague commonmark, published on 7 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-407
Which versions are affected by CVE-2026-86434?
2 open source packages ship an affected version, including league/commonmark.
| Package | Affected | Fixed in |
|---|---|---|
league/commonmark Packagist | From 2.0.0 | 2.9.0 |
league/commonmark Packagist | From 2.0.0 | 2.9.0 |
How do I fix CVE-2026-86434?
Upgrade league/commonmark to 2.9.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-86434 being exploited?
There is no known exploitation of CVE-2026-86434 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-86434, GHSA-jcmh-chhq-rh2x, GHSA-mh25-x5hq-wrqp, UBUNTU-CVE-2026-86434.