CVE-2026-86430 league/commonmark before 2.9.1 Denial of Service via parsing

High 8.7 Published 7 Sep 2026 Updated 8 Oct 2026

Attackers can cause denial of service by submitting crafted Markdown input that triggers excessive CPU consumption in the league/commonmark library.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-86430?

CVE-2026-86430 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 before 2.9.1 contain multiple denial of service vulnerabilities in fenced code block detection, reference link label lookup, and emphasis delimiter processing that perform super-linear work on crafted input. Attackers can submit specially crafted Markdown with long backtick runs, nested brackets, or delimiter sequences to consume disproportionate CPU time and prevent legitimate requests from completing.

Weakness: CWE-407

Which versions are affected by CVE-2026-86430?

2 open source packages ship an affected version, including league/commonmark. It affects 1 product.

Package	Affected	Fixed in
`league/commonmark` Packagist	From `0.6.0`	`2.9.1`
`league/commonmark` Packagist	From `0.6.0`	`2.9.1`

Products include Thephpleague Commonmark.

How do I fix CVE-2026-86430?

Upgrade league/commonmark to 2.9.1 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-2026-86430 being exploited?

There is no known exploitation of CVE-2026-86430 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-86430, GHSA-hxrj-xxg4-w3qp, GHSA-j8pm-gj4c-rq4x, UBUNTU-CVE-2026-86430.