league/commonmark: Quadratic-time denial of service when parsing crafted Markdown

published 2026-08-06 3 mo ago modified 2026-10-24 16 d ago also EUVD-2026-54143 , GHSA-2q4p-g7hv-5rgv

cvss: 7.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.6%; EPSS 0.6% chance of exploitation in 30 days, percentile 50%
public exploits: 0; none known

league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 0.6.0 until 2.9.0, specially crafted Markdown lines can cause the parser to have quadratic time complexity when converting, because several parsing paths repeatedly rescan growing portions of a line to translate between character positions and byte positions, and the Autolink extension can also copy and validate the remaining line at every URL-like prefix, allowing an attacker who can submit Markdown for conversion to consume disproportionate CPU time with a comparatively small request. This issue is fixed in 2.9.0.

Patch / fix

fixed in 2 distros 1 links 4

packagist league/commonmark 2.9.0
thephpleague commonmark 2.9.0

php-league-commonmark Debian 14 2.9.0-1

debian advisories DEBIAN-CVE-2026-71488

github.com/thephpleague/commonmark@a6ef6cd commit
github.com/thephpleague/commonmark@a70979e commit
github.com/thephpleague/commonmark@c97b02e commit
github.com/thephpleague/commonmark/releases/tag/2.9.0 release

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 3 mo ago (2026-08-07)

Exploit prediction

0.63% chance of exploitation in the next 30 days, higher than 50% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem	package	affected	fixed in
packagist	league/commonmark	>=0.6.0, <2.9.0	2.9.0

Affected products

thephpleague commonmark

Classification

Weakness

CWE-407 Inefficient Algorithmic Complexity
CWE-1050 Excessive Platform Resource Consumption within a Loop

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary