Vulnerability · Published Sep 7, 2026 · Assigned by VulnCheck

CVE-2026-86433commonmark 1.5.0 before 2.8.4 Denial of Service via Attributes

High 7.5

In plain English

A high-severity security flaw in commonmark from thephpleague. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 56.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

commonmark versions from 1.5.0 before 2.8.4 contain a denial of service vulnerability in the Attributes extension where AttributesListener::findTargetAndDirection() performs quadratic-time sibling list scanning. Unauthenticated attackers can submit approximately 32 KB of repeated attribute blocks to cause parsing to take over 5 seconds, exhausting server resources.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	php-league-commonmark	no fix listed	DEBIAN-CVE-2026-86433
Debian:13	php-league-commonmark	no fix listed	DEBIAN-CVE-2026-86433
Debian:14	php-league-commonmark	2.9.0-1	DEBIAN-CVE-2026-86433
Ubuntu:26.04:LTS	php-league-commonmark	no fix listed	UBUNTU-CVE-2026-86433

Affected products (as reported)

Vendor	Product	Versions
thephpleague	commonmark	1.5.0 – before 2.8.4

Weakness: CWE-407 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

GitHub Security Advisory (GHSA-g2gp-3wwq-f4ph) vendor-advisory
VulnCheck Advisory: commonmark 1.5.0 before 2.8.4 Denial of Service via Attributes third-party-advisory