---
id: CVE-2026-86431
title: >-
  league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1
  contain a cross-site scripting vulnerability in the AttributesExtension
summary: >-
  league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1
  contain a cross-site scripting vulnerability in the AttributesExtension.
  Prefixing an attribute name with a single U+000C form feed byte (e.g.
  {\x0Conclick="alert(…
severity: high
cvss: 7.2
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N'
cwe:
  - CWE-79
vendor: thephpleague
product: commonmark
affected:
  - 'commonmark >= 2.7.0, < 2.9.1'
patched:
  - commonmark 2.9.1
published: '2026-09-07'
updated: '2026-09-09'
sourceUpdated: '2026-09-09T16:20:39.770'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-86431'
references:
  - url: >-
      https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass
    label: disclosure@vulncheck.com
tags:
  - nvd
  - cve.org
epss: 0.00388
epssPercentile: 0.30169
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-08T13:30:04.102356Z'
ingestedAt: '2026-09-08T15:33:26.977Z'
---

## Overview

league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.

## Affected

- `commonmark >= 2.7.0, < 2.9.1`

## Remediation

Upgrade past the affected range:

- `commonmark 2.9.1`