Vulnerability · Published Sep 7, 2026 · Assigned by VulnCheck
CVE-2026-86431commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
High 7.2
In plain English
A high-severity cross-site scripting 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 0.4% (higher than 33% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | php-league-commonmark | no fix listed | DEBIAN-CVE-2026-86431 |
| Debian:13 | php-league-commonmark | no fix listed | DEBIAN-CVE-2026-86431 |
| Debian:14 | php-league-commonmark | 2.10.0-1 | DEBIAN-CVE-2026-86431 |
| Packagist | league/commonmark | 2.9.1 | GHSA-f8fg-pg57-v4j8 |
| Ubuntu:26.04:LTS | php-league-commonmark | no fix listed | UBUNTU-CVE-2026-86431 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| thephpleague | commonmark | 2.7.0 – before 2.9.1 |
Weakness: CWE-79 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
References
- GitHub Security Advisory (GHSA-f8fg-pg57-v4j8) vendor-advisory
- Patch Commit patch
- VulnCheck Advisory: commonmark before 2.9.1 XSS via AttributesExtension form feed bypass third-party-advisory