commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
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.
CVE-2026-86431
Primary source · Revision 1
Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.
Security context
Source reference: cve:CVE-2026-86431
commonmark
Source reference: /containers/cna/affected/0/product
- Affected versions
- Not stated in this record
- Fixed versions
- Not stated in this record
- Prerequisites
- Not stated in this record
Structured affected-version data
Original package identities and version rules from this source.
[
{
"vendor": "thephpleague",
"product": "commonmark",
"versions": [
{
"status": "affected",
"version": "2.7.0",
"lessThan": "2.9.1",
"versionType": "semver"
},
{
"status": "unaffected",
"version": "2.9.1",
"versionType": "semver"
}
],
"packageURL": "pkg:composer/league/commonmark",
"defaultStatus": "unaffected"
}
]- Related CVEs
- CVE-2026-86431
Source reference:
/cveMetadata/cveId
Publisher severity
high
Source reference: /containers/cna/metrics
- Event
- #12948 · Revision 1
- Added to Heist Feed
- Publisher date