Disclosure 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(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.
Source-reported weakness categories
CWE-79
Source-specific records & product guidance
Sources retain their own attribution and scoring. Follow the original record to confirm affected versions, fixed releases, and configuration conditions.
NIST National Vulnerability Database · NVD-CVE-2026-86431
Open original source · Updated Oct 08, 2026
Only CPE matches marked vulnerable=true are indexed. AND/OR platform conditions must be checked in the original NVD record.
| Vendor | Product / association | Version / bounds |
|---|---|---|
| thephpleague | commonmark | * {"versionStartIncluding":"2.7.0","versionEndExcluding":"2.9.1"} |
Original records & references
- NIST NVD record
- CVE Program record
- github.com — Patch
- github.com — Exploit, Mitigation, Vendor Advisory
- www.vulncheck.com — Patch, Third Party Advisory
PUBLISHED 2026-09-07T09:20:42-04:00
MODIFIED 2026-10-08T12:17:57-04:00
INGESTED 2026-10-10T20:50:44-04:00