commonmark before 2.9.1 XSS via AttributesExtension form feed bypass (CVE-2026-86431) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-86431

commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

Answer in brief

CVE-2026-86431 records a Unknown severity vulnerability in commonmark before 2.9.1 XSS via AttributesExtension form feed bypass. The current sources do not mark it as known exploited. The current feed maps thephpleague/commonmark (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 7, 2026Updated Sep 7, 2026Source checked Sep 7, 2026First seen by HOL Sep 7, 2026Material review Sep 7, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps thephpleague/commonmark (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
thephpleague/commonmarkgeneric	>=2.7.0 <2.9.1	2.9.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for thephpleague/commonmark.
Review the reported fix. Update thephpleague/commonmark to 2.9.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 7, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 7, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 7, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.