Reference page — cumulative record through Sunday, October 4, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2026-86431

CVE-2026-86431MEDIUM

thephpleague commonmark — commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

  AV  AC  AT  PR  UI  VC  VI  VA   CVSS    EPSS   %ile   KEV
   N   L   N   N   N   L   L   N    6.9   .0039   30.6     —

AFFECTED
  Product     Versions  Fixed
  commonmark  2.7.0 –   2.9.1

TIMELINE
  Sep 7   Reserved by VulnCheck
  Sep 7   Published (CNA: VulnCheck)
  Sep 9   EXPLOIT PUBLISHED — CVE-2026-86431 (thephpleague commonmark). Public exploit reference added.

CWE-79 · CNA: VulnCheck · CVSS v4.0 · 3 references · NVD status: Analyzed

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.

Lifecycle

Complete event history — 3 events, chronological
Date	Event	Detail
September 7, 2026	Reserved	Reserved by VulnCheck
September 7, 2026	Published	Published (CNA: VulnCheck)
September 9, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2026-86431 (thephpleague commonmark). Public exploit reference added.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
thephpleague	commonmark	—	2.7.0	2.9.1

Weaknesses

CWE-79

References (3)

https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea [Patch]
https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8 [Exploit, Mitigation, Vendor Advisory]
https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass [Patch, Third Party Advisory]

Authoritative record: CVE-2026-86431 at cve.org

Vendors: thephpleague

Weaknesses: CWE-79

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-86431 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Sunday, October 4, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.