TI-2026-59139 Aliases CVE-2026-86431 GHSA-m24x-mf52-x8gc

commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

LOW 37CWE-79Published 2026-09-07 · Updated 2026-10-08

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Attack requirements: None
Impact type: Cross-site scripting
Confidentiality · integrity · availability: Low · Low · None
Fix status: Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA. Impact type from CWE-79.

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00388 33rd percentileScored 2026-11-07

Public exploit Yes 1 exploit referenceTagged by NVD

Fix Fixed commonmark (thephpleague): 2.9.1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: Yes
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
thephpleague	commonmark	2.7.0 to before 2.9.1	2.9.1	Patch available	CNA

commonmark Vendorthephpleague Affected 2.7.0 to before 2.9.1 Fixed in 2.9.1 Patch available CNA

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:thephpleague:commonmark::::::::	2.7.0 to before 2.9.1	Vulnerable	thephpleague

cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:* Versions 2.7.0 to before 2.9.1 Vulnerable ·thephpleague

Remediation

Fix · CNA commonmark (thephpleague) Fixed in 2.9.1

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

Release · 2026-09-07
thephpleague commonmark 2.9.1
3 records Highest priority Priority MEDIUM 2 MEDIUM, 1 LOW
Other records, highest priority first
- TI-2026-59138 league/commonmark before 2.9.1 Denial of Service via parsingPriority MEDIUM 44
- TI-2026-59137 commonmark before 2.9.1 Denial of Service via SmartPunct and AttributesPriority MEDIUM 44
Show all 3 records →

Timeline

2026-10-02 Threatist record created from CVE-2026-86431
2026-10-03 Alias added: GHSA-m24x-mf52-x8gc

References

Exploit 1

GitHub Security Advisory (GHSA-f8fg-pg57-v4j8)

Patch 1

Patch Commit

TI-2026-59138 league/commonmark before 2.9.1 Denial of Service via parsing CVSS 4.0HIGH 8.7
TI-2026-59137 commonmark before 2.9.1 Denial of Service via SmartPunct and Attributes CVSS 4.0HIGH 8.7
TI-2026-59136 commonmark 1.5.0 before 2.10.0 Denial of Service via Attributes CVSS 4.0HIGH 8.7
TI-2026-25753 league/commonmark has an embed extension allowed_domains bypass CVSS 4.0MEDIUM 6.3
TI-2018-14605 Cross-site scripting (XSS) vulnerability in the PHP League CommonMark library versions… CVSS 3.0MEDIUM 6.1
TI-2019-07346 Cross-site scripting (XSS) vulnerability in the PHP League CommonMark library before… CVSS 3.0MEDIUM 6.1

See all 7 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences