CVE-2026-86431

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.

CVSS v3 7.2 HIGH

7.2^/10

CVSS v3 : HIGH

Vector :

Exploitability : 3.9 / Impact : 2.7

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact LOW

Integrity Impact LOW

Availability Impact NONE

Scope CHANGED

References

Link	Resource
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

Configurations

Configuration 1 (hide)

cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:*

History

No history.

Information

Published : 2026-09-07 13:20

Updated : 2026-09-09 16:20

NVD link : CVE-2026-86431

Mitre link : CVE-2026-86431

CVE.ORG link : CVE-2026-86431

JSON object : View

Products Affected

thephpleague

commonmark

CWE

CWE-79

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

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