thephpleague commonmark up to 2.9.0 AttributesExtension filterAttributes cross site scripting

Summaryinfo

A vulnerability identified as problematic has been detected in thephpleague commonmark up to 2.9.0. This affects the function AttributesHelper::filterAttributes of the component AttributesExtension. This manipulation causes cross site scripting. This vulnerability appears as CVE-2026-86431. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability classified as problematic was found in thephpleague commonmark up to 2.9.0. Affected by this vulnerability is the function AttributesHelper::filterAttributes of the component AttributesExtension. The manipulation with an unknown input leads to a cross site scripting vulnerability. The CWE definition for the vulnerability is CWE-79. The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. As an impact it is known to affect integrity. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-86431 since 09/07/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1059.007 according to MITRE ATT&CK.

Upgrading to version 2.9.1 eliminates this vulnerability.

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Productinfo

Vendor

• thephpleague

Name

• commonmark

Version

• 2.0
• 2.1
• 2.2
• 2.3
• 2.4
• 2.5
• 2.6
• 2.7
• 2.8
• 2.9.0

Website

• Product: https://github.com/thephpleague/commonmark/

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