Cross-site Scripting (XSS) Affecting php-league-commonmark package, versions *


Severity

Recommended
0.0
high
0
10

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Threat Intelligence

EPSS
0.39% (31st percentile)

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  • Snyk IDSNYK-ECHOLATEST-PHPLEAGUECOMMONMARK-19643818
  • published8 Sept 2026
  • disclosed7 Sept 2026

Introduced: 7 Sep 2026

CVE-2026-86431  (opens in a new tab)
CWE-79  (opens in a new tab)

How to fix?

There is no fixed version for Echo:latest php-league-commonmark.

NVD Description

Note: Versions mentioned in the description apply only to the upstream php-league-commonmark package and not the php-league-commonmark package as distributed by Echo. See How to fix? for Echo:latest relevant fixed versions and status.

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 Base Scores

version 3.1