league/commonmark DisallowedRawHtml XSS bypass in tag parsing

Severity: medium · CVSS 6.1 · Published 2026-09-30

The CommonMark Markdown parser's security extension fails to properly escape certain HTML tags when used with default settings, allowing attackers to inject malicious scripts. An attacker who can post Markdown content could execute arbitrary JavaScript in users' browsers through stored cross-site scripting (XSS), potentially compromising user accounts and stealing sensitive data.

Technologies: league/commonmark (Packagist). Vendors: League, Packagist.

Quick answers

Is This vulnerability exploited in the wild?
No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?
Medium severity, with a CVSS score of 6.1, published 30 September 2026.
Which products and versions are affected?
League Commonmark 1.3.0 through current release; league/commonmark (Packagist) >= 1.3.0, <= 2.10.1.
Is there a fix for This vulnerability?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is This vulnerability attacked?
It is attacked over the network, in a software library.
Am I affected?
Check whether you run League Commonmark or league/commonmark (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The DisallowedRawHtml extension uses a regex that requires at least one character after a tag name, but the HTML block parser accepts tag names at end-of-line. When a disallowed tag like &lt;script ends a line, the regex fails to match and the tag passes through unescaped; subsequent lines then supply attributes that the browser interprets as part of the tag. This is a bypass of the previous fix for GHSA-4v6x-c7xx-hw9f. The vulnerability requires html_input to be set to allow (default) and the DisallowedRawHtml extension to be active (enabled by default in GFM mode).

Affected products

  • league commonmark 1.3.0 through current release

Timeline

  • 2026-09-30: disclosed

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