league/commonmark contains a XSS vulnerability in Attributes extension

Summary

Cross-site scripting (XSS) vulnerability in the Attributes extension of the league/commonmark library (versions 1.5.0 through 2.6.x) allows remote attackers to insert malicious JavaScript calls into HTML.

Details

The league/commonmark library provides configuration options such as html_input: 'strip' and allow_unsafe_links: false to mitigate cross-site scripting (XSS) attacks by stripping raw HTML and disallowing unsafe links. However, when the Attributes Extension is enabled, it introduces a way for users to inject arbitrary HTML attributes into elements via Markdown syntax using curly braces.

As a result, even with the secure configuration shown above, an attacker can inject dangerous attributes into applications using this extension via a payload such as:

![](){onerror=alert(1)}

Which results in the following HTML:

<p><img onerror="alert(1)" src="" alt="" /></p>

Which causes the JS to execute immediately on page load.

Patches

Version 2.7.0 contains three changes to prevent this XSS attack vector:

• All attributes starting with on are considered unsafe and blocked by default
• Support for an explicit allowlist of allowed HTML attributes
• Manually-added href and src attributes now respect the existing allow_unsafe_links configuration option

Workarounds

If upgrading is not feasible, please consider:

• Disabling the AttributesExtension for untrusted users
• Filtering the rendered HTML through a library like HTMLPurifier