GHSA-3527-qv2q-pfvx league/commonmark contains a XSS vulnerability in Attributes extension

• Packagist league/commonmark: >= 1.5.0, < 2.7.0 → fixed in 2.7.0

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: ```md ``` Which results in the following HTML: ```html ``` 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