league/commonmark contains a XSS vulnerability in Attributes extension
Date: 2025-05
Affected: composer/league/commonmark
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2025-46734
CWE: CWE-79
CVSS: 6.4 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N)

### Summary Cross-site scripting (XSS) vulnerability in the [Attributes extension](https://commonmark.thephpleague.com/extensions/attributes/) 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 ![](){onerror=alert(1)} ``` Which results in the following HTML: ```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](https://commonmark.thephpleague.com/2.7/extensions/attributes/#configuration) - Manually-added `href` and `src` attributes now respect the existing `allow_unsafe_links` configuration option ### Workarounds If upgrading


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