CVE-2025-46734

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.

Workarounds

If upgrading is not feasible, please consider:

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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2025-46734 has a CVSS score of 6.4 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.7.0); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2025-46734? CVE-2025-46734 is a medium-severity cross-site scripting (XSS) vulnerability in league/commonmark (composer), affecting versions >= 1.5.0, < 2.7.0. It is fixed in 2.7.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2025-46734? CVE-2025-46734 has a CVSS score of 6.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of league/commonmark are affected by CVE-2025-46734? league/commonmark (composer) versions >= 1.5.0, < 2.7.0 is affected.
4. Is there a fix for CVE-2025-46734? Yes. CVE-2025-46734 is fixed in 2.7.0. Upgrade to this version or later.
5. Is CVE-2025-46734 exploitable, and should I be worried? Whether CVE-2025-46734 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-46734 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-46734? Upgrade league/commonmark to 2.7.0 or later.