GHSA-3527-qv2q-pfvx

### 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 is not feasible, please consider:

- Disabling the `AttributesExtension` for untrusted users - [Filtering the rendered HTML through a library like HTMLPurifier](https://commonmark.thephpleague.com/security/#additional-filtering)