CVE-2026-30838
CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names
Impact
### Impact The `DisallowedRawHtml` extension can be bypassed by inserting a newline, tab, or other ASCII whitespace character between a disallowed HTML tag name and the closing `>`. For example, `<script\n>` would pass through unfiltered and be rendered as a valid HTML tag by browsers. This is a cross-site scripting (XSS) vector for any application that relies on this extension to sanitize untrusted user input. All applications using the `DisallowedRawHtml` extension to process untrusted markdown are affected. Applications that use a dedicated HTML sanitizer (such as HTML Purifier) on the rendered output are not affected. ### Patches Fixed in 2.8.1. The regex character class `[ \/>]` was changed to `[\s\/>]` to match all whitespace characters that browsers accept as valid tag name terminators. ### Workarounds - Set the `html_input` configuration option to `'escape'` or `'strip'` to disable all raw HTML, though this is a broader restriction than the `DisallowedRawHtml` extension provides. - Pass the rendered HTML through a dedicated HTML sanitizer before serving it to users ([always recommended](https://commonmark.thephpleague.com/2.x/security/#additional-filtering))
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| composer | league/commonmark | >= 2.0.0, <= 2.8.0 | 2.8.1 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Thephpleague | Commonmark | before 2.8.1 |