CommonMark has DisallowedRawHtml extension bypass via whitespace in HTML tag names

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)