AIKIDO-2026-261475

TL;DR

The DisallowedRawHtmlRenderer filter uses a regex that requires a character after a disallowed tag name, so a raw HTML line containing only a bare tag name such as <script passes through unescaped. The GFM block parser opens an HTML block on that bare tag name with no trailing newline for the regex to match, and the next block's raw HTML is then interpreted by the browser as that tag's attributes. Anyone who can submit Markdown rendered with the shipped GFM defaults can use this to achieve stored XSS. The fix widens the regex to also match a disallowed tag name at the end of the string.