Citizen Short Description stored XSS vulnerability through wikitext

Summary

Short descriptions are not properly sanitized by the ShortDescription before being inserted as HTML using mw.util.addSubtitle, allowing any user to insert arbitrary HTML into the DOM by editing a page.

Details

The description provided by the user via the {{SHORTDESC:}} parser function is insufficiently sanitized by the sanitize() function, as html entities are decoded: https://github.com/StarCitizenTools/mediawiki-extensions-ShortDescription/blob/7244b1e8b5cb6dbd7e546c5be7fed8a56e33d065/includes/Hooks/ParserHooks.php#L147-L159 Via JS, the short description is then passed to mw.util.addSubtitle, which inserts it as raw HTML: https://github.com/StarCitizenTools/mediawiki-extensions-ShortDescription/blob/7244b1e8b5cb6dbd7e546c5be7fed8a56e33d065/modules/ext.shortDescription.js#L8 https://github.com/wikimedia/mediawiki/blob/96372101b3c579d9992e8a31a3ccd90a937cac47/resources/src/mediawiki.util/util.js#L552-L563

PoC

1. Enable ShortDescription
2. Make sure $wgShortDescriptionEnableTagline is set to true (this is the default)
3. Create a page and insert the following wikitext: {{SHORTDESC:<img src="" onerror="alert('shortdescription xss')">}}
4. Visit the page

Impact

Arbitrary HTML can be inserted into the DOM by any user, allowing for JavaScript to be executed.