Citizen vulnerable to stored XSS in sticky header button messages

Summary

The JS implementation for copying button labels to the sticky header in the Citizen skin unescapes HTML characters, allowing for stored XSS through system messages.

Details

In the copyButtonAttributes function in stickyHeader.js, when copying the button labels, the innerHTML of the new element is set to the textContent of the old element: https://github.com/StarCitizenTools/mediawiki-skins-Citizen/blob/f4cbcecf5aca0ae69966b23d4983f9cb5033f319/resources/skins.citizen.scripts/stickyHeader.js#L29-L41 This unescapes any escaped HTML characters and causes the contents of the system messages to be interpreted as HTML.

PoC

1. Edit any of the affected messages (citizen-share, citizen-view-history, citizen-view-edit, nstab-talk) to the following payload: <img src="" onerror="alert('Sticky Header Button XSS')">.
2. Visit any mainpage article in the wiki using the Citizen skin.

Impact

This impacts wikis where a group has the editinterface but not the editsitejs user right. By default, this is the case for the sysop group.