ImportDump is a mediawiki extension designed to automate user import requests. A user's local actor ID is stored in the database to tell who made what requests. Therefore, if a user on another wiki happens to have the same actor ID as someone on the central wiki, the user on the other wiki can act as if they're the original wiki requester. This can be abused to create new comments, edit the request, and view the request if it's marked private. This issue has been addressed in commit 5c91dfc and all users are advised to update. Users unable to update may disable the special page outside of their global wiki. See miraheze/mw-config@e566499 for details on that.
Exploitability
AV:NAC:HPR:LUI:NScope
S:UImpact
C:LI:HA:L6.4/CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:LOther