Wikipedia is not under attack. Some stupid admin running with full privileges unsandboxed ran a test that grabbed and ran random user scripts, and one of them just happened to be this 2 year old malicious script.

that's a common attack vector -- like leaving malware usb sticks on the ground, knowing an admin will pick it up and insert it.

Phabricator reveals the ops tasks that WMF admins perform, so attackers can drop malware in common locations and bet on them getting run from time to time.