When checking certain users (can give more info privately) via checkuser, it returns 127.0.0.1. Additionally, autoblocks are propagating from vandals to random other users due to the 127.0.0.1 edits.
This appears to only affect some logged-in users.
127.0.0.1 (block) (14:52, 23 September 2019 -- 15:12, 23 September 2019)  (~697 from all users)
This is on enwiki.
I cleared an autoblock on 127.0.0.1: https://en.wikipedia.org/w/index.php?title=Special:Log&offset=20190923192600&limit=1&type=&user=Reaper+Eternal