I just visited https://www.mediawiki.org/wiki/Special:NewMessages on MediaWiki 1.23wmf7 while not logged in and it was showing messages for me that I have not contributed to.
I checked the page source, and wgUser is "null".
I think the expected result is that Special:NewMessages never gives results for anonymous users.
Version: master
Severity: normal
URL: https://www.mediawiki.org/wiki/Special:NewMessages