MediaWiki before 1.18.5, and 1.19.x before 1.19.2 does not properly protect user block metadata, which allows remote administrators to read a user block reason via a reblock attempt.
https://phabricator.wikimedia.org/T41823
https://lists.wikimedia.org/pipermail/mediawiki-announce/2012-August/000119.html
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=686330