Mediawiki 1.16.5 was released [1] to correct two security flaws:

The first issue is yet another recurrence of the Internet Explorer 6
XSS vulnerability that caused the release of 1.16.4. It was pointed
out that there are dangerous extensions with more than four
characters, so the regular expressions we introduced had to be updated
to match longer extensions. (CVE-2011-1765)

For more details, see https://bugzilla.wikimedia.org/show_bug.cgi?id=28534

The second issue allows unauthenticated users to gain additional
rights, on wikis where $wgBlockDisablesLogin is enabled. By default,
it is disabled. The issue occurs when a malicious user sends cookies
which contain the user name and user ID of a "victim" account. In
certain circumstances, the rights of the victim are loaded and persist
throughout the malicious request, allowing the malicious user to
perform actions with the victim's rights. (CVE-2011-1766)

$wgBlockDisablesLogin is a feature which is sometimes used on private
wikis to prevent users who have an account from logging in and viewing
content on the wiki.

For more details, see https://bugzilla.wikimedia.org/show_bug.cgi?id=28639

[1] http://lists.wikimedia.org/pipermail/mediawiki-announce/2011-May/000098.html