If triggered by a logged-in administrator, under default settings the attacker can leverage the vulnerability to execute arbitrary code on the server via the plugin and theme editors.
Alternatively the attacker could change the administrator’s password, create new administrator accounts, or do whatever else the currently logged-in administrator can do on the target system.
If the comment text is long enough, it will be truncated when inserted in the database. The MySQL TEXT type size limit is 64 kilobytes so the comment has to be quite long.
The truncation results in malformed HTML generated on the page. The attacker can supply any attributes in the allowed HTML tags, in the same way as with the two other recently published stored XSS vulnerabilities affecting the WordPress core.
The vulnerability bears a similarity to the one reported by Cedric Van Bockhaven in 2014 (patched this week, after 14 months). Instead of using an invalid UTF-8 character to truncate the comment, this time an excessively long comment text is used for the same effect.
Enter the following as a comment:
<a title='x onmouseover=alert(unescape(/hello%20world/.source)) style=position:absolute;left:0;top:0;width:5000px;height:5000px AAAAAAAAAAAA [64 kb] ...'></a>
This was tested on WordPress 4.2, 4.1.2, and 4.1.1, MySQL versions 5.1.53 and 5.5.41.
Disable comments (Dashboard, Settings/Discussion, select as restrictive options as possible). Do not approve any comments.
The vulnerability was discovered by Jouko Pynnonen of Klikki Oy.
An up-to-date version of this document: http://klikki.fi/adv/wordpress2.html
-- Jouko Pynnonen <firstname.lastname@example.org> Klikki Oy - http://klikki.fi - @klikkioy