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 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 character to truncate the comment, this time an excessively long comment is used for the same effect.
The similar vulnerability released by Klikki in November 2014 could be exploited in the administrative Dashboard while the comment is still in the moderation queue. Some exploit attempts of this have been recently reported in the wild.
Solution : Update To Latest Version