Earlier this week WordPress 4.1.3. It fixed an XSS exploit discovered by a former student of mine. This week a new exploit – even in the new WordPress versions, including 4.2 – was uncovered.
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.
Here’s how it works:
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.
Until a patched WordPress is available, it is wise to disable comments. I guess (read: think but haven’t tested it yet) that configuring your MySQL server with STRICT_ALL_TABLES would also work, as it should then trigger an error when too much data is attempted to being inserted.