Last week I promised to follow up on a few XSS bugs that I found in WordPress. The vulnerabilities are fixed in WordPress 2.0.3, even though the release notes do not mention their existence. I think there are a number of useful lessons that can be drawn from them, so in this post I will describe some more details.
The goal of a classic XSS exploit is to run arbitrary Javascript, in the context of a another webpage, which retrieves the user’s cookies. With WordPress I will concentrate on the comment management interface. Here, the deletion button has a Javascript onclick event handler to display a confirmation dialog, which includes the comment author’s name. If malicious input can break out of the dialog box text, then when an administrator activates the button, the attacker’s Javascript is run, allowing access to the admin user’s cookies. I found two classes of bugs which allowed me to do this.