Since people are asking, this so-called alert on Security Focus appears to be completely false and has no information that an attacker or the WordPress developers could use. It is completely content-free, except for making claims that every version of WP since 2.0 is vulnerable.
Online, apparently, it’s fine for someone to run into a crowded theatre and yell “fire” and the less basis there is in fact the more people link to them. It’s not uncommon to see crying-wolf reports like the above several times in a week, and a big part of what the WP security team is sifting through things to see what’s valid or not.
A valid security report looks like this, it usually includes sample code and a detailed description of the problem. The WP security team was notified of the KSES problem and it was fixed in 2.5. You can impress your friends by saying whether a security report is valid or not, so it’s a good critical facility to pick up.
All that said, there is a wave of attacks going around targeting old WordPress blogs, particularly those on the 2.1 or 2.2 branch. They’re exploiting problems that have been fixed for a year or more. This typically manifests itself through hidden spam being put on your site, either in the post or in a directory, and people notice when they get dropped from Google. (Google will drop your site if it contains links they consider spammy, you’ll remember this is one of the main reasons I came out against sponsored themes.) Google has some guidelines as well, what to do if your site is hacked. If I were to suggest WordPress-specific ones, I would say:
Nivi