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.