wp 3.0.1 and hackers

Hello the community

Today was a new administrator on my site, just a matter with him, he came from himself and I didn’t get a mail to advertise me.
Could be it is the same security bug like with the 2.8 (not really beacause this one was to see by users) ? Don’t know.

I use wp 3.0.1 multisite with Buddypress 1.2.5.2
This guy was registred on my main site as administrator, under the name Benjamin Collins.

The site is : https://www.tby-liber.com (french)

I deleted his account but I think it’s just good for a short time. Can somebody help ?

I would update Buddypress to the 1.2.6. Could be the matter.

Know somebody about this ? Can somebody help me ?
Thanks
Thierry