If you use Wordpress then you might need to upgrade to version 2.1.2 pronto! There are a couple of huge holes in the code, apparently inserted by someone for the purpose of intrusion! Said otherwise, the source code was somehow compromised.
Someone inserted code into at least two files (themes.php and feed.php) and in a nutshell, someone could pass operating system commands via a URL where they then execute on the system using PHP's passthrough() function. Ouch!
If you downloaded Wordpress between February 25 and March 2 then you have a vulnerable copy.
If you can't upgrade to Wordpress 2.1.2 then you might be able to use .htaccess to insert mod_rewrite rules on Apache to prevent attacks. Something like this might help:
RewriteCond %{QUERY_STRING} (ix|iz)
RewriteRule .* - [F]
The best route is to upgrade though.
The problem was discovered by Ivan Fratric from Zagreb, Croatia, and posted to the Bugtraq mailing list.