[S]I'm not convinced Advanced Forum Rules is the attack vector for the latest round. Sites that have never used it have reportedly been attacked.[/S]
Retracted.
I found a hole in the cookie handling code due to the use of the PHP eval function.
I.e. the hacker pre-sets a cookie to contain malicious code, and the eval function runs it when it picks up the cookie content (that it was expecting to be something else).
Kym