Looks like a bot attack to me.
It relates to this article
http://www.vbulletin.com/forum/forum...-1-vbulletin-5
Apache Log below:
178.33.229.22 - - [05/Sep/2013:10:10:37 +0100] "GET /forum/core/install/upgrade.php HTTP/1.1" 404 613 "-" "-"
178.33.229.22 - - [05/Sep/2013:10:10:38 +0100] "GET /forum/install/upgrade.php HTTP/1.1" 404 613 "-" "-"
178.33.229.22 - - [05/Sep/2013:10:10:39 +0100] "GET /forums/core/install/upgrade.php HTTP/1.1" 404 613 "-" "-"
178.33.229.22 - - [05/Sep/2013:10:10:39 +0100] "GET /forums/install/upgrade.php HTTP/1.1" 404 613 "-" "-"
178.33.229.22 - - [05/Sep/2013:10:10:40 +0100] "GET /core/install/upgrade.php HTTP/1.1" 404 613 "-" "-"
178.33.229.22 - - [05/Sep/2013:10:10:41 +0100] "GET /install/upgrade.php HTTP/1.1" 200 13394 "-" "-"
66.96.183.79 - - [05/Sep/2013:10:10:45 +0100] "POST /install/upgrade.php HTTP/1.1" 200 279 "-" "-"