I always recommend forum owners hire Securi. I use them for all my sites. they monitor the sites for intrusions, and track down and repair successful malware / virus attacks on my sites. They have been fantastic for me and they monitor all my sites.

Well I ran two free scans on my forum and the showthread.php page, it says theres no security threat. *sigh* You sure this is correct?

This is something only a vB *expert* can deal with. I also have a person dedicated to security on my dedicated servers, but he isn't a vB pro. I fixed my problem but with no help from him. That said my servers are secure and I do not expect him to deal with vB issues.

Other than that, not much I can say other than:

How do you know it's showthread.php?

'cause the host was able to trace the activity of the hacker, and showthread.php was where he/she accessed to send spammails.

Ah. Well, it looks like only you and I have run into what ever it was/is. I haven't seen it mentioned anywhere by anyone else. I feel for you. I wish I could help you. I do hope if you find out what it is and how they're doing it you will let me and others know.

There is an email to friend feature on that page, are you sure your guests are not allowed to use it?

Check the blog usergroup permissions if yuou have it installed, also this blog post should be helpful:

http://www.vbulletin.com/forum/blogs...ve-been-hacked