GET request to register.php from cheap-24h.com: What the objective?
 

In my website logs I noticed referrals to my site from various web pages at www.cheap-24h.com. My apache logs showed they are GET requests directed at register.php of my forum. I am running Vbulletin version 3.8.4.

In the requests* after register.php?s= there is always a string of 32 letters and numbers (the post box would not let me paste these in).

As far as I can tell this has not caused any problems with my forums. None of the referrer IP addresses from my apache logs appear as the IP address of a registered user. Nevertheless I am wondering what the originator(s) of these requests is trying to accomplish. It must be something underhanded. I could not find any specific clues about it in a Google search - that I could understand anyway.

Could someone please explain to me what the objective of these GET requests might be* and if they could be a security threat?

Many Thanks!

Steve