Julian,
Whats happened is your site has been compromised by the link Juraj has provided. We have dealt with several clients who have had a similar experience. Eventually it will lead to a ransomware attack on your company/site when the access to your site is sold to the right person/attacker.
Upgrading to the latest version and hotfix you can possibly upgrade to is a great start. The second thing you need to do is look in the code base and find where that make.aspx call is being made. I'd suggest starting with the following files and checking the contents of those files:
Default.aspx*.ashx/CMSPages/PortalTemplate.aspx/CMSWebParts/*- Search for code similar to this in those pages (or your project in general). The
make.aspx page probably doesn't exist, but the code to show others it does may in those .aspx, and .ashx` pages.
I have to say though, removing the code DOES NOT FIX the problem. There is some other underlying code still residing on your server that will be executed, at some point and time, which will cause further problems. The best course of action is if you have CI/CD set up and backups, would be to spin up a new server, redeploy the site(s) and database(s).
On the couple instances we worked on, we found the breach was done anywhere from 3-7 months prior to us actually discovering this issue. What happens is an attacker finds access, messes around, sells the access to another attacker, they then mess around and do things, then they sell the access to another attacker. This cycle goes on and on until someone decides to do something that can possibly bring your whole site down, for good.