How could you add someone else's page to the site?

Question

MODX

Greetings, a page of the form sitename.com/page-3d-porno-monstri-online has appeared on the site, there is, of course, obscenity. No new pages were created in the admin panel of the site, everything is in order with the site. All styles, scripts, pictures, etc. the page pulls from an external resource. Tell me how this could happen, and most importantly, how to remove it?

Answer 1

Hacking an FTP account, exploiting vulnerabilities in the site engine, hacking an ssh account...
Here is a short list of possible ways....

Answer 2

The site is infected, somewhere there is a file that processes this, and possibly other similar addresses and displays spam content. Check the site, it is quite possible that somewhere else there are redirects from your site to others or js loading some kind of rubbish on the page.
If necessary, I can help with treatment.