When setting up custom domain for my Github Pages by following https://help.github.com/articles/setting-up-a-custom-subdomain/, I encountered the following warning:
Danger: Do not use wildcard DNS records (e.g. *.example.com) with GitHub Pages! A wildcard DNS record will allow anyone to host a GitHub Pages site at one of your subdomains.
While the warning is very clear: avoid wildcard DNS records, I would like to know how it is possible for another user to host their GitHub Pages using my subdomain even when I'm specifying my own domain
YOUR-GITHUB-USERNAME.github.io as CNAME value.