The other day someone claimed a hostname on a domain I own and it took me a while to track down how. After a lot of digging around, trying to figure out how the hijack was accomplished, it turns out it was via GitHub Pages.
When you set up a custom domain with GitHub pages, you have to point your domain at GitHub’s servers. There are a bunch of ways to do this, but if you use an A record, you need to be careful with your DNS settings. The site in question had a wildcard hostname (*) A record pointed at GitHub’s servers. At the time I’d set it up, that was the recommendation if you wanted all traffic to go to the same place.
Fast forward a few years and it’s become a known exploit of GitHub Pages: when wildcard hostnames are in play, any repo can add a CNAME file to their repository and claim ownership of a hostname belonging to that domain. GitHub even warns you not to do this anymore, but I hadn’t checked the docs in years. In my particular case, it was an archived domain that I don’t really use anymore, but I wouldn’t have been aware of the DNS hijack if the attacker hadn’t taken the step of claiming the domain on Google’s Webmaster Central.
Thankfully the fix was simple: Remove the wildcard A record and point the Apex domain at GitHub’s IP addresses.
If you use GitHub pages to host any of your own domains, I highly recommend auditing their DNS records to ensure this doesn’t happen to you. You can also use domain verification for GitHub Pages and organizations to further protect yourself.