Subdomain takeover vulnerabilities occur when a subdomain (subdomain.example.com) is pointing to a service (e.g. GitHub pages, Heroku, etc.) that has been removed or deleted. This allows an attacker to set up a page on the service that was being used and point their page to that subdomain. For example, if subdomain.example.com was pointing to a GitHub page and the user decided to delete their GitHub page, an attacker can now create a GitHub page, add a CNAME file containing subdomain.example.com, and claim subdomain.example.com.

You can read up more about subdomain takeovers here:

• https://labs.detectify.com/2014/10/21/hostile-subdomain-takeover-using-herokugithubdesk-more/
• https://www.hackerone.com/blog/Guide-Subdomain-Takeovers
• https://0xpatrik.com/subdomain-takeover-ns/

Claim the subdomain discreetly and serve a harmless file on a hidden page. Do not serve content on the index page. A good proof of concept could consist of an HTML comment served via a random path:

$ cat aelfjj1or81uegj9ea8z31zro.html
<!-- PoC by username -->

You can submit new services here: https://github.com/EdOverflow/can-i-take-over-xyz/issues/new?template=new-entry.md.

A list of services that can be checked (although check for duplicates against this list first) can be found here: EdOverflow#26.