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Github pages show "CNAME is already taken" although it is not. #22705

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The problem that happened here was a case of subdomain takeover, and is a serious issue. If you create a subdomain CNAME record in your domain’s DNS settings that points to a github username and don’t update the custom domain address in your repository’s  Settings > GitHub Pages within a stipulated amount of time, your subdomain could be taken over by someone else, and if you try to add that subdomain to a repository later, it will show that “The CNAME subdomain.domain.com is already taken”.

Patrik has written a lot about subdomain takeovers in his blog here: https://0xpatrik.com/

Hope this was useful.

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