As you may already know Spotify is only allowing new users via an invite from a Premium user. However when you enter your invitation code you are not sent to an encrypted page that can only be accessed if you have a cookie placed there because of a correct code, you are sent to a run-of-the mill web page, one that can be accessed by anyone.
This means that anyone who visits https://www.spotify.com/en/get-started/ can cut out the invite system.
Simples.
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