1. You sign into myaccount.google.com.
  2. Somewhere along the line your browser is redirected to https://accounts.youtube.com/accounts/SetSID?ssdc=1&sidt=<reallyLongBase64>&continue=https://myaccount.google.com/intro&tcc=1 probably inside an iframe.
  3. Youtube server sets some cookies.
  4. You click the youtube icon in myaccount.google.com and you're logged in without following the oauth2 flow.

How can this be achieved?

My best guess is that the sidt=<reallyLongBase64> parameter is the users id that should be logged in encrypted with a key private to google with an algorithm that takes time into account(so the request wouldn't be repeatable)



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