*[https://help.yahoo.com/kb/SLN5013.html Yahoo 2-step verification]
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*[https://www.facebook.com/help/148233965247823 Facebook Login Approval]
*https://support.twitter.com/articles/20170388# Twitter Login Verification]
[https://help.yahoo.com/kb/SLN5013.html Here's a list] of other popular online services that provide 2fa.
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