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I use Google's two factor authentication to send my phone text messages with a string of 6 numbers every time I log into a computer that isn't mine. It has worked well so far until today.

Today, I took too long to enter the code, which would be fine normally... except I clicked through faster than I should have and realized shortly after that the "trust this computer" box was checked the second time through. I took a look through the options for my Google account and saw no obvious place to revoke the trust.

This is absolutely not a computer I want trusted for any extended amount of time.

How do I untrust a computer from Google's two-factor authentication?

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migrated from superuser.com Oct 15 '12 at 13:30

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3 Answers 3

up vote 11 down vote accepted

Clear the browser's cookies (and perhaps cache/history).

'Trust this computer' option doesn’t work

For computers you trust, Google recommends you select the "Trust this computer" option when you are asked for a verification code. When you check this box, you won’t have to enter a code every time you sign in to your account from that computer.

This feature uses a cookie to remember your browser when you sign in. If you’re still being asked to enter a code despite selecting the “Trust this computer” option, this might be because your browser is set to delete cookies after a certain period of time, or is set to delete cookies every time you quit the browser. You may also be manually clearing cookies from your browser. (emphasis added)

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Ahhhh... So it's cookie-based? This is good to know. Thank you! –  Aeo Oct 15 '12 at 13:27
    
Do you have any references for this? It’s just a gut feeling, but I don’t think it’s just cookie-based. –  Alex Oct 15 '12 at 13:40
    
@Alex: Proof by experimentation; also, this Google Help page. –  grawity Oct 15 '12 at 13:47
    
Excellent! Thanks for the updated info. –  Alex Oct 15 '12 at 13:51
    

According to Google Support:

Removing trusted computers from your list

You can remove computers from your set of trusted computers at any time. To do so, sign in to your account and go to your 2-step verification settings page. Under the “Advanced” section, you’ll have the choice to remove the computer you’re using at the time or all other computers. If you choose to remove other computers from your trusted list, those computers will automatically sign out of your account, and you’ll be prompted for a verification code next time you try to sign in from them.

The 2-step verification settings page is at https://google.com/accounts/SmsAuthConfig

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I like this option much better as it seems far more secure. It is nice having the ability to unregister devices, but I wonder why a user can't SEE what devices are registered. If some unknown device keeps being added as a "registered device" even after unregistered all of them repeatedly, it would be nice to know about this. –  Aaron Klap Mar 31 at 18:19

Follow these steps:

Account > Security > 2-step Verification Setting > Registered Computers > Require Codes

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