New security measure blocks “anyone attempting to login as you from an unrecognized computer or browser”.
In order for users to login from a given computer, an access code (sent via email) must be entered into Steam upon your first attempt to access your account from an unfamiliar computer.
The system will also notify you of any attempts to login to your account from any computers that you have not ‘authorised’.
You can learn more about the new system from the official Steam FAQs.
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