Eric Rafaloff

My personal blog on software development and security

Home » Google Account Security and Number Portability

Google Account Security and Number Portability

By now, you may have read this story about someone having $8,000 worth of Bitcoin stolen due to a social engineering attack on their Verizon account. This was an unfortunate event and an urgent reminder that SMS-based 2FA isn’t secure. When you allow a second factor of authentication to occur over SMS, the proof isn’t that you have your phone. Rather, it’s that you are able to receive SMS messages sent to a certain number. The problem with this as a means of authentication can be summed up in two words: number portability. If an attacker can social engineer your mobile provider, they can port your number over to their own account and your 2FA provider would never know the difference.

This got me thinking. How secure is my Google account, even when locked down with 2FA via the Google Authenticator application? Would I be able to withstand an attack similar to the one that Cody Brown suffered?

As it turned out, I wouldn’t. A serious security concern appeared when I went through the account recovery flow for my Google account. The following events illustrate this:

  1. Start the login process on by entering my username. Click “Forgot password?”
  2. Be asked to “enter the last password you remember. Click “Try a different question.”
  3. Be asked to “enter a verification code”. Click “Try a different question.”
  4. Be asked to “get a verification code by text message at: (***) ***-**-XX.Since my cell phone number appears on my business cards and is public information as far as I’m concerned, this would hardly deter an attacker. By taking advantage of number portability, an attacker could steal my number.
  5. Be asked to “confirm the phone number you provided in your security settings: (***) ***-**XX.”Since I just received a text sent to this number, I obviously know this.
  6. Answer a security question of “What is my father’s middle name?” Skipping this forced me to specify the month and year my account was created. While the first security question is terrible, the second option isn’t all that much better as there are a very limited number of possible answers.
  7. Change my password.
  8. Login to my account.

That’s right. Despite using the Google authenticator application, I was able to effectively skip it and instead opt for receiving a text and answering a lame security question.

Now to be fair, Google discontinued security questions a while ago. However, they stick around in your account until you delete them. And that’s just one symptom of the problem here: Google’s account recovery flow falls back to other forms of verification that you may not even be aware of.

I get why Google designed the account recovery workflow to be this way. For the average user, getting access restored to their account may be more important than locking out adversaries. But for those of us who beg to differ, this can have disastrous consequences.

I urge you to review your 2-Step Verification and remove “Voice or text message” as an alternative second step, as well as any legacy credentials such as security questions. Only trust cryptographically secure 2FA. To prevent accidental lockout, store your 2FA recovery codes somewhere safe.

Your future self will thank you.

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Name: ericr