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'Adding a Phone Number To Your Google Account Can Make it Less Secure' (vijayp.ca)
You may think that adding a backup phone number to your account will make it prone to hack, but that is not always the case. Vijay Pandurangan, EIR at Benchmark (and formerly with Eng Site Lead at Twitter) argues that your phone number is likely the weakest link for many attackers (at least when they are trying to hack your Google account). He has shared the story of his friend who had his Google account compromised. The friend in this case, let's call him Bob, had a very strong password, a completely independent recovery email, hard-to-guess security questions, and he never logged in from unknown devices. Though Bob didn't have multi-factor authentication enabled, he did add a backup phone number. On October 1, when Bob attempted to check his email, he discovered that he was logged out of his Gmail account. When he tried to login, he was told that his password was changed less than an hour ago. He tried calling Verizon, and discovered that his phone service was no longer active, and that the attacker had switched his service to an iPhone 4. "Verizon later conceded that they had transferred his account despite having neither requested nor being given the 4-digit PIN they had on record." The attacker reset Bob's password and changed the recover email, password, name on the account, and enabled two-factor authentication. He got his account back, thanks to support staff and colleagues at Google, but the story illustrates how telco are the weakest link. From the article: Using a few old Google accounts, I experimented with Google's account recovery options and discovered that if a Google account does not have a backup phone number associated with it, Google requires you to have access to the recovery email account OR know the security questions in order to take over an account. However, if a backup phone number is on the account, Google allows you to type in a code from an SMS to the device in lieu of any other information. There you have it: adding a phone number reduces the security of your account to the lowest of: your recovery email account, your security questions, your phone service, and (presumably) Google's last-ditch customer service in case all other options fail. There are myriad examples of telcos improperly turning over their users' accounts: everything from phone hacking incidents in the UK to more recent examples. Simply put, telcos can be quite bad at securing your privacy and they should not be trusted. Interestingly, it appears that if two-factor-auth via SMS is enabled, Google will not allow your password to be reset unless you can also answer a security question in addition to having access to a phone number.
It doesn't really matter what that is, but if there's a way to "recover" your account, then it's by necessity, a way to completely bypass any other authentication you had. The more ways to recover the account, the more attack vectors there are.
It's why I hate "recovery questions", they're usually bad questions that anyone could find out, and if I use some other answer, then I'm likely to forget what it is anyway.
If I need a password to access the site, at least it's only one thing to remember, and only one point of weakness for an attacker.
So the big question is, which is more important? the ability to recover an account you've been locked out of? or the security of knowing nobody else can either?
Of course companies can really screw this up too. For instance Tumblr recently re-set everyone's passwords and forced them all to use their recovery option because their password database had been compromised. Anyone who did not have a working recovery option was completely screwed, even though their account was otherwise more secure.