However, as a user, I go back to the idea that I can't think of many companies I trust to only use my phone number as 2FA and not as identity verification. So I'm am skeptical that it is good to train users to trust SMS 2FA, because those same users will probably not be able to distinguish between 2FA and identity verification when they sign up for other services. It is better to teach users a simple rule (never give out your phone number) than a complex rule (only give out your number for this specific use-case).

The other big thing I just can't get past is that nearly everyone today has a smartphone that will run a 2FA app, and that even users who don't have a smartphone would be better served by getting codes delivered to their email. So sure, it's better than nothing. But there are even better options that exist that aren't that hard for us to switch to.

In practice, even if you know you're only going to use SMS for 2FA, I now lean towards saying you shouldn't use SMS at all. Treat email like the backup SMS option, and just get rid of phone numbers entirely.

Maybe the dynamics of that change for some developing countries? But Twitter, Facebook, and Amazon all know what country I live in. If they want to offer an SMS option for India because of some extenuating circumstance I can't think of, they should still have the good sense to at least discourage SMS verification for accounts that are based in the USA or Europe.

What do they do when their smartphone dies? The phone company makes sure your new phone has the same phone number, but you lose 2FA tokens in apps.

And I have no idea where the recovery codes that I printed on paper are since I last moved.

> even users who don't have a smartphone would be better served by getting codes delivered to their email

But then isn't that just one factor instead of two factors, because both their password for the service and their email password are just "something you know"? I'm assuming if they have no cell phone, they don't have a second factor to secure their email either.

It should be right next to the document that explains your loved ones how to get into your password manager, if you ever get hit by a bus.

> The phone company makes sure your new phone has the same phone number

This is exactly why 2-factor SMS is insecure. You mention later that email is something you know, instead of something you have. In the same way, if a company can transfer my number to a new phone without access to the original phone, then it's not really something I have.

The ease of number transfers are the problem. The reason why 2FA tokens aren't stored online and secured with a password is because they are designed to be something you have, not something you know.

For comparison, switching your number to Verizon only requires information that you know (account numbers, a SSN)[0], so it's just extra steps around a less secure password that you can't change or set yourself.

> But then isn't that just one factor instead of two factors

Expanding on the above -- yes, email is often going to be just another account secured with another password. In practice, hacking two accounts is often harder than hacking one, and in practice, I suspect breaking into someone's Gmail account is harder than stealing their phone number. Google offers much more comprehensive 2FA options than most other companies, and their automated security alerts also tend to be better.

But there's no reason for us to debate over how secure email is.

The situation we have today with companies like Amazon/Facebook/Twitter is one where I can already request a password reset without SMS. Companies are scared of strict 2FA methods because customers get locked out of their accounts. Very, very few of them are willing to take that risk, so email will virtually always be an option. SMS is being added on top of that system -- it's not replacing it.

Here's Twitter's account recovery help page[1]:

> If you do not receive anything back, get help with Twitter via SMS or use the email password reset option.

So if you consider email to be a weak link in identity verification/2FA, adding SMS verification as a secondary option alongside email still doesn't do anything to increase your security. In fact, even if SMS was as secure as email, forcing you to monitor two authentication methods instead of just one would still be less secure.

I'm not advocating email is perfect, I'm just advocating that SMS is less secure than email, and that since companies are already comfortable trusting email, they can continue to rely on that.

Of course if you really want to set up 2FA to literally be 'something you have', then you need to accept that things you have can be lost. And if you're not willing to make that compromise, at least email accounts are harder to hack than phone numbers, because the most common email providers are probably more resistant than Verizon to social engineering attacks.

[0]: https://www.verizonwireless.com/support/local-number-portabi...

[1]: https://help.twitter.com/en/managing-your-account/forgotten-...

> Twitter [...] So if you consider email to be a weak link in identity verification/2FA, adding SMS verification as a secondary option alongside email still doesn't do anything to increase your security.

I agree, and your comment's parent (my comment's grandparent) specifically went out of its way to agree: "as you said, many big tech companies say they have 2FA, when they really are just giving you two ways of logging in, where one of those ways is incredibly insecure."

> switching your number to Verizon only requires information that you know (account numbers, a SSN) [...] hacking two accounts is often harder than hacking one, and in practice, I suspect breaking into someone's Gmail account is harder than stealing their phone number

I think the number of people who go to the trouble of using like, a Yubikey or something for their Gmail but won't use it for anything else is vanishingly small. People opting for password + SMS 2FA (NOT the SMS 1FA that let @jack get hacked) are probably using the same thing for their email.

I'm sure it's true that it's easier to steal someone's phone number than break into their Gmail account, but afterwards you can go into Verizon's physical store with your physical government-issued ID and get your phone number back. That's not an option with a Gmail account.

No one is saying any of these are perfect, and everyone agrees SMS is less secure than email. The question is whether password + SMS 2FA is less secure than email 1FA, or whether password + email 2FA with no account recovery pathway is workable—doubtful, and definitely not.

Let's agree that out of password + SMS 2FA, password + email 2FA, and email + SMS 2FA, the first one is the weakest link, because SIM-jacking is terrifyingly easy and people choose terrible passwords. Just for account recovery, though, email + SMS 2FA still provides security benefits over email 1FA (you can guarantee a second factor, even if it's a weak factor, whereas you actually have no idea how strongly or weakly their email account is protected, you're just assuming) and usability benefits over email + TOTP apps/Yubikeys/paper backup codes.

Agreed, but I can't think of a single company, anywhere, that offers what you're talking about. Everyone offers SMS and email as separate options, both of which separately unlock your account.

If either Twitter, Facebook, Amazon, or Facebook required both email and SMS access to recover an account, I'd agree that there could be some value there. But (to the best of my knowledge) they don't. So the debate over whether or not SMS verification is better than nothing is hard for me indulge, when (again to the best of my knowledge) virtually no company is using SMS account recovery in a way that provides real value over 1FA email.

Maybe Lyft is an example? But the last time I used Lyft, I'm pretty sure I could get access to my account with only my phone, no password/email required. I'm not 100% sure Lyft even requires an email to sign up.

> That's not an option with a Gmail account.

I've never been in this scenario, so I'll have to take your word for it, but this seems strange to me. Could I really not fax or mail a government ID to Google to get access to my account?

Assuming this is right though, we again run into the same problem.

I lose access to my password and email. Is a company comfortable letting me reauthenticate with only an SMS message?

If yes, then we have 1-factor authentication over pure SMS.

If no, then we have to be comfortable with the idea that losing your email/password might mean losing your account, or going through a complicated recovery process involving government IDs.

I think you're right. I thought Vanguard or my bank did, but no, it's email or SMS plus personal info like SSN, birthdate, zipcode (LOL, information that no one has on me, thanks Equifax!).

> Could I really not fax or mail a government ID to Google to get access to my account?

To what address? Just plop it down at 1600 Amphitheatre Pkwy? I've never heard of Google offering any account support whatsoever for a free private Gmail account, have you?

I do personally know people who have just given up on accounts they lost access to (they claim they didn't forget the password) and just created a new Gmail account. Not the most technically literate person, but still, that's who support is supposed to be for. But it's a free service, so.

> then we have to be comfortable with ... going through a complicated recovery process involving government IDs

What? The whole point I've been trying to get across is that unlike TOTP or email, you can get your phone number back through a "complicated recovery process involving government IDs", which as an advantage. That's not a tradeoff to be comfortable with, that's an upside.

> I lose access to my password and email.

Why would we design for this? Unless there's a particular reason to think password and email are likely to be lost simultaneously (which I can't think of, unlike say, smartphone TOTP app + phone number), then we should either design for losing any combination of 2 auth methods, or not worry about combos.

By contrast, to me it could make sense to design a system so that you can lose any 1 of 3 things but still be able to log in with the remaining 2 makes sense (e.g. password, email, phone number). But you're right that most services are effectively just email 1FA, and many are SMS 1FA too which we all agree is utterly broken.

Don't know in the USA but here in Europe you have to confirm/prove that you own the number being moved from one operator to another.

Edit: I suppose phone numbers can at least be changed, but that doesn't really solve the problem for 2fa as it might for SS numbers.

What is this supposed to mean? What is the point of having a phone number if you never give it out?

I suppose you can make outbound calls only with own number sending turned off, but that sort of diminishes the purpose of owning a phone number?

In other words, if a company tells you they need your number for account security, refuse to give it to them. Of course you can give out a phone number for general contact purposes.

I think it's easier to teach people phone numbers can't be used for security, period, then to teach them that phone numbers can be used for security in one very specific case -- especially since companies like Facebook/Twitter rarely use terms like 2FA when asking users for their number. They just say stuff like, "you'll use this to gain access to your account" or, "we'll use this to help keep your account secure."

It wasn't my intention to make it sound like users should never tell anyone their number for any reason, but I can definitely see how it might read that way.

Alice is tricked into visiting fakebank.example a site which looks exactly like Alice's real bank. She enters her username and password, the site says it will send her an SMS, behind the scenes Alice's details are plugged into realbank.example automatically, triggering an SMS to Alice from her real bank.

Alice types the SMS code into fakebank.example, it stalls a little bit, very common for banks, meanwhile it plugs the SMS code into the realbank.example site and successfully logs in as Alice. Cool. The fakebank.example site finally gives an error. "Code 418. We're sorry, this service is temporarily wormhole phasers galactic transwarp. Please try again later". Alice is annoyed but decides to try again in an hour. By then her account will be empty.

WebAuthn / U2F work fine here, because they don't give Alice the opportunity to mistake this for her real bank, the FIDO token will cheerfully mint credentials for fakebank.example which are no use to log in as Alice on realbank.example. There's no "I'm sure" step, no "Actually this is my real bank", no opportunity for human error to betray her. The bad guys still annoy Alice with their bogus site, but they don't get working login credentials.

I had an account with Bank of America and a prepaid AT&T plan. I was about to move abroad so I went to my local bank office and asked if there was anything I needed to do to prepare for this. "Nothing in particular" I was told.

After moving abroad I opened another bank account in that country and tried to transfer my money over to said account. However in order to transfer the money out I had to enroll (I wasn't already) in sms 2FA. As at&t prepaid does not have roaming I wasn't able to enroll using it. And only American numbers were accepted. I got on the phone (a different one) with at&t to see if roaming could be enabled. "-Nah, there is no roaming for prepaid only for postpaid. -Can you sign me up for that? -No, only in a store"

Now I weighted my options. I could fly back to the US to resolve it, if I could get another visa. I could fly to Mexico or Canada where there is roaming. In the end I went with the advice the bank's support gave me: I enrolled a friends number. This does not feel secure at all, but what you gonna do?

Moral of the story: If you move abroad, better make sure your sim supports roaming, because you're gonna get locked out of stuff otherwise.

If it's truly always 2FA, including two-factor password reset (need SMS verification code in addition to email verification code), than it's probably better than nothing. But otherwise, it's actually a major exposure you're opening up for yourself, especially if you otherwise have good password practices (no re-use, and ideally pseudo-randomly generated from a password manager).