Malicious SS7 activity and why SMS should never by used for 2FA
jna at retina.net
Sun Apr 18 21:05:41 UTC 2021
I’m sorry - I think we miscommunicated here.
I was not advocating for TOTP or HOTP for SMS - in fact I’m completely against SMS being used for multi factor auth at all.
Sent from my iPhone
> On Apr 18, 2021, at 12:48, William Herrin <bill at herrin.us> wrote:
> On Sun, Apr 18, 2021 at 12:03 PM John Adams <jna at retina.net> wrote:
> > On top of this most TOTP and HOTP systems have additional security checks like blocking reuse of codes, rate-limiting of guesses, and in some cases acceptance of earlier codes (in TOTP) if the clock skews too far that make them much stronger options which decreases security but is certainly more of a convenience factor.
> Hi John,
> On a site, the symmetric key used to generate the TOTP code is stored in the same database as the user's password. Unencrypted or with readily reversible encryption since unlike a password it can't be verified by comparing ciphertext. Your protection is that every site uses a different TOTP key, just like you're supposed to use a different password, so compromise of a single site doesn't broadly compromise you elsewhere. It can also be captured with malware on your phone, the same place an adversary will sniff your password, which -will- broadly compromise you if you're also entering the passwords on your phone.
> None of these authentication schemes are magic. They all have attack vectors with varying degrees of difficulty, none of which are particularly harder than breaking a well chosen password. 2FA doesn't solve this. All it does is require an adversary to break -two- completely different authentication schemes in close enough proximity that you won't have closed the first breach before they gain the second. That's it. That's all it does.
> While attacks on SMS are certainly practical, stop and think for a moment on how you would scale them up and break 10000 accounts per day. Got a plan where you're not caught in the first two days? No, you don't.
> SMS is not a strong authentication factor. When used well, it's not intended to be. It's meant to require an adversary to do enough extra work after having already captured your password that unless they're specifically targeting you, the odds favor discovering and correcting the original breach before much harm can be done. For that use and that use only, it performs about as well as TOTP.
> If you can reset your email password with an SMS message and reset your bank password with an email then SMS has been misused as a very weak single factor authentication process. Not because SMS offers weak authentication (that's all it's meant to offer) but because it was used incorrectly in a process that needed strong authentication.
> Bill Herrin
> William Herrin
> bill at herrin.us
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