nanog mailing list archives
Re: Malicious SS7 activity and why SMS should never by used for 2FA
From: William Herrin <bill () herrin us>
Date: Sun, 18 Apr 2021 12:47:36 -0700
On Sun, Apr 18, 2021 at 12:03 PM John Adams <jna () 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. Regards, Bill Herrin -- William Herrin bill () herrin us https://bill.herrin.us/
Current thread:
- Malicious SS7 activity and why SMS should never by used for 2FA Eric Kuhnke (Apr 17)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Tim Jackson (Apr 17)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mel Beckman (Apr 17)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Dan Hollis (Apr 17)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mark Tinka (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mel Beckman (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mark Tinka (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA John Adams (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA William Herrin (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA John Adams (Apr 18)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mel Beckman (Apr 17)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Tom Beecher (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mel Beckman (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mark Tinka (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA William Herrin (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA John Adams (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA John Levine (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mark Tinka (Apr 19)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mike (Apr 20)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Mel Beckman (Apr 20)
- Re: Malicious SS7 activity and why SMS should never by used for 2FA Tim Jackson (Apr 17)