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mszary | 3 years ago
In both cases you (an admin) control how many attempts you wish to accept before sacrificing the usability over security (temporarily disabling the card and forcing you to call the bank and disabling 6-digits OTP in favour of ie. 12-digits OTP).
Your real-world analogy is, this time, too simplified :) Let me counter-argument with my example:
You are in a a possession of 1 million user emails and request OTPs for all of them (let's assume no other security measures are in place). If you hit all of the 1M records with 3 attempts of OTP - according to my simple experiment - you might hit (on avg) 3.18 accounts.
Of course, in real life, your attack would be mitigated on other (non-application) level.
Please find my ugly code for that experiment: https://pastebin.com/S2ufabhU
PS. When OTP size is increased to 7 digits - the avg account hit drops to 0.33. For 8 digits - it drops to 0.02. (please mind I'm using lousy RNG :)
PS2. What I'm trying to emphasize is that there's nothing wrong with OTP-auth like this if properly implemented with other basic anti-bruteforce techniques. You could get "better" results with simple password spraying.
aaaaaaaaaaab|3 years ago
1. I don’t have any problem with OTP-based magic links, if they’re implemented carefully. As you’ve just demonstrated, a 6-digit OTP with 3 retries allowed is not sufficient at scale.
2. I’ve also demonstrated that OP’s proposal to augment the 6-digit code with a “secret” token gives you no additional security. It can only protect against someone peeking over your shoulder to snatch your OTP.
3. As you’ve just demonstrated, increasing the OTP length will mitigate the problem, as it increases entropy. For peace of mind, I’d recommend 64 bits of entropy, which can be achieved with a 13-letter alphanumeric code (case insensitive), or with 5 words from a dictionary of 10 000.
mszary|3 years ago
ad 1 - IMO, it's still sufficient at scale with some basic infra. hardening
ad 2 - AFAIK the "secret" was never meant to protect from brute-force, but rather mitigate threats from actors controlling the email part
ad 3 - again, let's be more pragmatic - no one would use it if it required typing 13-letters OTP :) there are other ways to mitigate the potential attack you've described.
Best.