4 ms·

So if I have 700k usernames I can pwn one of them with 50% probability. Cool! With 3 attempts allowed I only need 230k usernames, even better! The "secret" st

So if I have 700k usernames I can pwn one of them with 50% probability. Cool! With 3 attempts allowed I only need 230k usernames, even better! The "secret" stored in the browser doesn't protect you from this, since I'm not stealing someone's code; I'm logging in from my own browser. (Of course I'm doing this via a botnet, so you won't notice it by IP address)

Isinlor 4y ago
With passwords you will get even higher probability if you just try 100 most popular passwords allowed by the service on hundreds of thousands of users. Try making password creation too difficult and now password reset will be the default authentication for a lot of users anyway.
- aaaaaaaaaaab 4y ago
  The 100 most popular passwords should already be blacklisted during registration, so it wouldn't be possible to set them as your password. I have nothing against password-reset-as-authentication aka "magic links", when done properly. And by properly I mean not using an ultra-low entropy auth code, like the parent is proposing (6 decimal digits, aka 20 bits of entropy).
joshstrange 4y ago
I'm pretty sure you are missing how they implementing this. It sounds like at time of the request to send the code the browser either generates and sends to the server (or gets from the server) a longer/more-complicated secret. Both that secret (which is probably account-specific, the one you requested login to) and the 6 digit code are required. You can't just guess the code, the code has to be combined with another account-specific token. By regenerating longer token each time a 6-digit token is required and only allowing something like 3 guesses per longer token (Before regenerating and sending a new 6-digit code) you have a secure system.
- aaaaaaaaaaab 4y ago
  No, actually I'm pretty sure you are missing why their system is insecure. But sure, let me explain it: Step 1: acquire a list of 230 000 usernames/emails. Step 2: for each username/email, launch a browser and request a login code. The login code is sent to the users' email address, and the super-ultra-secret token is stored in the browser. Step 3: for each username/email, do 3 random guesses from their respective browsers. Each guess is a Bernoulli trial with P = 10^-6. Thus the probability of at least one success is 1 - (1 - 10^-6)^(230000 * 3) = 49.8% I hope this made it clear for you. I think the fallacy in your reasoning was assuming that only legitimate users can get the "secret" token, which is false. It is true for things like debit cards, where the token is stored on the card itself, hence the 4-digit PIN with 3 tries allowed is sufficient. In this case however a "secret" token is handed out to anyone who requests a code. It protects you from someone peeking into your mailbox and stealing your code, but it fails against brute forcing codes on a large number of users.
  - mszary 4y ago
    You're over complicating this. In fact it might provide a similar kind of protection the physical chip provides for debit cards. Let me explain: 1. Someone (anyone) initiates the login flow 2. The user record in a DB gets enriched with - the 6-digit OTP (sent via email) - the "secret" (shared with the initiating browser) - the current session failed attempts counter (default: 0, max: 3) - the total failed attempts counter since last successful login (default: 0, max: 20) Let's assume that you've initiated the session (hence own the "secret"), but have no access to the OTP - you will always have 3 attempts to guess the OTP before the session gets reset (a new "secret" + new OTP). So after the 3rd unsuccessful attempt you start from scratch. And at that point other means of protection might come into play based on the second counter (throttling the auth. requests on the WAF/fronting server, blacklisting IP addresses, etc.) The number of accounts you're targeting does not matter - in fact it may help other systems mentioned catch your malicious attempts sooner.
    - aaaaaaaaaaab 4y ago
      >In fact it might provide a similar kind of protection the physical chip provides for debit cards. Let me explain: You didn’t explain how this gives you any protection akin to a physical chip on a debit card… Let me illuminate the problem with a real-world analogy: this system is like me walking into a bank with a list of account names, and the bank issuing me debit cards for those accounts with random PIN codes, no questions asked. I’d have 3 attempts to guess the PIN, then the ATM swallows the card. Same as in OPs login system! Still, don’t you think banks would be silly to do this? >So after the 3rd unsuccessful attempt you start from scratch. You don’t start from scratch. You do 3 attempts per user, period. >The number of accounts you're targeting does not matter It does matter. The entropy of their six-digit OTP is small enough that pwning at least one account from a pool of users is feasible. As I’ve shown above, the number of users necessary for pwning at least one account with 50% probability is log(0.5)/log((1 - 10^-[digits])^[attempts per user]). In our case digits = 6, and attempts per user = 3, which gives us ~231000 users. Which isn’t too many for a website.

Isinlor 4y ago

With passwords you will get even higher probability if you just try 100 most popular passwords allowed by the service on hundreds of thousands of users. Try making password creation too difficult and now password reset will be the default authentication for a lot of users anyway.

aaaaaaaaaaab 4y ago
The 100 most popular passwords should already be blacklisted during registration, so it wouldn't be possible to set them as your password. I have nothing against password-reset-as-authentication aka "magic links", when done properly. And by properly I mean not using an ultra-low entropy auth code, like the parent is proposing (6 decimal digits, aka 20 bits of entropy).

joshstrange 4y ago

I'm pretty sure you are missing how they implementing this. It sounds like at time of the request to send the code the browser either generates and sends to the server (or gets from the server) a longer/more-complicated secret. Both that secret (which is probably account-specific, the one you requested login to) and the 6 digit code are required. You can't just guess the code, the code has to be combined with another account-specific token. By regenerating longer token each time a 6-digit token is required and only allowing something like 3 guesses per longer token (Before regenerating and sending a new 6-digit code) you have a secure system.

aaaaaaaaaaab 4y ago
No, actually I'm pretty sure you are missing why their system is insecure. But sure, let me explain it: Step 1: acquire a list of 230 000 usernames/emails. Step 2: for each username/email, launch a browser and request a login code. The login code is sent to the users' email address, and the super-ultra-secret token is stored in the browser. Step 3: for each username/email, do 3 random guesses from their respective browsers. Each guess is a Bernoulli trial with P = 10^-6. Thus the probability of at least one success is 1 - (1 - 10^-6)^(230000 * 3) = 49.8% I hope this made it clear for you. I think the fallacy in your reasoning was assuming that only legitimate users can get the "secret" token, which is false. It is true for things like debit cards, where the token is stored on the card itself, hence the 4-digit PIN with 3 tries allowed is sufficient. In this case however a "secret" token is handed out to anyone who requests a code. It protects you from someone peeking into your mailbox and stealing your code, but it fails against brute forcing codes on a large number of users.
- mszary 4y ago
  You're over complicating this. In fact it might provide a similar kind of protection the physical chip provides for debit cards. Let me explain: 1. Someone (anyone) initiates the login flow 2. The user record in a DB gets enriched with - the 6-digit OTP (sent via email) - the "secret" (shared with the initiating browser) - the current session failed attempts counter (default: 0, max: 3) - the total failed attempts counter since last successful login (default: 0, max: 20) Let's assume that you've initiated the session (hence own the "secret"), but have no access to the OTP - you will always have 3 attempts to guess the OTP before the session gets reset (a new "secret" + new OTP). So after the 3rd unsuccessful attempt you start from scratch. And at that point other means of protection might come into play based on the second counter (throttling the auth. requests on the WAF/fronting server, blacklisting IP addresses, etc.) The number of accounts you're targeting does not matter - in fact it may help other systems mentioned catch your malicious attempts sooner.
  - aaaaaaaaaaab 4y ago
    >In fact it might provide a similar kind of protection the physical chip provides for debit cards. Let me explain: You didn’t explain how this gives you any protection akin to a physical chip on a debit card… Let me illuminate the problem with a real-world analogy: this system is like me walking into a bank with a list of account names, and the bank issuing me debit cards for those accounts with random PIN codes, no questions asked. I’d have 3 attempts to guess the PIN, then the ATM swallows the card. Same as in OPs login system! Still, don’t you think banks would be silly to do this? >So after the 3rd unsuccessful attempt you start from scratch. You don’t start from scratch. You do 3 attempts per user, period. >The number of accounts you're targeting does not matter It does matter. The entropy of their six-digit OTP is small enough that pwning at least one account from a pool of users is feasible. As I’ve shown above, the number of users necessary for pwning at least one account with 50% probability is log(0.5)/log((1 - 10^-[digits])^[attempts per user]). In our case digits = 6, and attempts per user = 3, which gives us ~231000 users. Which isn’t too many for a website.