Comment by bradgessler | Hacker News Reader

Comment by bradgessler | Hacker News Reader

I wrote a Rails plugin for magic links at https://github.com/rocketshipio/nopassword that doesn’t suffer from many of the problems I’m seeing in the comments.

The big thing is I only use a 6 digit numerical code that people have to copy and paste or type into the browser which they’re authenticating. I looked at stuffing a token into a URL, but it’s not a good idea because the email client may try opening the link to preview it or it may try opening the link in the wrong app/browser, such as an in-app browser.

That may sound super insecure, but the 6 digit code is half the secret that’s needed to authenticate. The browser that the person is using to login has a much longer complex secret that must be included with the code. Additionally, this combination must be authenticated within a set number of attempts, 3 by default, within a certain timeframe, 5 minutes by default.

My motivations for creating this, instead of using something like devise with passwords, is because I have seen soooooo many non-technical people get tripped up by passwords. I know there’s sign-in with Google, MS, etc. via OAuth, but I wanted to give people a way to login to web applications without being under the watchful eye of big tech.

I’m currently using it in production for all of my Rails apps, like https://legiblenews.com/email_authentication/new

A better description of why and how it works at https://github.com/rocketshipio/nopassword

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)

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.

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).

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.

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.

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.

>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.

> You didn’t explain how this gives you any protection akin to a physical chip on a debit card…

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.

So you actually agree with me.

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.

Seems like it :) but I would be more pragmatic and didn't bash the system for the reasons you brought up :)

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.

OP here… this was a great thread. Here’s a few takeaways, which you may not like, but I think are worth writing out.

1. This def needs hardening. I currently only use it for really low stakes stuff. I need to make that clearer in the README.

2. The next level of hardening I was planning on implementing is rate limiting code requests per email address. I figure I can step up the time-outs between code requests pretty aggressively to slow down brute forcing the code. The ATM that eats the card is a great analogy.

3. Maybe I missed it in the thread, but nobody mentioned that this from of auth makes it evident to the persons being attacked that something is happening if they have a bajillion code requests in their email inbox. There’s a lot of other problems that creates for these auth scheme, like filling a users inbox beyond its quota, triggering high volume breakers for SMTP relay services like AWS SES, etc. that wouldn’t be good.

For those watching, if you’re a security researcher feel free to open a GitHub issue on that repo and I’ll address it.

You should take this nopassword repo down. It's a good example of why you shouldn't "roll your own" in InfoSec, and it would honestly make me skeptical of any of your other security work. It looks like you may not be using it for anything sensitive, but I'm worried that someone else might try to.

As an alternative, there are many hosted services (e.g. Auth0) that are well-regarded, and I'm sure there are some self-hosted options. There seems to be at least one way to do magic links on top of Keyclock[1].

1. https://github.com/p2-inc/keycloak-magic-link

Yep. Promoting shit like this on a public forum is borderline malicious.

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