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So attackers cannot password spray. This is typically after attackers has gotten access to the latest database breach, and are just blindly trying username/password combinations.

A simple ratelimit takes care of that. Plus, it's not like attackers would be easily defeated by a CAPTCHA anyway --- there are services selling batches of valid tokens, likely generated by actual humans or very close emulations thereof, for ReCAPTCHA.




CAPTCHA is not a fool proof, it is just the first layer in of defence in the signup/login form. CAPTCHAS increases the cost of password spraying, attackers can't simply fire up Hydra. They'll need additional tools and services which costs money.

Captcha solving service also has other costs than just the money it costs. It adds time costs and additional resource usage on the machines it is running on. A quick look at a service[1] shows that the average response for a challenge was 40 seconds (this value changed a lot when refreshing the page). The attacker has now gone from the 200ms range per attempt to several seconds, slowing the down a lot. This gives defenders additional time to respond, it is also a useful metric for detecting malicious logins.

[1] https://anti-captcha.com/mainpage


Rate limit by what? IP? Botnet traffic will originate at random IPs.


By the account. 3 failed login attempts in a row, and you disallow further logins for 30 seconds.

This should waste less time than reCAPTCHAs. I know it's not 1:1 in terms of pros/cons, but it gets a good subset of the advantages without the key disadvantages mentioned above.


First, that's a bit user-hostile (and suddenly a DoS-vector; I can prevent a site's users from logging in by continuously firing bad password attempts).

Secondly, botnets can, and presumably do, randomize which accounts they try, too.


So rate-limiting is "user-hostile", but permanently hell-banning someone because their network is considered "seedy" is user-friendly?

Incidentally, you still need rate-limiting if you use Google's CAPTCHA. If you don't rate-limit CAPTCHA endpoint, an attacker can DDoS you (especially if your server-side captcha component uses low-performance single-threaded HTTP client). Furthermore, an attacker within the same AS as their target can purposefully screw over their account by performing attacks on Google's services until the reputation of the network hits rock bottom.


reCAPTCHA is a rate-limiting measure. Google handles all the heavy-lifting and attacker protection for you, and the slow fade you see in the video is that rate-limiting in action. But if you get a clean CAPTCHA result back from them, then that client is very unlikely to be an automated attacker. It's super easy and scales really well.

Conveniently, normal users with typical browser configurations get nothing but the animated checkbox. For nearly everyone, the whole experience is simple and easy. The only people who get inconvenienced are the low-grade privacy enthusiasts who think that preventing tracking is the path to Internet safety. Ironically, "tracking" is literally the mechanism by which legitimate users can be distinguished from attackers, so down that road lies a sort of self-inflicted hell for which the only sensible solution is to stop hitting yourself.


so down that road lies a sort of self-inflicted hell for which the only sensible solution is to stop hitting yourself.

"Be a good little sheeple and do what Big Brother Google says." Fuck no.


So I can lock you out of your account with 3 attempts from any IP address?


For a minute usually. Prevents flooding. Not a bad approach unless the account is constantly hit. In those cases two factor auth makes sense.


This is obviously a bad idea. It costs nothing for an attacker to send 3 http requests, every minute, every hour, all day. They could lock your account basically forever. IP filtering and locking accounts are terrible ways of preventing password spraying.


> By the account. 3 failed login attempts in a row, and you disallow further logins for 30 seconds.

...congratulations, I just locked out all of your users. Have a nice day.


How did you get the email addresses of all my users, which are used as login name?


From that messed up email from support that leaked them. Or I assumed that you'll have a big cross-section with some other site that leaked.

This is not theory, this is hard-earned experience. Locking-out people is bad, the most that's acceptable is rate limiting to a once every few seconds.




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