Let's assume CAPTCHA is enabled with account lock out control (after five continuous failed attempts, the account will be locked for 15 min) on a system.

Is brute force still a probable threat?

  • If users can pick their own user names, and there's no password strength policy, then yes, an attacker can brute force common user names and passwords and even limited to 5 passwords per account in 15 minutes, would be likely to break into a few. A password strength policy would help. – paj28 Oct 7 at 21:03
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    With password security, the ultimate goal is to make it difficult for an attacker to access user accounts even if they have a copy of your database. – Justin Lardinois Oct 8 at 6:30
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    Consider that if you ban IPs for too long, denial of service might become an issue when the IP is shared (for example in local networks behind a single public IP). You actually said that "the account" will be locked for 15 minutes, I hope you meant IP, otherwise that's even worse (a bot could try to log in repeatedly and fail, and the account will keep on being locked). – reed Oct 8 at 22:57
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    Lock-out is a great start, the obvious next step is incremental increase of the time. 15 minutes. 30 minutes. 2 hours. 4 hours. A day. – Mast Oct 9 at 13:48
up vote 65 down vote accepted

The protections you describe are good ones that you should consider, but there can still be weaknesses:

  • Many CAPTCHAS can be solved by robots, or you can easily pay people to solve them en masse for you (there are companies selling that service).
  • Account lock out is a good idea, but if you do it based on IP someone with access to a botnet could retry login on a single account from different IP:s until they get in.
  • Offline brute force is still a problem if your database gets leaked. If the attacker has access to the password hash, they can try all they want on their own system. That's why you should use good hashing.
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    Many CAPTCHAS can be solved by robots: this is particularly true nowadays, with deep learning and neural networks, which can have better scores than average humans at this kind of tasks. Captchas are on the verge of being useless. – spi Oct 8 at 14:58
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    Good salted (and perhaps peppered as well) hashing. – Monty Harder Oct 8 at 22:21
  • @spi what about reCAPTCHA? – theonlygusti Oct 9 at 12:26
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    @theonlygusti I fail reCAPTCHA's on average 30% of the time, and I'm a software engineer blah blah. Don't use them, they're pointless anymore and just cause people more pain-and-suffering than it's worth. (I keep dropping services that I can when I hit a reCAPTCHA anymore, they're just too painful to deal with. I spent 30+ minutes in one session on a single one because the Google algorithm was wrong, in fact.) – 202_accepted Oct 9 at 14:07
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    @ChristopherSchultz And how does PBKDF2 (etc) not fit the definition of a cryptographic hash function? It is given a user input (the password), and it produces a fixed-length output (which it is hard to deduce the input from, is likely to be different for different inputs, etc). – Martin Bonner Oct 10 at 15:28


it depends on how you define "brute force".

A lockout after X incorrect attempts is great for protecting an account where an attacker is going after a single target.

There's another scenario where the attacker has picked a few common passwords "password, password123, etc." And rather than attacking a single user, they try their 4 common passwords on every account they know of in your system.

User: Jim
PW: password, password123, letmein, secret

User: Bob
PW: password, password123, letmein, secret

User: Alice
PW: password, password123, letmein, secret

This is more common in scenarios where attackers are looking to harvest credentials for resale on the darknet, or make lateral moves to other services where passwords may have been reused.

I suggest you add something in place to count the rate of overall invalid logins, rather than just on a per account or IP level.

  • easily countered by proxies or botnets. – Sebastian Nielsen Oct 9 at 17:14
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    @sebastian I think you may have misunderstood what I am saying. You should be aware of the rate of overall invalid logins, not just those from the same IP. You could use 10,000 proxies, and I would still notice that the overall rate indicated a coordinated attack. In that case I might investigate and see if I could find an identifier of the attack (IP range, country, request format, browser agent, history of interaction such as whether they ever loaded the home page, etc.) Count. next comment – Daisetsu Oct 9 at 17:23
  • cont. Then weed out the coordinated attack that way in a transparent to the attacker, leaving them assuming they are still attempting loving while their traffic is really being automatically denied. – Daisetsu Oct 9 at 17:23
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    This kind of attack is called a "password spray" (for the sake of searching for it), and it could be mitigated somewhat by either a password strength requirement, or more ideally a "you used a common password, I'm not going to allow that" policy. (see: updated NIST password guidelines pages.nist.gov/800-63-3/sp800-63b.html section – Vidia Oct 9 at 21:12

It is a threat in a different sense. If you lock accounts for 15 minutes after 5 unsuccessful attempts, then you've effectively built-in a DoS mechanism.

Assume I don't really want to break in, but I'm fine with just causing havoc, no problem. Just do a few thousand logins per second with random usernames. Hey, I'll not even bother doing the CAPTCHA, who cares. All I want is to fail and lock up.

A better strategy than a fixed amount of time after a fixed number of failures might be quadratic (or exponential) growth. Some AVM routers do that. First login failure, you have 15 second lockout, next failure you have 30, etc etc. This is much less hassle to legitimate users, and much more trouble to attackers.
In order to make DoS harder, you would need a kind of recipe involving the IP address as well as the account name, capping the maximum delay per account-IP pair to a tolerable value. Otherwise, a legitimate user could still be DoSed easily and indefinitively. The exponential growth deals better with the "infinite number of attempts" problem, though.

Actually finding a username-password pair online by brute force is, well, assuming people aren't stupid, practically hopeless. Unluckily, people are stupid, so you cannot assume they won't have one of the top-ten-most-stupid passwords, and you must assume it's doable. So, yes, there is a bit of a threat there, too. In particular because while it may be hard to target one user on one server, on a purely username-based control system, you can target a thousand users on that same server in parallel no problem (each scoring only a single fail!) and you can do that on a thousand servers in parallel. And, it doesn't really cost you anything to keep this script running for weeks (months, years...), retrying every 15-20 minutes.

So, while for the individual account your chances as attacker are very small, as numbers add up to, well, virtually infinity you are bound to hit someone, somewhere, eventually, it's unavoidable. Since otherwise it's trivial to try a thousand users in parallel, it should be clear that you also need to consider IP addresses in your calculation. Even so, it doesn't give 100% protection against a botnet with a few thousand bots, but it sure makes the attack somewhat less effective, requiring more work and management. More work is good.

You cannot win the race once you are a serious target, but the harder you make an attacker's work, the more likely it is the attacker chooses someone else (who's an easier target) to begin with.
It's very much the same thing as locking your front door instead of leaving it wide open. A burglar can easily break your window, and there's finally nothing you can do to prevent someone from entering. But given the choice of an open door at the neighbour's house and having to smash your window, he will likely choose the easier way. Fewer expenses, same profit.

  • "If you lock accounts for 15 minutes after 5 unsuccessful attempts, then you've effectively built-in a DoS mechanism."; aren't most lockout systems today built with blocking at the IP level, not just username? – ravemir Oct 11 at 13:42

Yes, it is still a threat, because:

  • CAPTCHA's are very quickly approaching the point of security theatre. There are multiple services out there which can break them (for a price), and it's becoming more and more difficult to find problems that can be trivially solved by a human but not a computer.
  • Brute-force attempts against a single user are not the only potential attacks you have to deal with. It's not uncommon for attacks to try the same set of passwords against a list of known account names. It's also not too unusual for certain services that are likely to have a handful of well known usernames (SSH for example) to see user mapping attacks.
  • Unless you're enforcing some form of password quality checking, it's reasonably likely that a brute force attempt will not need enough tries for 5 tries every 15 minutes to slow it down enough.

Ideas for improving what you've proposed:

  • As mentioned above, enforce password quality. In an ideal situation, allow for things like the XKCD password generation method to work (see XKCD #936 for info on that), and better yet, make sure that any valid Unicode character is accepted.
  • Return exactly one failure code for an authentication failure due to invalid credentials, instead of having different ones for invalid usernames and invalid passwords. This is really important, because it protects against user mapping attacks.
  • Provide MFA support. This is actually not hard to do correctly if you take some time to set it up. TOTP MFA methods (such as what the Google Authenticator app and Steam's MFA system provide) are generally pretty easy to get working and are reasonably secure. U2F is also pretty secure but requires more work to set up. Regardless, if you go this way, allow for multiple MFA methods (ideally a mix of different types). Avoid anything that requires sending the login codes via email (it's absolutely not secure) or SMS (it's better than email, but can have a really long delay before the code is received). Any of your users who enable MFA are then functionally protected from most brute force attacks.
  • Don't just use a static lockout arrangement. Use an adaptive approach instead, where the time the account remains locked out is based on how many times it's been recently locked out. A simple exponential scaling system with an upper limit on the lockout time will work well enough. For example, make the time equal 15 * 2^n minutes with a cap of 2 hours, where n is the number of previous lockouts in the past 24 hours (first attempt is a 15 minute lockout, second is 30, third is 60, fourth and subsequent are 120).

There is such a thing as low speed brute force attacks, designed specifically to break into accounts with timeouts or lockouts.

If the attacker can figure out your thresholds (which he can by trial runs), he can write a bot to stay just under that threshold.

Of course, this limits the number of combinations he can try in a given time period, which is why these kinds of attacks often run for months or years and are unlikely to compromise accounts with reasonably long passwords.

So, in combination with a sane password policy (that's a different topic, here I'll just say that complexity != security and length > complexity) and a solid implementation of your described system, you can reduce the probability of a compromise considerably. In most cases, enough that the remaining risk is well within your risk acceptance limit.

Rate limit logins, account locking, etc are good to stop any economically feasible brute force attack against a login screen, but that is not necessarily how the attack is done.

Quite often accounts are compromised because the brute force attack isn't done against the login screen itself (which is limiting) but against a copy of the data. If an attacker can gain access to the data via a compromised server or some other means, the brute force attack is really about downloading the accounts and passwords, and then breaking the encryption on a much more powerful machine.

  • You have described the difference between an online vs an off-line brute force attack, but the question is about online attacks. – schroeder Oct 9 at 15:37
  • @schroeder I did answer about online attacks - see the first paragraph. The question never explicitly limits the question to only an online attack. I answered in a way that would raise awareness if it isn't already know. Security is such a complex and sensitive issue that your assumption cannot be safely made. – Paul Oct 9 at 19:18
  • Your first line only says "good to stop any economically feasible brute force attack", but you do not explain why – schroeder Oct 10 at 13:35

Brute force does not need to use much "force". Brute force could run for days and be a tiny, but persistent drop after drop after drop. I would consider captcha as a non issue for any determined attacker.

Even with your constrains you implied that these limits only apply to a single account. So if I know there are multiple accounts, I can still automate the process to keep trying.

I wont be able to brute force the whole possible keyspace with your constrains, but I will be able to force the top 1000 passwords per account within very slightly over 2 days.

Given that a list of top-1000 passwords would most likely cover a reasonable percentage of your users, you should be able to gain access to your system quite soon.

So can you defend against it?

Limit the trial per IP? -> Vector to avoid it: Botnets / VPN

So lets add "impossible travel" to the list? (User logs in from Germany and US within a minute)

So how about same IP tries various users?

One thing to consider is the value of the resource you try to protect, and which additional steps you want to take to secure it. Another quite safe addition to the strength of your system is a 2nd factor. But these can cause an additional cost for you, depending on what you use and on how many authentications you need to make. For example as a stand alone Service Azure will charge 1,4 $ per 10 authentications. Or you could use some kind of free service or a "Grid" system with unique data per user.

Is brute force still a probable threat?

"Probable" depends on how tasty a target you are. If you're a desirable target, then yes, they're a threat.

While a timeout with a rate limit and lockout will take care of brute force, since they would only get X tries in Y minutes, it's a huge problem since it allows outside attackers to lock out your users with almost no effort.

In this case you have a choice between threats. You're deciding that in exchange for protecting individual accounts, an attacker can lock out the users. It's a different attack than actually stealing/changing data, but it's still an attack.

A better solution would be to require strong passwords and Two-Factor Authentication and have no lockout.

If you do both of these, your accounts will be reasonably safe and your users won't be locked out.

The vulnerability here is reduced significantly since the attacker will need to steal and crack your password database and 2FA secrets to gain access, but by the time they're in deep enough to do that, they don't actually need the user accounts anymore.

This really all depends on what you're protecting. If it's a Wordpress blog and the users don't get to comment on your latest post, it's not a big deal. If your site contains financial or medical or security records, it's a huge deal.

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