Should I impose a maximum length on passwords?

Question

I can understand that imposing a minimum length on passwords makes a lot of sense (to save users from themselves), but my bank has a requirement that passwords are between 6 and 8 characters long, and I started wondering...

• Wouldn't this just make it easier for brute force attacks? (Bad)
• Does this imply that my password is being stored unencrypted? (Bad)

If someone with (hopefully) some good IT security professionals working for them are imposing a max password length, should I think about doing similar? What are the pros/cons of this?

Answer 1

Passwords are hashed to 32, 40, 128, whatever length. The only reason for a minimum length is to prevent easy to guess passwords. There is no purpose for a maximum length.

The obligatory XKCD explaining why you're doing your user a disservice if you impose a max length:

Answer 2

A maximum length specified on a password field should be read as a SECURITY WARNING. Any sensible, security conscious user must assume the worst and expect that this site is storing your password literally (i.e. not hashed, as explained by epochwolf).

In that that is the case:

1. Avoid using this site like the plague if possible. They obviously know nothing about security.
2. If you truly must use the site, make sure your password is unique - unlike any password you use elsewhere.

If you are developing a site that accepts passwords, do not put a silly password limit, unless you want to get tarred with the same brush.

[Internally, of course your code may treat only the first 256/1024/2k/4k/(whatever) bytes as "significant", in order to avoid crunching on mammoth passwords.]

Answer 3

Allowing for completely unbounded password length has one major drawback if you accept the password from untrusted sources.

The sender could try to give you such a long password that it results in a denial of service for other people. For example, if the password is 1GB of data and you spend all your time accept it until you run out of memory. Now suppose this person sends you this password as many times as you are willing to accept. If you're not careful about the other parameters involved this could lead to a DoS attack.

Setting the upper bound to something like 256 chars seems overly generous by today's standards.

Answer 4

First, do not assume that banks have good IT security professionals working for them. Plenty don't.

That said, maximum password length is worthless. It often requires users to create a new password (arguments about the value of using different passwords on every site aside for the moment), which increases the likelihood they will just write them down. It also greatly increases the susceptibility to attack, by any vector from brute force to social engineering.

Answer 5

Setting maximum password length less than 128 characters is now discouraged by OWASP Authentication Cheat Sheet

https://www.owasp.org/index.php/Authentication_Cheat_Sheet

Citing the whole paragraph:

Longer passwords provide a greater combination of characters and consequently make it more difficult for an attacker to guess.

Minimum length of the passwords should be enforced by the application. Passwords shorter than 10 characters are considered to be weak ([1]). While minimum length enforcement may cause problems with memorizing passwords among some users, applications should encourage them to set passphrases (sentences or combination of words) that can be much longer than typical passwords and yet much easier to remember.

Maximum password length should not be set too low, as it will prevent users from creating passphrases. Typical maximum length is 128 characters. Passphrases shorter than 20 characters are usually considered weak if they only consist of lower case Latin characters. Every character counts!!

Make sure that every character the user types in is actually included in the password. We've seen systems that truncate the password at a length shorter than what the user provided (e.g., truncated at 15 characters when they entered 20). This is usually handled by setting the length of ALL password input fields to be exactly the same length as the maximum length password. This is particularly important if your max password length is short, like 20-30 characters.

Answer 6

One reason I can imagine for enforcing a maximum password length is if the frontend must interface with many legacy system backends, one of which itself enforces a maximum password length.

Another thinking process might be that if a user is forced to go with a short password they're more likely to invent random gibberish than an easily guessed (by their friends/family) catch-phrase or nickname. This approach is of course only effective if the frontend enforces mixing numbers/letters and rejects passwords which have any dictionary words, including words written in l33t-speak.

Answer 7

One potentially valid reason to impose some maximum password length is that the process of hashing it (due to the use of a slow hashing function such as bcrypt) takes up too much time; something that could be abused in order to execute a DOS attack against the server.

Then again, servers should be configured to automatically drop request handlers that take too long. So I doubt this would be much of a problem.

Answer 8

I think you're very right on both bullet points. If they're storing the passwords hashed, as they should, then password length doesn't affect their DB schema whatsoever. Having an open-ended password length throws in one more variable that a brute-force attacker has to account for.

It's hard to see any excuse for limiting password length, besides bad design.

Answer 9

The only benefit I can see to a maximum password length would be to eliminate the risk of a buffer overflow attack caused by an overly long password, but there are much better ways to handle that situation.

Answer 10

Ignore the people saying not to validate long passwords. Owasp literally says that 128 chars should be enough. Just to give enough breath space you can give a bit more say 300, 250, 500 if you feel like it.

https://www.owasp.org/index.php/Authentication_Cheat_Sheet#Password_Length

Password Length Longer passwords provide a greater combination of characters and consequently make it more difficult for an attacker to guess.

...

Maximum password length should not be set too low, as it will prevent users from creating passphrases. Typical maximum length is 128 characters. Passphrases shorter than 20 characters are usually considered weak if they only consist of lower case Latin characters.

Answer 11

Storage is cheap, why limit the password length. Even if you're encrypting the password as opposed to just hashing it a 64 character string isn't going to take much more than a 6 character string to encrypt.

Chances are the bank system is overlaying an older system so they were only able to allow a certain amount of space for the password.

Answer 12

My bank does this too. It used to allow any password, and I had a 20 character one. One day I changed it, and lo and behold it gave me a maximum of 8, and had cut out non-alphanumeric characters which were in my old password. Didn't make any sense to me.

All the back-end systems at the bank worked before when I was using my 20 char password with non alpha-numerics, so legacy support can't have been the reason. And even if it was, they should still allow you to have arbitrary passwords, and then make a hash that fits the requirements of the legacy systems. Better still, they should fix the legacy systems.

A smart card solution would not go well with me. I already have too many cards as it is... I don't need another gimmick.

Answer 13

If you accept an arbitrary sized password then one assumes that it is getting truncated to a curtain length for performance reasons before it is hashed. The issue with truncation is that as your server performance increases over time you can't easily increase the length before truncation as its hash would clearly be different. Of course you could have a transition period where both lengths are hashed and checked but this uses more resources.

Answer 14

Try not to impose any limitation unless necessary. Be warned: it might and will be necessary in a lot of different cases. Dealing with legacy systems is one of these reasons. Make sure you test the case of very long passwords well (can your system deal with 10MB long passwords?). You can run into Denial of Service (DoS) problems because the Key Defivation Functions (KDF) you will be using (usually PBKDF2, bcrypt, scrypt) will take to much time and resources. Real life example: http://arstechnica.com/security/2013/09/long-passwords-are-good-but-too-much-length-can-be-bad-for-security/

Answer 15

Should there be a maximum length? This is a curious topic in IT in that, longer passwords are typically harder to remember, and therefore more likely to get written down (a BIG no-no for obvious reasons). Longer passwords also tend to get forgotten more, which while not necessarily a security risk, can lead to administrative hassles, lost productivity, etc. Admins who believe that these issues are pressing are likely to impose maximum lengths on passwords.

I personally believe on this specific issue, to each user their own. If you think you can remember a 40 character password, then all the more power to you!

Having said that though, passwords are fast becoming an outdated mode of security, Smart Cards and certificate authentication prove very difficult to impossible to brute force as you stated is an issue, and only a public key need be stored on the server end with the private key on your card/computer at all times.

Answer 16

Longer passwords, or pass-phrases, are harder to crack simply based on length, and easier to remember than requiring a complex password.

Probably best to go for a fairly long (10+) minimum length, restricting the length useless.

Answer 17

Legacy systems (mentioned already) or interfacing outside vendor's systems might necessitate the 8 character cap. It could also be a misguided attempt to save the users from themselves. Limiting it in that fashion will result in too many pssw0rd1, pssw0rd2, etc. passwords in the system.

Answer 18

One reason passwords may not be hashed is the authentication algorithm used. For example, some digest algorithms require a plaintext version of the password at the server as the authentication mechanism involves both the client and the server performing the same maths on the entered password (which generally won't produce the same output each time as the password is combined with a randomly generated 'nonce', which is shared between the two machines).

Often this can be strengthened as the digest can be part computed in some cases, but not always. A better route is for the password to be stored with reversible encryption - this then means the application sources need to be protected as they'll contain the encryption key.

Digst auth is there to allow authentication over otherwise non-encrypted channels. If using SSL or some other full-channel encryption, then there's no need to use digest auth mechanisms, meaning passwords can be stored hashed instead (as passwords could be sent plaintext over the wire safely (for a given value of safe).

Answer 19

Just 8 char long passwords sound simply wrong. If there ought to be a limit, then atleast 20 char is better idea.

Answer 20

I think the only limit that should be applied is like a 2000 letter limit, or something else insainly high, but only to limit the database size if that is an issue