Should I impose a maximum length on passwords?

Question

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:

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.]

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.

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.

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.

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.