Common passwords blacklist

Any system that implements password authentication must check whether the passwords are not too common. Every system faces the brute-force attacks that try one or another list of most common password (and usually succeed, by the way). The system must have a capability to slow down an attacker by any means available: slowing down system response every time an unsuccessful authentication is detected, blocking an account for a short time after a number of unsuccessful authentication attempts or throwing up captchas.

However, even the most sophisticated system fails if the user’s password is the most common word: “password”. The attacker simply succeeds then at once because that is likely to be the first word tried. So we need a system for blacklisting passwords that are thought of as most likely to be tried in a dictionary brute-force attack. This may be annoying for users of the system who may prefer to use a simple word as a password but this is the reality – any simple word used as a password is likely to be a security hole and must be banned.

While implementing the user login plugin for CakePHP I came across this simple question. Where do we get the password lists to check the newly entered passwords against? And here is a resource I can recommend: 62K Common Passwords by InfoSec Daily. Depending on your system’s speed you could use a smaller file of 6 MB, a 1.5 GB file that should take care of most common passwords or fuse the files into your own list.