Ubuntu OS must require the change of at least eight characters when passwords are changed.

If the operating system allows the user to consecutively reuse extensive portions of passwords, this increases the chances of password compromise by increasing the window of opportunity for attempts at guessing and brute-force attacks.

The number of changed characters refers to the number of changes required with respect to the total number of positions in the current password. In other words, characters may be the same within the two passwords; however, the positions of the like characters must be different.

If the password length is an odd number, the number of changed characters must be rounded up. For example, a password length of 15 characters must require the change of at least eight characters.

Verify Ubuntu OS requires the change of at least eight characters when passwords are changed by using the following command:

     $ grep -i difok /etc/security/pwquality.conf 
     difok = 8  

If "difok" is less than "8", is commented out, or is missing, this is a finding.

Configure Ubuntu OS to require the change of at least eight characters when passwords are changed.

Add or modify the following line in the "/etc/security/pwquality.conf" file:

difok = 8