Vulnerability Discussion

If the Ubuntu 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 then number of changed characters must be rounded up. For example, a password length of 15 characters must require the change of at least 8 characters.

Check

Verify the Ubuntu operating system requires the change of at least 8 characters when passwords are changed.

Determine if the field "difok" is set in the "/etc/security/pwquality.conf" file with the following command:

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

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

Fix

Configure the Ubuntu operating system to require the change of at least 8 characters when passwords are changed.

Add or update the "/etc/security/pwquality.conf" file to include the "difok=8" parameter:

difok=8