The system must require at least eight characters be changed between the old and new passwords during a password change.
Requiring a minimum number of different characters during password changes ensures that newly changed passwords should not resemble previously compromised ones. Note that passwords which are changed on compromised systems will still be compromised, however.
Note: The "difok" parameter will indicate how many characters must differ. The DoD requires eight characters differ during a password change. This would appear as "difok=8".
If “difok” is not found or is set to a value less than “8”, this is a finding.
Fix Text (F-19375r462386_fix)
The pam_cracklib module's "difok" parameter controls requirements for usage of different characters during a password change.
Edit /etc/pam.d/system-auth and /etc/pam.d/password-auth adding "difok=[NUM]" after pam_cracklib.so to require differing characters when changing passwords, substituting [NUM] appropriately. The DoD requirement is 8.