Understanding Password History
The following figure shows the password policy available on a Windows system. You can see that it’s possible to set up a password policy to ensure that users create passwords with a minimum number of characters (Minimum Password Length), and to ensure that they change their password periodically (Maximum Password Age). However, two of the settings sometimes confuse people:
- Enforce Password History
- Minimum Password Age
These two settings work together to prevent users from reusing passwords.
Many users would rather use a single password forever because it’s easier to remember. A basic password policy with a Maximum Password Age setting forces users to change their password, but many users have tried to adapt by switching between two passwords. They’ll use one password until they’re forced to change, and then use the other password until they’re forced to change again, and continuously switch back and forth between these two passwords.
The Enforce Password History setting will remember previous passwords used and prevent users from reusing any passwords in the history. In the figure, this setting is configured to remember the last 24 passwords so users cannot reuse the same password until they’ve used 24 other passwords.
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A determined user may decide he simply wants to change his password repeatedly until he’s used up 24 passwords and can get back to his original password. If the Minimum Password Age is not configured, the user can do this in one sitting. However, if the Minimum Password Age is set to something like 1 day, then the user will not be able to change the password until a day has passed. Combined with a history of 24 passwords, it would take a user 25 work days to get back to their original password. This is enough to discourage most users from reusing the same policy
Password policies help ensure users create strong password policies, and these two settings help ensure they don’t circumvent the security.