In order to improve Plesk security, you can set the minimum password strength (in Tools & Settings > Security Policy > Password strength) for all passwords in the system. Password strength is a measure of how resistant a password is to various types of attacks. Generally, the strength of a password depends on its complexity and overall length.
When any Plesk user (the administrator, a customer, reseller, or additional user) sets a new password in Plesk (for example, creates a new mail account or changes the password of an FTP user), they are required to adjust the password until it meets the minimum password strength. Note that if you increase the minimum strength, already existing passwords will not be affected by the new requirements.
We recommend that you force your users to employ stronger passwords. However, note that such passwords are harder to remember.
Plesk supports five levels of password strength that allow you to get the right balance between required protection and password complexity: Very weak, Weak, Medium, Strong, and Very strong.