As part of our audit the users have to set passwords with certain length.
Under Jenkins --> Configure Global Security --> I can enable security and select Jenkins own user database. But I haven't found a way to set the rules for password.
Is there any plugin that could help?
The obvious answer is not to do this directly in Jenkins.
Use an external source for authentication (and authorisation) such as an LDAP, where you can properly enforce password quality policies in a centralised manner, avoiding the operational overhead of maintaining different databases of users and passwords.
