This is an issue that have been out for quite a while, but I see it a lot when checking customers environments, so it’s always good to remind yourself about this kind of important things.
Through GPP, you have the ability to do lots of stuff, one of them is fiddling with builtin local accounts on computers and even change password on those.
It actually gets worse from there… with the script which can be downloaded here, you can extract ALL passwords from ALL GPP’s you have since they are stored in XML files located on the sysvol in your domain.
OK, who have access to read those files then?
Authenticated Users, which means whoever that has an account in your domain can do this.
Conclusion: do not change password with GPP unless you do it and remove the policy directly after the policy has successfully been applied.
In this blog post series from Microsoft PFE’s, you find a great solution to handle and store local admin passwords in a secure, yet manageble way.