Now surely there are ways to prevent this sort of security breach. Applying proper identity management and access controls could prevent a major breach. Fist, old accounts that still have access to the network can be cleaned up and properly disabled immediately upon employee discharge. Additionally, using access control with credential delegation can prevent users who only need access to critical systems for a short period of time from gaining unnecessary access outside that time frame.
Another issue is that sometimes users with highly privileged access can create their own access by making extra Domain Administrator accounts in Active Directory. This can be monitored with identity management software that scans Active Directory and compiles lists of newly created user accounts with escalated privileges. Another way to prevent the creation of extra administrator accounts by a potential inside attacker would be by forcing them to go through an approval process using workflow management to create new administrator accounts.
In conclusion there are plenty of ways that identity and access management tools can prevent insiders from getting privileges they were not granted. Whether it be through using access control with credential delegation, identity management with automated notifications and account cleanup or even via workflow by preventing one person from delegating their own administrator accounts. One or multiple of these options can prevent devastating insider attacks from afflicting your organization.