Every business is different. Some older and more established organizations have networks and infrastructure that have evolved through the years without security being a priority, and IT shops have traditionally just bolted on new technology without properly configuring it and/or decommissioning the old tech.
Even startups who begin their lives in the cloud still have some local technology servers or infrastructure that need constant care and feeding.
Some of the themes I see, and the most common mistakes made by companies, are:
1. No patch strategy or a strategy that is driven more by concerns over network unavailability and less on actual information assurance and security posture.
2. Not understanding [of] what normal traffic looks like on their networks and/or relying on software tools. Usually too many of them overlap and are misconfigured. The network architecture is the company’s pathway to security or vulnerability with misconfigured tools.
3. Relying too much on backups, and believing that a backup is enough to protect you. Backups that were not segmented from the network, were only designed to provide a method of restoring a point in time, and were never designed to be protected from an attacker. Backups need to be tested regularly to ensure the data is complete and not corrupted.