Custom Windows ISOs are a primary vector for malware distribution. Since the modifier has root-level access to the OS image during creation, they can embed:
This article dives deep into the history, features, risks, and legacy of this underground operating system. windows xp dark edition v7 iso
Since this is a feature for an ISO build, the integration process would be handled via winnt.sif or a specialized $OEM$ script: Custom Windows ISOs are a primary vector for