A compatibility layer is software, middleware, or an interface that lets two different systems work together when they were not designed to do so. It can translate data formats, mimic missing APIs, or bridge old drivers and new applications. In practice, a browser-facing scanner workflow or a hardware shim may act as a compatibility layer between aging equipment and a modern operating system.
In cyber security, compatibility layers matter because they keep legacy tools usable, but they also add complexity. Every new translation step can create bugs, weaken visibility, or expand the attack surface. Defenders use compatibility layers to preserve access to old hardware, applications, or protocols while they plan a migration. Attackers may try to exploit them by hiding malicious activity inside trusted translation paths or by abusing weak legacy interfaces that the layer must still support.