An expression engine is the part of an application that evaluates small pieces of user-defined logic, such as conditions, field mappings, or simple calculations. Instead of running full code, it is meant to process limited expressions safely and return a result the app can use.
In cyber security, expression engines matter because they sit on a boundary between user input and privileged application behavior. If the engine is poorly sandboxed, an attacker may escape the intended limits, access internal objects, read secrets, or reach operating-system features. In workflow and automation tools, that can turn a low-privilege editor account into a path toward command execution or data exposure. Defenses include strict sandboxing, allowlists of supported functions, removing access to host objects, and testing for parser or evaluation bypasses before deployment.