A service boundary is the interface where one software service or component interacts with another. It may be defined by an API, a message queue, a database connection, or another agreed way to exchange data and requests. The boundary also marks which component is responsible for particular behavior, such as validating input or handling failures.
In cyber security, service boundaries matter because they are points where trust, data, and control pass between systems. Attackers may target weak authentication, overly broad permissions, unsafe input handling, or unprotected data at these interfaces. Defenders can reduce risk by enforcing access controls, validating requests, encrypting sensitive traffic, and monitoring interactions. Clearly documenting boundaries also helps teams understand dependencies and investigate how a failure or compromise could spread.