What is the trust boundary?

When data crosses from one "system" to another, draw a trust boundary line. This would certainly be anyplace where data traverses a network interface, or passes between two processes. Data entering from across a trust boundary indicates every place where the receiving systems need to validate the inbound data. Data crossing a trust boundary also means that you have identified a place where you should examine the security of the data. It doesn't mean you must encrypt or that you must authenticate, but as you analyse the connection for vulnerabilities, you may discover that encryption and/or authentication remediates the issues.