A trusted boundary is the point in a system where data stops being assumed safe and must be validated, authenticated, or otherwise checked before use. It separates internal inputs, such as data produced by your own code, from external or attacker-controlled inputs, such as network traffic, files, queues, or session blobs.
This matters because many exploits begin when software crosses that boundary without verification. In Ruby deserialization, for example, calling Marshal.load on untrusted data breaks the boundary: the program may rebuild objects and trigger methods the attacker did not create. Defenders use trusted boundaries to decide where parsing, schema checks, allowlists, MACs, and type restrictions must happen. Clear boundaries reduce gadget-chain risk, prevent privilege crossing, and make it easier to spot dangerous sinks that should never receive raw user input.