Validate Untrusted Input at Trust Boundaries
Applications constantly receive data they did not create: HTTP parameters, uploaded metadata, webhook payloads, queue messages, imported files, configuration from external systems, and values read from shared storage. The security problem is not that every external value is malicious. The problem is that application code can make unsafe assumptions about values whose shape, size, meaning, or origin has not been established. Boundary validation reduces that risk by checking untrusted data before the rest of the application relies on it. The goal is simple: turn vague external input into explicit internal invariants.