message-filtering capability

A security control function, implemented in boundary protection devices such as gateways, guards, or firewalls, that regulates where information can travel within a system and between systems by inspecting and acting on message content itself — for example, through keyword searches or document characteristics — rather than on network-layer header attributes alone. Enforcement mechanisms compare security attributes associated with information (data content and data structure) and source/destination objects, and respond appropriately — blocking, quarantining, or alerting — when they encounter information flows not explicitly permitted by policy. In practice, flow control restrictions enforced this way include keeping export-controlled information from being transmitted in the clear, restricting unauthorized web requests, and limiting information transfers between organizations based on data structures and content.

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