message-filtering capability

A control mechanism implemented in boundary protection devices — such as firewalls, guards, or cross-domain solutions — that inspects the *payload content* of communications (e.g., keyword matching, structural analysis, or document characteristics) to decide whether to allow, block, or transform an information flow, as distinguished from shallower approaches that act only on envelope or header metadata. In NIST's information-flow-enforcement framework, it sits alongside packet-filtering as one of the two primary enforcement modes, where packet-filtering acts on header information while message-filtering acts on message content; together they implement the policy that "regulates where information can travel within a system and between systems." Practical uses include blocking export-controlled information from leaving in the clear, restricting data transfers between organizations based on data structures and content, and enforcing boundary rules between security or privacy domains. The trustworthiness of the hardware, firmware, and software components performing that filtering is itself a security concern, and the control family extends to advanced cross-domain filtering techniques