Cross-Domain Solution (CDS)

A CDS enforces Mandatory Access Control (MAC) policies, not discretionary ones. It makes an authoritative decision based on security labels (e.g., Top Secret, Secret, Unclassified) attached to both subjects (users) and objects (data). This ensures data transfers comply with a formal security model, typically Bell-LaPadula (no read up, no write down) or Biba (integrity-focused), preventing unauthorized downgrades or leaks.

Unlike a network firewall that inspects packet headers, a CDS performs full-stack, deep content inspection. It deconstructs files and message formats to validate data integrity and eliminate threats:

• Format Validation: Verifies a file's magic bytes match its extension to prevent masquerading attacks.
• Active Content Stripping: Removes embedded macros, JavaScript, and zero-width characters that could harbor steganographic data.
• Schema Validation: Checks structured data against a strict, pre-approved schema, discarding any malformed or extraneous fields.

CDS architectures are defined by their data flow topology. A Unidirectional Gateway (often hardware-enforced via a data diode) physically guarantees data can only travel from a low to a high domain, making exfiltration impossible. A Bidirectional CDS allows controlled two-way communication but requires two independent, logically separated inspection paths—one for each direction—each enforcing its own distinct security policy to prevent a bypass.

A fundamental principle is the complete break of the OSI protocol stack. The CDS terminates the TCP session from the source network at the application layer, extracts the payload, and initiates a brand-new session to the destination. This prevents blind tunneling attacks where malicious Layer 2/3 traffic is encapsulated within an allowed protocol. The source and destination networks never share a direct link-layer connection.

Every byte crossing the boundary generates a cryptographically signed audit record. The CDS logs the source IP, destination IP, user identity, file hash, security labels, and the full sanitized payload. These logs are typically streamed to a dedicated, immutable audit server in the high-security domain to provide non-repudiation and support forensic analysis in the event of a spillage investigation.

CDS deployments support two operational modes. Automated Release allows data that passes all automated checks to cross instantly, essential for real-time sensor data. Manual Review (or 'High Assurance Guard' mode) places flagged or high-risk transfers into a quarantine queue. A trusted human analyst must visually inspect the sanitized content and explicitly approve the release, providing a final cognitive check against complex data correlation attacks.