How Cross Domain Solutions Support Zero Trust Architectures

Zero Trust has become the organizing principle for federal cybersecurity modernization. Executive directives, NSA guidance, and CISA’s maturity model have all pushed agencies toward a posture where no user, device, or session is trusted by default—regardless of where it originates. What gets less attention is how that posture intersects with the physical and logical boundaries that already exist across classified, controlled, and unclassified environments. That intersection is where cross domain solutions become operationally significant.

Many organizations approach Zero Trust as a network and identity problem. They invest in micro-segmentation, enforce multi-factor authentication, implement continuous session validation, and work toward least-privilege access controls. All of that matters. But in defense and intelligence environments—and increasingly in civilian agencies handling sensitive data—there are hard enforcement boundaries that exist between domains at entirely different classification levels. Zero Trust principles do not dissolve those boundaries. They reinforce why those boundaries need to be managed with precision rather than just assumed to be handled by perimeter policy.

The challenge is that legacy approaches to cross domain data transfer often involved manual processes, sneakernet-style workflows, or point-in-time approvals that created both security gaps and operational bottlenecks. Those approaches were designed before Zero Trust was a framework and before agencies were expected to move data at the speed modern missions require. Fitting those workflows into a Zero Trust architecture requires more than policy language—it requires technical enforcement at the data transfer layer that aligns with how Zero Trust actually functions.

This article examines how Cross Domain Solutions function as technical enforcement mechanisms within Zero Trust architectures—not as separate compliance tools, but as active components of a broader security posture. It covers segmentation strategy, policy enforcement, data flow validation, and the operational realities that programs encounter when trying to integrate CDS into modern Zero Trust frameworks.

Understanding this relationship is not academic. For organizations responsible for RMF packages, interconnection agreements, security architecture reviews, or mission system integration, it has direct implications for program risk, audit outcomes, and operational continuity.

Zero Trust Principles and Domain Boundaries

Zero Trust is built on the premise that access decisions should be made continuously, based on verified identity, device posture, behavioral context, and data sensitivity—not based on network location. The architectural model assumes that perimeter-based trust is no longer sufficient and that every access request should be evaluated against policy before it is permitted.

What this model does not address is the transfer of data between domains that operate under fundamentally different classification or sensitivity levels. Zero Trust frameworks are primarily concerned with controlling access within a domain—or across domains that share a common policy authority. When data needs to move between a classified network and an unclassified environment, or between two networks with different accreditation boundaries, the rules change significantly. Those transfers are not governed by identity and access management policy alone. They are governed by content-level inspection, transfer rules approved through formal security review processes, and in many cases by hardware-enforced data diodes or filter stacks that sit between networks.

This is not a gap in Zero Trust theory. It is a recognition that domain boundaries in federal environments carry legal, regulatory, and national security implications that extend beyond what any single framework is designed to handle. Zero Trust addresses how access is controlled. CDS addresses what data is permitted to cross a hard boundary, under what conditions, in what direction, and with what level of content validation before transfer is allowed. These are complementary functions, not competing ones.

The critical issue is that many organizations planning Zero Trust architectures do not map those architectures against existing or planned CDS requirements early enough. They build out identity infrastructure, deploy micro-segmentation, instrument endpoints, and then encounter the boundary problem late in the program when classified and unclassified workflows need to connect. At that point, CDS becomes a schedule problem rather than a design decision.

What Cross Domain Solutions Actually Enforce

A Cross Domain Solution is a formally evaluated and accredited system—hardware, software, or a combination—that enforces policy governing the transfer of information between security domains. The core function is content inspection and filtering: determining whether a given piece of data is permitted to cross a boundary based on predefined rules, and blocking or holding anything that does not meet those rules.

In practice, this means that a CDS operates below the application layer in many respects. It does not simply route data—it inspects it. It looks at file types, metadata, embedded content, classification markings, and rule-based attributes before making a transfer decision. Some implementations enforce directionality at the hardware level, meaning that data physically cannot flow in the prohibited direction regardless of software configuration. That kind of enforcement is important in high-assurance environments where software-only controls are considered insufficient for the threat model.

From a Zero Trust perspective, this maps directly to the principle of explicit verification. Zero Trust requires that every access request be evaluated against policy. CDS requires that every data transfer request be evaluated against filter rules before it is permitted. The underlying philosophy is the same—deny by default, permit only what is explicitly authorized. The difference is the enforcement layer and the authority structure behind the policy.

CDS filter policies are not configured at the discretion of the system administrator. They are developed, reviewed, and approved through a formal governance process—typically through the program’s Authorizing Official and coordinating security officials—and changes to those policies go through configuration control. This is a higher level of rigor than most Zero Trust policy enforcement mechanisms currently require, and it reflects the sensitivity of the environments where CDS is deployed. For programs working through the RMF process, those approved filter policies become part of the security authorization package and are subject to ongoing audit.

Directionality and Hardware Enforcement

One of the distinguishing features of certain CDS implementations is hardware-enforced unidirectionality—commonly implemented as data diodes. These devices ensure that data can only flow in one direction at the physical layer. No software configuration can override that constraint. This is significant in environments where a high-side network must push data to a lower classification level but must never receive data from that lower level. The threat model in those cases includes sophisticated adversaries who might attempt to exploit software vulnerabilities in a bidirectional interface to exfiltrate data upward. A hardware-enforced boundary eliminates that attack surface entirely.

Zero Trust architectures do not have an equivalent control. They can enforce strict access policy and session-level validation, but they cannot physically prevent bidirectional communication at the hardware layer. This is one area where CDS provides assurance that Zero Trust frameworks do not replicate, and it is a key reason why the two are treated as complementary rather than substitutable in high-assurance environments.

Segmentation Strategy and the CDS Layer

Segmentation is one of the foundational technical mechanisms of a Zero Trust architecture. The goal is to limit lateral movement by dividing the network into smaller, policy-enforced zones where access between zones requires explicit authorization. In a mature implementation, this extends to workload-level segmentation, where individual applications or services are isolated and can only communicate with explicitly permitted peers.

Segmentation within a single accreditation boundary—even a complex one—is a network and policy engineering problem. Teams use software-defined networking, microsegmentation platforms, and access control policies to enforce those zone boundaries. But when the boundary in question separates two networks with different classification levels, different accreditation authorities, or different legal handling requirements, network segmentation alone is insufficient. The enforcement gap is not at the routing or switching layer. It is at the data content layer, which is exactly where CDS operates.

Consider an operational scenario: an intelligence fusion environment where analysts need to share sanitized reporting from a classified network to an unclassified collaboration platform. Network segmentation can isolate the two networks from each other. Identity and access management can ensure only authorized users initiate transfer requests. But neither control can inspect the content of the data being moved and validate that it does not contain embedded classified material or metadata that should not cross that boundary. That inspection function belongs to the CDS.

In well-designed architectures, CDS sits at the intersection of two segmented domains and acts as the policy enforcement point for cross-boundary data movement. The segmentation controls ensure that no traffic reaches the CDS except through authorized paths. The CDS then applies content-level rules before data is permitted to transit. Together, they create a layered enforcement model: network segmentation limits where data can travel, and CDS determines whether it is permitted to cross the domain boundary at all.

This layered model is one of the practical reasons why organizations pursuing robust cross domain solutions as part of their security architecture find that their Zero Trust investments compound in value rather than conflict. The two frameworks reinforce each other when they are designed together.

Integrating CDS Into a Zero Trust Architecture

Practical integration of CDS into a Zero Trust architecture is an exercise in early coordination and sustained governance. The programs that do it well share a common characteristic: they identified the domain boundary requirements during the architecture phase, not during implementation or accreditation.

The acquisition pathway for CDS is long. Approved solutions require formal evaluation through NSA and the National Information Assurance Partnership process. Depending on the transfer use case, filter development and approval can take months. Installation requires coordination across network operations, security operations, and the program office. None of that timeline compresses well under schedule pressure, and programs that discover the CDS requirement late routinely face delays that affect mission delivery dates.

Policy Alignment Across Authorities

One of the underappreciated integration challenges is policy alignment across multiple authorizing authorities. In a Zero Trust architecture, the policy authority for access decisions typically sits with the organization’s identity provider and policy engine—often a single team managing a common framework. CDS filter policies, by contrast, may require approval from multiple AOs depending on which networks the CDS connects. A solution connecting a DoD classified network to a partner unclassified environment may have two separate authorizing chains, each with its own review cadence and documentation requirements.

Zero Trust policy engines typically do not have visibility into CDS filter policy decisions. The two policy frameworks are maintained separately, by different teams, under different governance structures. That separation is intentional—CDS accreditation cannot be delegated to the same policy authority managing identity-based access decisions. But it creates a coordination burden that programs need to anticipate and staff for. The ISSO supporting the CDS boundary needs to maintain fluency in both the CDS accreditation requirements and the broader Zero Trust policy framework in order to ensure they remain aligned.

Audit Logging and Event Correlation

Zero Trust architectures depend heavily on logging and continuous monitoring. Every access request, session event, and policy decision should be captured and correlated to support detection, investigation, and compliance reporting. CDS also generates significant audit log volume—every transfer attempt, every filter decision, every blocked transfer is a log event. The question for architects is whether CDS log data flows into the organization’s SIEM or remains siloed within the CDS management infrastructure.

Integrating CDS audit logs into enterprise security monitoring adds value in both directions. Security operations teams gain visibility into cross-domain transfer activity that would otherwise be opaque. CDS administrators gain the benefit of correlated context—seeing, for example, that a high volume of blocked transfers correlates with a specific user session that is also exhibiting anomalous behavior on the identity side. That correlation capability is only possible if both log streams reach a common analytics platform, and it requires early planning around log format compatibility, classification handling of log data, and SIEM ingestion pipelines.

Zero Trust Controls Compared to CDS Enforcement

Understanding where Zero Trust controls end and CDS enforcement begins is essential for building architectures that do not have unexamined gaps. The following comparison reflects operational deployment realities across federal environments.

Common Mistakes

Programs consistently make the same planning and execution errors when attempting to integrate CDS into a Zero Trust architecture. These are not theoretical failure modes—they show up in program reviews, accreditation delays, and post-deployment operational incidents with enough regularity that they warrant direct attention.

Treating CDS as a late-stage procurement action. CDS accreditation timelines are measured in months, not weeks. Programs that defer the CDS decision until the system is otherwise ready to deploy routinely push their operational delivery dates by six months or more. The filter policy development process, the AO review cycle, the physical installation, and the interconnection agreement negotiation all run in parallel and all take time. Those activities need to begin in the architecture phase, not the fielding phase.

Assuming Zero Trust controls cover the boundary. Some architects treat Zero Trust as a comprehensive security framework that handles all access and data transfer scenarios. In environments with classified domains, this assumption is incorrect and creates real gaps. Zero Trust policy engines do not inspect data content for classification markings. They do not provide hardware-enforced directionality. They do not carry the accreditation weight required for classified-to-unclassified transfers under applicable security policies. CDS is not optional in those environments.

Under-resourcing policy management. CDS filter policies are living documents. Mission requirements change, data types evolve, and new use cases emerge after fielding. Every change to filter policy requires configuration control, AO coordination, and often regression testing. Programs that staff for installation and then assume the CDS runs itself will find that policy drift creates both security and operational problems. The transfer queue fills with blocked exceptions. Workarounds emerge. The disciplined boundary the CDS was meant to enforce becomes inconsistent in practice.

Siloing CDS logs from enterprise monitoring. A CDS that logs transfer decisions but does not feed those logs into the organization’s security monitoring infrastructure provides incomplete situational awareness. Security operations teams cannot correlate CDS events with identity anomalies, endpoint behavior, or network indicators. That correlation gap matters during incident response, when understanding the full sequence of events across the boundary is essential to determining scope and impact.

Failing to document ownership boundaries. In multi-stakeholder environments, the question of who owns the CDS—the network operations team, the program office, or the security team—is often left unresolved until something breaks. Ownership boundaries for the physical hardware, the filter policy, the accreditation documentation, the log management, and the change control process all need to be defined explicitly before the system is fielded. Ambiguity in these boundaries creates sustainment risk and tends to surface at the worst possible moment.

Recommended Federal Approach

Organizations that successfully integrate CDS into Zero Trust architectures approach both frameworks as part of a unified security design rather than as separate engineering tracks. The practical steps that distinguish those programs from ones that struggle are consistent across environments.

Start with a domain boundary map. Before any architecture work begins, document every domain boundary in the environment—what classification levels are involved, what data needs to flow across each boundary, in what direction, and how frequently. This map becomes the foundation for both the CDS use case definition and the segmentation design. It also surfaces boundary complexity that might not be visible until well into implementation if this step is skipped.

Align the CDS acquisition timeline with the program schedule. Once use cases are defined, initiate the CDS acquisition and filter policy development process in parallel with Zero Trust infrastructure deployment. Do not sequence them. The Zero Trust components—identity, segmentation, endpoint controls—can deploy on a faster timeline. CDS accreditation should be running concurrently so it is ready when the broader architecture is ready for integration testing.

Integrate CDS into the segmentation design explicitly. Define the network zones such that no path to the CDS exists except through the authorized ingress points. The segmentation controls should funnel cross-domain transfer traffic to the CDS deliberately, with no alternate routes. This removes ambiguity about where the enforcement boundary sits and ensures that the CDS inspection function cannot be bypassed through network misconfiguration.

Build CDS log integration into the SIEM architecture. Plan for CDS audit log ingestion as a first-class monitoring requirement, not an afterthought. Define the log format requirements, the classification handling approach for log data, and the correlation use cases that the security operations team needs to support. This planning is significantly easier to do before the SIEM is deployed than after. Visit Anchor Defense resources for additional guidance on security architecture planning considerations.

Staff for sustainment, not just fielding. Budget and workforce planning should account for the ongoing policy management function from day one. Identify who owns filter policy updates, what the change control process is, how often periodic reviews will occur, and what the escalation path is when a blocked transfer needs rapid resolution. These are operational decisions that have direct mission impact if they are not made in advance.

Treat the CDS as part of the Zero Trust policy framework, not separate from it. The goal is not to manage two independent security programs—one for Zero Trust and one for CDS. It is to maintain a coherent security posture that spans both the access control layer and the data transfer enforcement layer. That requires regular coordination between the teams responsible for each, shared situational awareness through integrated logging, and consistent policy governance that recognizes both frameworks as components of a single architecture. Organizations looking to structure that coordination can engage directly through Anchor Defense to work through environment-specific requirements.

Zero Trust and CDS are not in tension. They address different problems at different enforcement layers, and both are necessary in environments where domain boundaries carry classification or sensitivity implications. Programs that understand this relationship early and design for it deliberately will have fewer surprises at accreditation, fewer operational bottlenecks after fielding, and a more defensible security posture over the full lifecycle of the system.