What Are Cross Domain Solutions (CDS) for Security?

Everything is connected these days, from cloud platforms to enterprise networks and critical infrastructure.

But it’s not all roses. This interconnectedness can threaten security, as it expands your attack surface and creates more pathways for breaches to spread, particularly in relation to national security. Traditional resolution approaches often fall short. Air gapping is a common response in defence, critical infrastructure, and industrial control systems. This isolates critical systems to reduce cyber risk, but it can also limit the efficiency and agility that connectivity provides in terms of secure data transfer.

What we need is a middleman; something that allows data to move where it’s needed without exposing sensitive environments to unnecessary risk. 

This is where cross-domain security solutions come in, bridging the gap between security and usability by enabling safe, controlled data transfer across environments with different security levels without exposing critical systems to unnecessary risk.

Importantly, the term “cross-domain solution” does not necessarily refer to a single product or appliance. In line with guidance from the National Cyber Security Centre (NCSC), CDS is increasingly understood as a security architecture made up of multiple controls, technologies, and processes that work together to govern and protect information exchange across trust boundaries.

But what exactly are cross domain solutions (CDS)? Let’s discuss their importance for security, their key types, and things to consider when choosing a cross-domain solution for your organisation.

Cross-domain solutions are security architectures designed to enable safe, policy-enforced data exchange between networks operating at different security classification or trust levels.

Rather than relying on a single control, a CDS typically combines technologies such as content inspection, access controls, data sanitisation, encryption, monitoring, and sometimes specialised hardware to securely govern information flows between domains.

Their purpose is to prevent unauthorised data transfer, reduce the risk of cross-domain compromise, and stop malware or sensitive information from moving between environments unchecked. This enables organisations to safely exchange information between classified and unclassified systems while maintaining strict security controls.

Importantly, cross-domain access solutions can be bidirectional or unidirectional; information can move both ways between systems. Some environments require tightly controlled two-way communication, while others enforce one-way transfer only for stronger isolation. It’s all about controlled and risk-managed connectivity.

This is greatly significant in modern business environments, where we increasingly need to share data across security boundaries while maintaining strict access controls and preventing data leaks.

For any organisation needing to balance critical information sharing with stringent security requirements (that’s most of us, let’s be honest), a cross-domain security solution is essential.

But certain scenarios particularly suggest this solution may be of benefit to you. 

Firstly, they can help in multi-level security environments where users with different clearance levels need controlled access to shared information.

They’re also advantageous for any business trying to bridge IT and OT networks for monitoring and analytics without exposing operational systems to cyber risk.

Lastly, if you’re in a high-security industry such as finance or healthcare, where sensitive data must remain isolated but still accessible in a controlled way for business needs, a CDS may provide the best security for you.

Think of CDSs as a checkpoint between separate networks operating at different security levels networks with different security levels. Rather than allowing unrestricted direct communication between the two, a cross-domain solution sits in between, inspecting and governing every piece of data that crosses the boundary. 

Data is checked against strict security policies, which can include:

• File type validation
• Content scanning (to remove malicious code or sensitive leaks)
• Data format checking

Some CDSs will even modify the data so it can safely move between environments, such as by stripping metadata or converting formats. 

Only once approved can data be released into the destination network.

It’s basically like airport customs for your business information.

Cross-domain security is so effective because it adopts a defence-in-depth approach, which employs layered defences to protect data rather than relying on a single security control. 

This multi-layered approach provides assurance that if one security mechanism fails, others remain active to prevent unauthorised data transfers. But what are the protective measures CDS solutions use?

• Physical isolation: Often, strict separation is enforced between networks of different classification levels, using dedicated hardware to ensure no direct connection exists
• Content inspection: All data is scanned deeply for malware, policy violations and unauthorised content before transfer is permitted
• Data sanitisation: Files are sanitised during transfer, stripping out metadata, active content, and potential threats, then rebuilt into safe, trusted formats
• Encryption: Encrypting data protects it as it moves between domains, while digital signatures ensure its integrity by confirming it hasn’t been altered in transit
• Policy controls: Detailed security policies determine what data can be transferred based on classification, user permissions, and content analysis results

Cross-domain solutions are often discussed in terms of their operational role, such as transfer solutions, access solutions, and multi-level solutions.

• Transfer solutions focus on securely moving data between environments with different trust levels.
• Access solutions enable users to securely access systems or information across security domains.
• Multi-level solutions support environments where users and systems with different classification levels operate within a shared architecture under strict policy enforcement.

In practice, these CDS models are rarely delivered through a single technology alone. Instead, organisations typically implement them using a combination of software controls, hardware appliances, guards, data diodes, and hybrid architectures, depending on their security, assurance, and operational requirements.

Below are some of the most common CDS deployment approaches.

1. Software CDS

As the name suggests, a software CDS only uses software to transfer data between security domains, using security protocols and algorithms to ensure that data is transferred safely and to enforce security policies.

This type is the more flexible and cost-effective solution. They can be deployed on your existing infrastructure and don’t require specialised devices to function (simple, right?) 

The only downside of this is that, because this type of CDS relies on software controls, it can be more exposed to vulnerabilities such as misconfiguration, software exploits, or system compromise if not properly maintained and governed. As a result, they may offer lower assurance than hardware-enforced or hybrid architectures in high-security environments.

2. Hardware CDS

As you may have guessed, a hardware CDS uses physical hardware devices to transfer data between different security domains (e.g., a data diode).

They act as a guard, standing as intermediaries between networks with different security classifications and allowing information to be transferred between them without compromising the security of either network. These devices enforce security policies to ensure that data is only transferred in a secure and controlled manner.

Compared to software CDS solutions, hardware types are more secure and are often used in high-security environments where the protection of sensitive data is critical (think government or military organisations). They are purpose-built to be tamper-proof and resistant to attacks.

So, what’s the drawback? They can be expensive to implement and maintain, often requiring specialised expertise to operate, and they lack the flexibility of software-based solutions. 

3. Hybrid CDS

If you want a healthy mix of the benefits of software and hardware CDS, why not combine the two? This is what a hybrid CDS solution is. But this model is not just beneficial; it’s often essential in order to meet stringent assurance requirements, particularly in government and defence environments.

This type gives increased cybersecurity of the hardware while offering the flexibility of the software. 

They can be deployed on existing infrastructure and are more cost-effective than hardware alone, but still enforce security policies and ensure that data is transferred securely.

It’s a ‘best of both worlds’ option, but hybrid CDS solutions may require specialised expertise to operate, and they can be more complex to configure and maintain than software-based solutions.

Thinking of implementing a cross-domain security solution? It requires some careful planning and consideration before diving into deployment. 

Start by assessing the data classification requirements, security policies, and regulatory frameworks that govern your organisation, whether in commercial, government and defence, manufacturing, or other industry contexts. This will give you a better idea of which security controls will be necessary for your specific use case.

Here are several other critical factors that you should carefully evaluate to ensure the system meets all of your needs:

Data Flow Requirements and Direction

It’s important to clearly define your organisation’s data requirements, including its types and volume, and whether it can flow bidirectionally or just unidirectionally. Only some solutions will support complex bidirectional workflows. 

Additionally, different solutions handle structured and unstructured data differently, so you should be clear on what formats need to pass through the domain boundary.

Do you need real-time data transfer, or can you tolerate delays? Some CDS solutions introduce latency due to deep inspection and sanitisation.

Integration and Compatibility

CDS tools don’t operate in isolation; they must sit between existing networks and applications, meaning they need to align with established architectures rather than force organisations to redesign them. 

A good cross domain data transfer solution will integrate smoothly with your existing IT infrastructure and workflows. When selecting a solution, consider its compatibility with your current environment. 

If it can’t be directly integrated, evaluate the APIs available. This determines whether it can adapt to existing workflows or whether it will introduce additional complexity and maintenance overhead over time.

Performance and Scalability

Two further factors to reflect on are the solution’s throughput capabilities and ability to scale with organisational growth. When thinking about this, don’t just consider your current needs but aim to foresee future expansion plans. 

Once you feel confident in your needs in these areas, conduct performance testing, including stress testing under various loads and evaluation of how the system handles peak usage periods.

Operational Complexity and Management

Some CDS solutions will require more configuration and ongoing management than others, so your capacity is an important factor in which tool you choose. This includes not only the availability and skill level of personnel, but also the time and resources required to maintain the system effectively over its lifecycle.

Complexity in these areas also significantly impacts the total cost of ownership, as solutions that require specialised expertise may create operational risks and increase expenses.

4Secure provides sovereign, vendor-agnostic cross-domain solutions (CDS) that allow you to inspect, verify, and securely transfer every piece of data without compromising operations.

Prevent data leakage, stop malicious content, and enforce mission-specific information policies with ease.

Our solutions are built around our TrustedFilter® enterprise data transfer software with data diodes for enhanced security where needed.

Designed in alignment with National Cyber Security Centre (NCSC) security principles for cross-domain solutions, you can reduce cyber threats, assure compliance, and find the confidence to share information when and where it matters most.