I was asked about domain joining Hyper-V hosts. My initial thought was “absolutely not” but I realized I didn’t have a good reason not to. What follows is a brief overview of some very quick research and some thoughts about it.

Hyper-V hosts: should you join them to a domain or not? This choice might seem straightforward given the perks of domain-joining, such as easier access controls and centralized administration. It’s crucial though to weigh these benefits against the increased risks of the join, especially when we have the spector of ransomware attacks looming over every network. Let’s look at this decision, take a look at the historical context, current Microsoft recommendations, and some associated security implications.

Why People Hesitate: A Historical Perspective

Historically, many administrators opted to keep Hyper-V hosts out of their domains to limit security exposures, and prevent some compatibility issues that existed in Server 2008 and 2008R2. Some of the thinking around compatibility has trickled over into newer versions of Windows Server, despite Microsoft solving the underlying problems. From a security standpoint it was simple: if a host wasn’t on the domain, it was harder for attackers, who had compromised a domain user or administrator, to access the host and spread malware or ransomware.

• Simplified Security Landscape: The arguement goes that Standalone hosts have fewer security complexities to manage, which could lead to a smaller chance of misconfiguration and therefore fewer vulnerabilities.
• Reduced Attack Vectors: Without domain credentials, attackers theoretically have a harder time escalating privileges and moving laterally (from one machine to another) across the network.

Microsoft’s Current Stance: Why Join?

Despite these historical hesitations, Microsoft now recommends domain-joining your Hyper-V hosts. This shift is driven by enhanced security measures in Active Directory and Windows Server, improved management capabilities, and the integration benefits that come with being part of a domain:

• Better Resource Management: Domain-joined hosts can leverage centralized Group Policy settings to manage resources and security configurations efficiently.
• Streamlined Administration: Using Active Directory to handle authentication simplifies the overall process of managing permissions and can reduce administrative overhead.
• Advanced Security Tools: Modern Windows Servers equipped with Active Directory offer robust tools and features that enhance security, even in a domain environment.
• Better Monitoring: Authentications and other events are easier to monitor within a domain environment.

Weighing the Risks: Increased Attack Surface

Joining a Hyper-V host to a domain does indeed open up new risks, particularly with ransomware:

1. Lateral Movement and Credential Theft

Domain-joined machines are prime targets for lateral movement strategies used in ransomware attacks. Here’s how:

• Pass-the-Hash and Other Credential Attacks: These common tactics become more concerning in a domain environment where stealing one set of credentials can give attackers access to multiple resources.
• Worm-Like Ransomware: Strains like WannaCry use network vulnerabilities to spread; these are more dangerous in networked environments where machines, including Hyper-V hosts, are interconnected.

2. Configuration Complexity and Human Error

Adding Hyper-V hosts to a domain means they’re now part of a much larger and more complex security setup managed through Group Policies. Misconfigurations in these policies can inadvertently expose hosts to attacks. We’ve seen things like passwords in cleartext present in GPOs, the Domain Users group having overly permissive login settings in GPOs, and just general misconfiguration creep that occurs due to poorly managed domain environments.

How to Safely Domain-Join Hyper-V Hosts

Given the risks and benefits, if you choose to domain-join your Hyper-V hosts, and you should domain join them, consider these best practices to secure them:

1. Administrative Tiers

Use a tiered approach to manage credentials and permissions:

• Tier 0: Domain admins, restricted to critical tasks and never used on lower-tier systems.
• Tier 1: Server admins, separate from Tier 0 to limit access rights.
• Tier 2: Workstation admins, isolated from higher tiers to prevent credential misuse.

2. Principle of Least Privilege

Always ensure that users and services operate with the minimum necessary privileges to reduce the risk of malware exploitation. Just because a service needs to access certain things that a normal user cannot, don’t just add it into Domain Admins. We see this frequently and it is commonly exploited.

3. Robust Monitoring and Quick Response

Deploy monitoring tools to detect and respond to unusual activities quickly, which can be indicative of a breach or an ongoing ransomware attack. One advantage of deploying Hyper-V over other hypervisor solutions is the ability to run an EDR tool such as SentinelOne or Crowdstrike.

4. Regular Patches and Updates

Keep all systems, especially Hyper-V hosts and associated virtual machines, updated with the latest security patches to defend against exploitation of known vulnerabilities.

5. Network Segmentation

Limit potential ransomware spread by segmenting the network. Critical assets, like Hyper-V hosts, should be isolated from general user networks.

Final Thoughts

Domain-joining Hyper-V hosts can make life easier but also opens up new vulnerabilities that need to have some controls put in place, particularly to avoid ransomware attacks. Implementing stringent security practices and continuously monitoring the health and security of your network will help maintain the integrity of your Hyper-V hosts while reaping the benefits of domain integration. You should join your Hyper-V hosts to the domain. Just make sure you have a plan and backups should disaster strike.