What is Tier Zero (Tier 0)

Every organization has certain assets that are far more critical, and far more sensitive than the rest. A breach of these assets can have catastrophic consequences, such as total system takeover, regulatory penalties and operational paralysis. In cybersecurity terms, these high-value assets are classified as Tier zero (Tier 0).

Tier 0 includes all the core systems and identities that have the highest level of access and control across the IT environment. Examples can include: Active Directory (AD) domain controllers, identity management systems, RBAC tools and cloud admin accounts. Because these assets sit at the very top of the trust hierarchy, they demand the highest level of protection.

Tier 0 assets hold the keys to everything in your ecosystem. Here’s why you must have a formal Tier 0 security policy:

• Epicenter of control: Tier 0 assets, like identity systems, govern authentication and authorization across the entire enterprise. If attackers get access to these assets, they can control user permissions, access sensitive data and move laterally across the network.
• Primary target for sophisticated attackers: Adversaries know that a Tier 0 compromise paves the most direct path to control over the entire network, whether for espionage, financial gain or disruption. Therefore, they use targeted techniques that help them penetrate this layer, such as Kerberoasting and Pass-the-Hash.
• Enables better incident response: When Tier 0 is clearly defined and well-protected, it becomes easier to detect unusual behavior at the core level. This helps security teams respond faster and with more precision. For example, if a monitoring tool detects a sudden login from a Tier 0 account outside business hours, an investigation can be initiated immediately.
• Improves compliance and audit readiness: Frameworks like NIST CSF and ISO 270001 require organizations to identify and protect their most critical assets. A clear Tier 0 list can help meet these requirements.

Next, here’s a step-by-step guide on how to implement Tier 0:

1. Start by identifying systems, accounts and services that have direct or indirect control over your environment. This can include: domain controllers, Active Directory services, privileged identity management systems, backup servers for Tier 0 assets, directory sync tools and any account or system that can change authentication or authorization settings.

2. Then, isolate your Tier 0 systems and accounts from lower-tier systems. Here are some tips in this regard:

   • No Tier 0 accounts should be used on Tier 1 or 2 systems (e.g., admin shouldn’t check email or browse the web).
   • Set up dedicated administrative workstations to manage AD and other Tier 0 systems.
   • Use micro-segmentation to strictly control which systems can talk to Tier 0 assets.
   • Apply SSL inspection at network boundaries to detect and block malicious activity directed towards Tier 0 systems.

3. Use identity governance tools and policies to:

   • Enforce least privilege access
   • Set up just-in-time (JIT) access for admins
   • Review and remove unused or excessive permissions regularly
4. Put strong monitoring in place around Tier 0 assets. For example, you can enable auditing for all privileged activity, use a SIEM or log management tool to track access and changes, and set up alerts for unusual activities (e.g. data exfiltration attempts).
5. Ideally, only a small, trusted group of people should have access to Tier 0. Train these people on strict operational protocols, security best practices and critical “never-do” actions.
6. Regularly review and audit system configurations and simulate attacks to see how well the controls hold up.

Zero trust and Tier 0 are not the same thing, but they can work well together.

Zero trust mandates that no one is trusted by default, even if they’re inside the network. Tier 0 is about inventorying your most important systems and protecting them at the highest level. When you combine the two, you’re not only locking down your most critical assets, but also making sure that every request to access them is verified, justified, logged and controlled.

For example, before anyone accesses a domain controller (a Tier 0 system), they must go through MFA, device checks and possibly just-in-time access approval. This setup makes it much harder for attackers to carry out a Tier 0 attack, even if they get inside your network.

Ideally, every organization should have a Tier 0 model in place. No matter the size, there are always some assets, like admin accounts, identity providers or core systems, that are too important to be treated the same as everyone else.

That said, certain types of organizations stand to benefit even more from implementing Tier 0:

• Organizations that use Active Directory or Entra ID. These directories are the backbone of authentication and access control, which makes them high-value targets.
• Companies that handle sensitive customer or financial data. A Tier 0 model helps protect the systems that store or process this information.
• Enterprises with a large number of privileged users. More admins mean more risk, so isolating critical accounts becomes even more important.
• Organizations with remote or hybrid work setups. Remote access increases the attack surface, especially around identity and admin tools.
• Businesses going through compliance audits. Tier 0 helps demonstrate that the most sensitive assets are clearly identified and well protected.

Static, isolationist security controls for Tier 0 assets can lead to challenges such as operational friction, false confidence and blind spots. Adaptive security can solve these challenges by adjusting defenses based on context and risk, instead of applying the same rules to every situation.

Here are some adaptive security measures to consider:

• Adaptive authentication adjusts access requirements based on factors like location, device, time or behavior. For example, an admin logging in from a new country will be asked for extra verification.
• Behavior-based monitoring learns normal admin patterns and flags actions that fall outside of them.
• Just-in-time (JIT) access reduces standing privileges by granting access to Tier 0 systems only when needed and only for a short time.
• Risk-based alerting filters out noise and focuses attention on truly risky actions, like unusual account movements near Tier 0 assets.

As threats grow more advanced, Tier 0 will likely become a central part of how organizations manage cyber risk. Here’s what we can expect to see in the future:

• AI-driven threat detection will play a bigger role in monitoring Tier 0 systems for subtle signs of compromise.
• Automated access decisions will use risk scores to allow or block admin actions in real time.
• Tier 0 classification will expand beyond its current scope to include cloud-native tools, APIs and application containers.
• A greater number of cyber insurance policies may start to require proof of Tier 0 protections as part of risk assessments.
• Security frameworks will likely offer more detailed guidance on how to manage Tier 0 assets as a core control area.