Best Practices for Active Directory Rebuild and OU Design

Microsoft guidance for an AD rebuild and OU redesign focuses on least privilege, tiered administration, and clear separation of duties, while ensuring the structure supports delegation and Group Policy.

1. Best practices for rebuilding an Active Directory environment

• Reduce the attack surface and enforce least privilege:
  • Implement a tiered administrative model with absolute minimum privileges for each role.
  • Discover, review, and audit privileged identities; remove excessive privileges.
  • Enforce MFA for administrative accounts, even if MFA isn’t yet broadly deployed.
  • Use time-based privileged roles and approval processes for elevation.
  • Enable and configure all available auditing for privileged identities (enable/disable, password reset, modifications).
• Use Privileged Access Workstations (PAWs):
  • Don’t administer PAWs from less-trusted hosts.
  • Require MFA to access PAWs.
  • Keep PAWs on supported OS versions and protect them physically.
• Understand attack paths and high-risk accounts/applications:
  • Prioritize monitoring identities and systems that pose the most risk.
  • Eradicate password reuse, including across OS boundaries.
  • Enforce policies that reduce risk (no internet browsing from secured workstations, avoid shared local admin accounts, etc.).
  • Reduce applications installed on domain controllers; eliminate unnecessary apps and move remaining workloads off DCs where possible.
• Ensure recovery readiness:
  • Maintain immutable backups of Active Directory.
  • Use a regular backup schedule and store backups in cloud-based or off-site locations per the disaster recovery plan.
• Periodically assess security posture:
  • Conduct an Active Directory Security Assessment and validate guidance from the assessment.
  • Run assessments at least annually.

2. OU structure aligned with tiered administration (Tier 0/1/2)

• Design OU structure primarily to enable delegation of administration and control of policy application.
• Assign OU owners (data administrators) for each OU; they manage objects and subtrees but don’t control the directory service itself. This separates service administration from data administration and reduces the number of high-privilege service admins.
• Use OUs to provide administrative autonomy and control visibility of objects, while recognizing that OUs don’t isolate from service administrators.
• For each domain, design and document the OU structure with:
  • OU name and type (for example, account OU vs. resource OU).
  • OU owner.
  • Parent OU.
  • Origin/rationale for the OU.
• A tiered model typically maps to separate OU trees for Tier 0 (DCs, PAWs, highly privileged accounts), Tier 1 (servers and their admins), and Tier 2 (workstations and standard users), with delegation and GPOs aligned to those boundaries.

3. Separating users, workstations, servers, and service accounts

• Use account OUs and resource OUs:
  • Account OUs: contain user, group, and computer objects. Create an account OU for each domain and delegate control to OU owners.
  • Resource OUs: contain resources and the accounts responsible for managing those resources. Create resource OUs based on each group’s autonomy requirements.
• Within this model:
  • Place user accounts in account OUs, separated by function or tier (for example, standard users vs. administrative users).
  • Place workstations and servers in separate OUs, typically aligned to their security tier and management responsibility.
  • Place service accounts in dedicated account OUs, with ownership and delegation aligned to the teams that manage the associated applications.

4. GPO structure and linking for security boundaries

• First design the OU structure for delegation and visibility; then create additional OU structures for Group Policy application.
• Use separate OUs (and GPOs) for different tiers and object types to avoid cross-tier exposure:
  • Link security-hardening GPOs at the appropriate tier OU (for example, Tier 0 DCs and PAWs) and avoid linking them across tiers.
  • Use OU-level GPOs instead of domain-level GPOs when enforcing tier-specific controls.
• Ensure auditing GPOs and policies for privileged identities are applied consistently to OUs that contain administrative accounts and PAWs.

5. Managing and placing service accounts, including with PAM

• Treat service accounts as privileged identities when they have elevated access:
  • Place them in dedicated account OUs with clear OU ownership.
  • Apply least privilege and avoid excessive rights.
  • Enforce strong credential hygiene (no reuse, strong policies) and auditing.
• Integrate with privileged access management by:
  • Enforcing MFA or controlled access where applicable (for interactive or semi-interactive service identities).
  • Using time-based privileged roles and approval processes for elevation.
  • Monitoring and auditing all changes to service accounts.

6. Reference architectures and design guidance (cert-based auth and privileged access)

• For environments that cannot fully move to the cloud, Microsoft recommends:
  • A tiered least-privilege administrative model.
  • Use of PAWs for privileged operations.
  • Strong auditing and monitoring of privileged identities.
  • Immutable AD backups and regular security assessments.
• For comprehensive guidance on securing on-premises AD, including privileged access controls and integration with broader identity strategies, use the “Best Practices for Securing Active Directory” guidance as the primary reference.
• When integrating certificate-based authentication (such as smart cards/CAC), align certificate issuance and usage with the tiered model and PAW strategy, ensuring that Tier 0 identities and PAWs are protected with the strongest authentication and auditing.

Documenting and periodically reviewing the OU and security design, combined with regular AD security assessments, helps maintain security, scalability, and long-term maintainability.

References:

• Best practices for securing Active Directory
• Enhanced Security Admin Environment
• Creating an Organizational Unit Design
• Cloud-First identity management: Guidance for IT architects