Active Directory & GPO: Administration and Security Best Practices

A poorly designed Active Directory or a tangle of conflicting GPOs is one of the most painful technical debts an IT team can inherit. Here's how to design it right from the start — or clean up what you have.

OU Structure: The Foundation

Your Organizational Unit (OU) hierarchy determines how GPOs apply and how delegation works. Get it wrong and you'll be fighting it for years.

Anti-pattern: Organizing by Location or Department Only

domain.local
├── Paris
│   ├── IT
│   ├── Sales
│   └── HR
└── Lyon
    ├── IT
    └── Sales

Problem: If Paris IT and Lyon IT need the same GPO, you apply it to both OUs — or use WMI filters. Duplication and drift.

Recommended: Functional + Type Split

domain.local
├── _Computers
│   ├── Workstations
│   │   ├── Standard
│   │   └── Privileged
│   ├── Servers
│   │   ├── Production
│   │   └── Dev-Test
│   └── Kiosk
├── _Users
│   ├── Employees
│   │   ├── IT
│   │   ├── Sales
│   │   └── HR
│   ├── ServiceAccounts
│   └── Admins
└── _Groups

Why this works:

• GPOs on _Computers\Workstations apply to all workstations regardless of office
• Service accounts are separated → easier auditing
• Admins OU can have stricter GPOs (no logon scripts, no personal apps)

Naming convention for OUs: Use a prefix like _ or OU- to distinguish OUs from other AD objects in queries.

GPO Design Principles

1. One GPO, One Purpose

Every GPO should do one thing and have a name that reflects it:

GPO: Computers-Firewall-Baseline
GPO: Computers-Bitlocker-Standard
GPO: Users-Desktop-Restrictions
GPO: Users-Proxy-Settings
GPO: Users-Mapped-Drives-Sales

Never create a "catch-all" GPO called "IT Policy" that contains 40 different settings. When you need to change the firewall config, you don't want to risk touching mapped drives.

2. Computer vs. User Settings

GPOs have two sides — Computer Configuration and User Configuration. Keep them separate:

Best practice: Disable the unused side of each GPO.

• If a GPO only contains Computer Configuration settings → disable User Configuration (GPO Properties → Details tab)
• This speeds up Group Policy processing

3. GPO Inheritance and Blocking

By default, GPOs from parent OUs apply to child OUs. You can control this with:

• Block Inheritance on an OU: child OU ignores parent GPOs. Use sparingly — creates confusion.
• Enforced on a GPO link: forces the GPO to apply even if child has Block Inheritance. Use for security-critical policies.
• Security Filtering: by default, GPOs apply to "Authenticated Users". Replace with a specific group for targeted application.

# Check which GPOs apply to a specific computer
gpresult /scope computer /r
 
# Detailed HTML report
gpresult /h C:\gpo-report.html
 
# Force immediate GPO refresh on a remote machine
Invoke-GPUpdate -Computer "PC-EXAMPLE" -Force

Essential Security GPOs

GPO: Password Policy (Default Domain Policy)

Note: Password policy for domain accounts must be in the Default Domain Policy (or use Fine-Grained Password Policies for specific groups).

Computer Configuration → Windows Settings → Security Settings → Account Policies → Password Policy

Minimum password length: 12
Password complexity: Enabled
Maximum password age: 90 days
Minimum password age: 1 day
Enforce password history: 24 passwords

Fine-Grained Password Policy for Admins (stricter):

# Create a PSO (Password Settings Object) for admins
New-ADFineGrainedPasswordPolicy -Name "PSO-Admins" `
    -Precedence 10 `
    -MinPasswordLength 16 `
    -PasswordHistoryCount 24 `
    -MaxPasswordAge "60.00:00:00" `
    -ComplexityEnabled $true `
    -ReversibleEncryptionEnabled $false
 
# Apply to the Domain Admins group
Add-ADFineGrainedPasswordPolicySubject "PSO-Admins" -Subjects "Domain Admins"

GPO: Account Lockout

Computer Configuration → Windows Settings → Security Settings → Account Policies → Account Lockout

Account lockout threshold: 5 invalid attempts
Account lockout duration: 30 minutes
Reset account lockout counter after: 30 minutes

GPO: Windows Firewall Baseline

Computer Configuration → Windows Settings → Security Settings → Windows Defender Firewall

Domain Profile: On
Private Profile: On
Public Profile: On (block all inbound by default)

Inbound rules:
- Allow: RDP from Management VLAN only (IP filter)
- Allow: ICMP (ping) from Management VLAN
- Block: SMB from external (port 445)

GPO: Disable Legacy Protocols

Computer Configuration → Administrative Templates → Network → Lanman Workstation
  → Enable insecure guest logons: Disabled

Computer Configuration → Windows Settings → Security Settings → Local Policies → Security Options
  → Network security: LAN Manager authentication level: Send NTLMv2 response only. Refuse LM & NTLM
  → Minimum session security for NTLM SSP: Require NTLMv2 session security, 128-bit encryption

GPO: Removable Media Restrictions

Computer Configuration → Administrative Templates → System → Removable Storage Access
  → All Removable Storage classes: Deny all access: Enabled

(If USB exceptions needed, use device installation policies or a whitelist by hardware ID)

User Rights and Privilege Management

Restrict Local Administrator Access

# GPO: Computer Configuration → Preferences → Control Panel Settings → Local Users and Groups
# Create/Update a local group: Administrators
# Members: DOMAIN\Domain Admins, DOMAIN\IT-LocalAdmins-Group
# Action: Update (replaces existing members)

This ensures only authorized AD groups are local admins — local accounts can't accumulate.

Audit Policy

Computer Configuration → Windows Settings → Security Settings → Advanced Audit Policy

Account Logon:
  Credential Validation: Success, Failure

Account Management:
  User Account Management: Success, Failure
  Security Group Management: Success

Logon/Logoff:
  Logon: Success, Failure
  Logoff: Success

Object Access:
  File System: Failure (on sensitive shares only)

Privilege Use:
  Sensitive Privilege Use: Success, Failure

System:
  Security State Change: Success, Failure

Scripting AD Administration

# Create a new user in the correct OU
# Password typed at the prompt: never written in clear text in a script
New-ADUser `
    -Name "Dupont, Jean" `
    -GivenName "Jean" `
    -Surname "Dupont" `
    -SamAccountName "j.dupont" `
    -UserPrincipalName "j.dupont@domain.local" `
    -Path "OU=Employees,OU=_Users,DC=domain,DC=local" `
    -AccountPassword (Read-Host -AsSecureString "Initial password") `
    -ChangePasswordAtLogon $true `
    -Enabled $true
 
# Add to groups
Add-ADGroupMember -Identity "GRP-VPN-Users" -Members "j.dupont"
Add-ADGroupMember -Identity "GRP-Sales" -Members "j.dupont"

# Disable account and move to Disabled OU on offboarding
$user = "j.dupont"
Disable-ADAccount -Identity $user
Move-ADObject -Identity (Get-ADUser $user).DistinguishedName `
    -TargetPath "OU=Disabled,OU=_Users,DC=domain,DC=local"

# Find all inactive computers (not logged in for 90 days)
$cutoff = (Get-Date).AddDays(-90)
Get-ADComputer -Filter {LastLogonDate -lt $cutoff -and Enabled -eq $true} `
    -Properties LastLogonDate | Select-Object Name, LastLogonDate

AD Administration Checklist

Structure:

• OU hierarchy reflects functional groups, not just org chart
• Computers and Users are in separate OU branches
• Service accounts are in a dedicated OU with audited access
• Unused accounts are in a Disabled OU (not deleted immediately)

GPO:

• Each GPO has a single, documented purpose
• GPOs are named clearly: [Scope]-[Purpose]-[Target]
• Unused half (computer or user) is disabled in GPO properties
• Password policy is in Default Domain Policy or PSO
• Audit policy captures logon successes/failures

Security:

• Admin accounts are separate from daily-use accounts
• LM/NTLM v1 disabled — NTLMv2 or Kerberos only
• USB restrictions in place for non-IT staff
• Local admin group controlled by GPO (no individual exceptions)

Conclusion

Active Directory is the backbone of your Windows environment — its design impacts security, manageability, and scalability for years. Invest time in a clean OU structure, name your GPOs descriptively, and keep each GPO focused on a single concern. The few hours spent designing this properly will save hundreds of hours of troubleshooting later.

Useful Resources:

• Microsoft AD Best Practices
• Group Policy Overview
• Fine-Grained Password Policies