# Forest functional level should be retrievable

The forest functional level determines which Active Directory features are available across all domains in the forest.

Source: https://maester.dev/docs/tests/AD-FOR-01

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<div class="test-byline"><div class="test-byline-avatars"><a class="test-byline-avatar test-byline-avatar--author" href="/contributors/soulemike" title="Michael Soule · Original author"><img src="https://github.com/soulemike.png" alt="Michael Soule" width="28" height="28" loading="lazy" decoding="async" /></a></div><div class="test-byline-meta"><span class="test-byline-text">Contributed by <a href="/contributors/soulemike">Michael Soule</a></span><a class="test-byline-link" href="/contributors">All contributors →</a></div></div>

<p class="test-part">Part of <a href="/docs/tests/ad">Active Directory</a> · Active Directory · Domain and forest · <a href="/docs/tests">All checks</a></p>

```powershell title="Run this check"
Invoke-Maester -TestId AD-FOR-01
```

## Overview

The forest functional level determines which Active Directory features are available across all domains in the forest. Higher functional levels unlock important forest-wide security capabilities:

- **Windows Server 2016+**: Enables features like privileged access management (PAM) across the forest
- **Windows Server 2012 R2+**: Provides access to forest-wide authentication policies and silos
- **Global Features**: Some features require forest-wide consistency to function
- **Security Posture**: Running at lower levels means missing modern security features

#### Control Type

**Preventive**

#### Security Recommendation

Aim to maintain your forest at the highest functional level supported by all domain controllers:

1. **Verify Compatibility**: Ensure all DCs in all domains support the target level
2. **Test Applications**: Verify critical applications work at the higher level
3. **Plan Maintenance Window**: Schedule the upgrade appropriately
4. **Document Changes**: Record the upgrade for audit and compliance purposes

#### How the Test Works

This test retrieves the current forest functional level from Active Directory along with basic forest information including the root domain and domain count.

#### Related Tests

- `Test-MtAdDomainFunctionalLevel` - Retrieves the domain functional level
- `Test-MtAdForestDomainCount` - Counts domains in the forest

#### Related links

- [Microsoft Learn: Active Directory Domain Services functional levels](https://learn.microsoft.com/windows-server/identity/ad-ds/active-directory-functional-levels)
- [ANSSI Active Directory checkpoints: Insufficient forest and domains functional levels](https://www.cert.ssi.gouv.fr/uploads/ad_checklist.html#vuln_functional_level)

## Details

<details>
<summary>Every attribute of this check</summary>

| Field | Value |
| --- | --- |
| Test ID | AD-FOR-01 |
| Severity | Info |
| Suite | Active Directory |
| Product | Active Directory |
| Feature | Domain and forest |
| Area | Infrastructure hardening |
| Resource type | Tenant |
| MITRE ATT&CK | None |
| PowerShell test | [Test-MtAdForestFunctionalLevel](https://github.com/maester365/maester/blob/main/tests/ad/domain/Test.AD-FOR-01.ps1) |
| Services | ActiveDirectory |
| Permissions | ActiveDirectory:Ldap:crossRef:ReadProperty |

</details>

## Source

- Test: [`tests/ad/domain/Test.AD-FOR-01.ps1`](https://github.com/maester365/maester/blob/main/tests/ad/domain/Test.AD-FOR-01.ps1)
- Documentation: [`tests/ad/domain/Test.AD-FOR-01.md`](https://github.com/maester365/maester/blob/main/tests/ad/domain/Test.AD-FOR-01.md)