# Computer SPN unknown service class details should be investigated

Computer SPN unknown service class details should be investigated. A Maester check for Active Directory, Service principal names.

Source: https://maester.dev/docs/tests/AD-SPN-04

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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 · Service principal names · <a href="/docs/tests">All checks</a></p>

```powershell title="Run this check"
Invoke-Maester -TestId AD-SPN-04
```

## Overview

Detailed information about unknown SPNs enables security teams to:

- **Investigate effectively**: Know exactly which computers have unknown SPNs
- **Assess scope**: Understand how widespread an unknown service is
- **Track down owners**: Identify the computers to contact administrators
- **Document exceptions**: Build a list of authorized custom SPNs

This granular visibility is essential for maintaining SPN hygiene and security.

#### Control Type

**Operational**

#### Security Recommendation

For each unknown SPN service class discovered:
1. Identify the service owner or application team
2. Determine if the service is legitimate and necessary
3. Verify the SPN is registered on the correct account (not a user account for a computer service)
4. Document approved custom SPNs in your security baseline
5. Remove or correct unauthorized SPNs

#### How the Test Works

This test analyzes all computer SPNs, compares service classes against a known database, and provides a detailed breakdown of unknown service classes including which computers have them and how many instances exist.

#### Related Tests

- `Test-MtAdComputerSpnUnknownCount` - Counts unknown service classes
- `Test-MtAdUserSpnUnknownDetails` - Details of unknown user account SPNs
- `Test-MtAdComputerSpnServiceClassUsage` - All service class usage breakdown

## Details

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

| Field | Value |
| --- | --- |
| Test ID | AD-SPN-04 |
| Severity | Info |
| Suite | Active Directory |
| Product | Active Directory |
| Feature | Service principal names |
| Area | Infrastructure hardening |
| Resource type | Computer |
| MITRE ATT&CK | None |
| PowerShell test | [Test-MtAdComputerSpnUnknownDetails](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-04.ps1) |
| Services | ActiveDirectory |
| Permissions | ActiveDirectory:Ldap:computer:ReadProperty |

</details>

## Source

- Test: [`tests/ad/spn/Test.AD-SPN-04.ps1`](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-04.ps1)
- Documentation: [`tests/ad/spn/Test.AD-SPN-04.md`](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-04.md)