# Computer SPN unknown service class count should be retrievable

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

Source: https://maester.dev/docs/tests/AD-SPN-03

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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-03
```

## Overview

Unidentified SPN service classes can represent security risks:

- **Shadow IT**: Unknown services may be running without IT's knowledge or approval
- **Misconfigurations**: Typos or incorrect SPN registrations can cause authentication issues
- **Malicious services**: Attackers may register SPNs for rogue services
- **Legacy cleanup**: Old application SPNs may remain after decommissioning

Identifying unknown SPNs allows security teams to investigate and validate whether these services are legitimate and properly secured.

#### Control Type

**Operational**

#### Security Recommendation

When unknown SPN service classes are identified:
- Investigate each unknown service class to determine its purpose
- Verify if the service is authorized and necessary
- Check if the SPN is registered on the correct account
- Remove SPNs for unauthorized or decommissioned services
- Document any custom or third-party SPNs for future reference

#### How the Test Works

This test compares discovered SPN service classes against a database of known SPNs (including standard Windows services, common enterprise applications, and database systems). Service classes not in the known list are flagged as unknown.

#### Related Tests

- `Test-MtAdComputerSpnUnknownDetails` - Provides detailed information about unknown SPNs
- `Test-MtAdComputerSpnServiceClassCount` - Counts all distinct service classes
- `Test-MtAdUserSpnUnknownCount` - Identifies unknown SPNs on user accounts

## Details

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

| Field | Value |
| --- | --- |
| Test ID | AD-SPN-03 |
| 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-MtAdComputerSpnUnknownCount](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-03.ps1) |
| Services | ActiveDirectory |
| Permissions | ActiveDirectory:Ldap:computer:ReadProperty |

</details>

## Source

- Test: [`tests/ad/spn/Test.AD-SPN-03.ps1`](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-03.ps1)
- Documentation: [`tests/ad/spn/Test.AD-SPN-03.md`](https://github.com/maester365/maester/blob/main/tests/ad/spn/Test.AD-SPN-03.md)