# (L2) Ensure users installing Outlook add-ins is not allowed

The default role assignment policy does not grant users the roles that let them install Outlook add-ins.

Source: https://maester.dev/docs/tests/CIS.M365.6.3.1

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<div class="test-byline"><div class="test-byline-avatars"><a class="test-byline-avatar test-byline-avatar--author" href="/contributors/jeremyfinney-lgtm" title="jeremy finney · Original author"><img src="https://github.com/JeremyFinney-lgtm.png" alt="jeremy finney" 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/jeremyfinney-lgtm">jeremy finney</a></span><a class="test-byline-link" href="/contributors">All contributors →</a></div></div>

<p class="test-part">Part of <a href="/docs/tests/cis">CIS</a> · Exchange Online · Outlook · <a href="/docs/tests">All checks</a></p>

```powershell title="Run this check"
Invoke-Maester -TestId CIS.M365.6.3.1
```

## Overview

The default role assignment policy does not grant users the roles that let them install Outlook add-ins.

An Outlook add-in can read and change mail. A malicious or over-privileged add-in, installed by a user who was persuaded to, gives an attacker access to the mailbox without a password. Restricting installation to administrators lets you review the add-in first. An add-in runs inside Outlook each time it starts (T1137.006), and one that can read mail can collect it through the Exchange APIs for someone else (T1114.002).

The test checks the **My Custom Apps**, **My Marketplace Apps** and **My ReadWriteMailbox Apps** roles in the default role assignment policy, which is the policy CIS audits. Custom role assignment policies are outside this recommendation's audit scope. Users who need an add-in can get one deployed by an administrator through Integrated apps.

#### Remediation action

1. Sign in to the [Exchange admin center](https://admin.exchange.microsoft.com).
2. Go to **Roles** > **User roles** > [**Default Role Assignment Policy**](https://admin.exchange.microsoft.com/#/individualpermissions).
3. Clear **My Custom Apps**, **My Marketplace Apps** and **My ReadWriteMailbox Apps**.
4. Select **Save changes**.

#### Related links

* [Exchange admin center - User roles](https://admin.exchange.microsoft.com/#/individualpermissions)
* [Specify the add-ins that users can install](https://learn.microsoft.com/exchange/clients-and-mobile-in-exchange-online/add-ins-for-outlook/specify-who-can-install-and-manage-add-ins)
* [Manage add-ins in the Microsoft 365 admin center](https://learn.microsoft.com/microsoft-365/admin/manage/manage-deployment-of-add-ins)
* [CIS Microsoft 365 Foundations Benchmark v7.0.0 - 6.3.1](https://www.cisecurity.org/benchmark/microsoft_365)

## Details

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

| Field | Value |
| --- | --- |
| Test ID | CIS.M365.6.3.1 |
| Severity | Medium |
| Suite | CIS |
| Product | Exchange Online |
| Feature | Outlook |
| Area | Applications and workload identities |
| Resource type | Policy |
| User impact | Medium |
| Implementation cost | Low |
| MITRE ATT&CK | T1137.006 Add-ins, T1114.002 Remote Email Collection |
| Supports benchmark | CIS Microsoft 365 Foundations Benchmark (version 7.0.0), requirement 6.3.1 |
| Benchmark level | L2 |
| Benchmark profiles | E3 Level 2 |
| Supports controls | cis-m365:6.3.1, nist-800-53-r5:CM-7 |
| PowerShell test | [Test-MtCisUserCannotInstallOutlookAddIn](https://github.com/maester365/maester/blob/main/tests/cis/Test.CIS.M365.6.3.1.ps1) |
| Services | ExchangeOnline |

</details>

## Source

- Test: [`tests/cis/Test.CIS.M365.6.3.1.ps1`](https://github.com/maester365/maester/blob/main/tests/cis/Test.CIS.M365.6.3.1.ps1)
- Documentation: [`tests/cis/Test.CIS.M365.6.3.1.md`](https://github.com/maester365/maester/blob/main/tests/cis/Test.CIS.M365.6.3.1.md)