(L2) Ensure additional storage providers are restricted in Outlook on the web

Contributed by Stefan Wey with 2 co-contributorsAll contributors →

Part of CIS · Exchange Online · Outlook · All checks

Run this check

Invoke-Maester -TestId CIS.M365.6.5.3

Overview

6.5.3 (L2) Ensure additional storage providers are restricted in Outlook on the web

This setting allows users to open certain external files while working in Outlook on the web. If allowed, keep in mind that Microsoft doesn’t control the use terms or privacy policies of those third-party services.

Ensure AdditionalStorageProvidersAvailable is restricted on the default OWA policy.

The check passes only when the default policy returns False. A missing value does not demonstrate that storage providers are restricted and therefore does not pass.

Rationale

By default, additional storage providers are allowed in Office on the Web (such as Box, Dropbox, Facebook, Google Drive, OneDrive Personal, etc.). This could lead to information leakage and additional risk of infection from organizational non-trusted storage providers. Restricting this will inherently reduce risk as it will narrow opportunities for infection and data leakage.

Impact

The impact associated with this change is highly dependent upon current practices in the tenant. If users do not use other storage providers, then minimal impact is likely. However, if users do regularly utilize providers outside of the tenant this will affect their ability to continue to do so.

Remediation

PowerShell

1. Connect to Exchange Online using Connect-ExchangeOnline.
2. Run the following PowerShell command:

Set-OwaMailboxPolicy -Identity OwaMailboxPolicy-Default -AdditionalStorageProvidersAvailable $false

Default Value

AdditionalStorageProvidersAvailable : True

Related links

• Set-OwaMailboxPolicy
• 3rd party cloud storage services supported by Office apps
• Microsoft Secure Score - Restrict third-party storage services
• CIS Microsoft 365 Foundations Benchmark v7.0.0 - Page 454

Details

Every attribute of this check

Field	Value
Test ID	CIS.M365.6.5.3
Severity	Medium
Suite	CIS
Product	Exchange Online
Feature	Outlook
Area	External access and sharing
Resource type	Tenant
User impact	Low
Implementation cost	Low
MITRE ATT&CK	T1567.002 Exfiltration to Cloud Storage, T1204.002 Malicious File
Supports benchmark	CIS Microsoft 365 Foundations Benchmark (version 7.0.0), requirement 6.5.3
Benchmark level	L2
Benchmark profiles	E3 Level 2, E5 Level 2
Supports controls	cis-m365:6.5.3, nist-800-53-r5:AC-20, nist-800-53-r5:CM-7
PowerShell test	Test-MtCisExoAdditionalStorageProvider
Services	ExchangeOnline

Source

• Test: tests/cis/Test.CIS.M365.6.5.3.ps1
• Documentation: tests/cis/Test.CIS.M365.6.5.3.md