# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-M365-MAIL-008
title: Block additional storage providers in Outlook on the web
finding:
  title: Outlook on the web lets users attach and save files with personal cloud storage
  description: Users can attach files from, or save attachments to, Dropbox, Google Drive, Box or a personal OneDrive, outside the organization's control.
status: active
profiles: [hardened]
risk:
  severity: medium
assessment: manual
tags: [exchange-online, outlook, owa-policy, cloud-storage, data-leakage]

description: |
  Every Outlook on the web mailbox policy in use has `AdditionalStorageProvidersAvailable` set to
  `$false`. Users can then only attach files from the device or from the organization's OneDrive
  and SharePoint, and cannot connect third-party or personal storage accounts in Outlook on the web
  and the new Outlook.
rationale: |
  A connected personal storage account is an easy path to move work files out of the
  organization, outside data loss prevention, retention and audit. Files shared from third-party
  storage are also not scanned by Safe Attachments the way OneDrive and SharePoint files are.
impact: |
  Users who used Box, Dropbox or Google Drive in Outlook on the web lose that option. Teams that
  exchange files with partners through those services must download and re-attach them, or use
  SharePoint sharing.

audit:
  manual: |
    1. Connect to Exchange Online PowerShell as at least a Global Reader.
    2. Run (read-only):
       ```powershell
       Get-OwaMailboxPolicy | Format-Table Name, IsDefault, AdditionalStorageProvidersAvailable
       Get-CASMailbox -ResultSize Unlimited | Group-Object OwaMailboxPolicy | Select-Object Name, Count
       ```
    3. Fail if a policy used by at least one mailbox (an empty `OwaMailboxPolicy` means the default
       policy) has `AdditionalStorageProvidersAvailable` set to `True`.

remediation:
  manual: |
    1. Connect to Exchange Online PowerShell as an Exchange Administrator.
    2. For each policy in use:
       ```powershell
       Set-OwaMailboxPolicy -Identity OwaMailboxPolicy-Default -AdditionalStorageProvidersAvailable $false
       ```
  rollback: |
    Set `AdditionalStorageProvidersAvailable` back to `$true` on the policy.

references:
  - title: Set-OwaMailboxPolicy
    url: https://learn.microsoft.com/powershell/module/exchangepowershell/set-owamailboxpolicy
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
  - title: Outlook on the web mailbox policies in Exchange Online
    url: https://learn.microsoft.com/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/outlook-web-app-mailbox-policies
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
mappings:
  - framework: cis-m365
    framework_version: "7.0.0"
    id: "6.5.3"
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-4
    relation: related
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-20
    relation: related
threats:
  - framework: mitre-attack
    id: T1567
related_controls:
  - id: CB-M365-MAIL-004
    type: related
  - id: CB-M365-MAIL-005
    type: related
  - id: CB-M365-TENANT-017
    type: related
  - id: CB-M365-COLLAB-016
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
introduced_in: 0.1.0
created: 2026-10-24
updated: 2026-10-24