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

Information

Specify the administrators and users who can install and manage add-ins for Outlook in Exchange Online

By default, users can install add-ins in their Microsoft Outlook Desktop client, allowing data access within the client application.

Attackers exploit vulnerable or custom add-ins to access user data. Disabling user-installed add-ins in Microsoft Outlook reduces this threat surface.

Solution

To remediate using the UI:

- Navigate to Exchange admin center https://admin.exchange.microsoft.com.
- Click to expand Roles select User roles.
- Select Default Role Assignment Policy.
- In the properties pane on the right click on Manage permissions.
- Under Other roles uncheck My Custom Apps, My Marketplace Apps and My ReadWriteMailbox Apps.
- Click Save changes.

To remediate using PowerShell:

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

$policy = "Role Assignment Policy - Prevent Add-ins"
$roles = "MyTextMessaging", "MyDistributionGroups", `
"MyMailSubscriptions", "MyBaseOptions", "MyVoiceMail", `
"MyProfileInformation", "MyContactInformation", "MyRetentionPolicies", `
"MyDistributionGroupMembership"

New-RoleAssignmentPolicy -Name $policy -Roles $roles
Set-RoleAssignmentPolicy -id $policy -IsDefault

# Assign new policy to all mailboxes
Get-EXOMailbox -ResultSize Unlimited | Set-Mailbox -RoleAssignmentPolicy $policy

If you have other Role Assignment Policies modify the last line to filter out your custom policies

Impact:

Implementing this change will impact both end users and administrators. End users will be unable to integrate third-party applications they desire, and administrators may receive requests to grant permission for necessary third-party apps.

See Also

https://workbench.cisecurity.org/benchmarks/24619

Item Details

Category: CONFIGURATION MANAGEMENT, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|CM-10, 800-53|CM-11, 800-53|SC-18, CSCv7|5.1

Plugin: microsoft_azure

Control ID: 48f780e79c57aca4ddfd7613055761ec444f0d5ae42747a50da881616ef617e1