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 ReadWriteMailboxApps
 - 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.