PowerShell: Securing App-Registration with Application Access Policy

In the rapidly evolving landscape of Entra ID, the use of App-Registrations has become increasingly prevalent, empowering users to seamlessly leverage PowerShell and the Microsoft Graph API for fundamental tasks in Exchange-Online, Entra ID, Intune, and related domains.

However, the challenge arises in the potential for App-Registrations to possess expansive access, allowing them to target all objects within a designated scope, such as all mailboxes, when set to "Application Scope" permission.

Addressing this concern, organizations can implement Application Access Policies, offering meticulous control over access permissions for specific resources like Calendars, Contacts, Mail, and Mailbox settings. Notably, these policies can only be configured through PowerShell, necessitating the ExchangeOnlineManagement Module for execution.

A crucial step in this process involves designating a mail-enabled security group as the policy target, ensuring a focused approach to access control. Although the policy cannot be applied to an individual mailbox, having a lone member in the security group is acceptable. Creating such a group is straightforward with the provided PowerShell command:

$groupParams = @{
    Name = "AAP_AppReg_SG"
    Alias = "AAPAppRegSG"
    Type = "security"
    PrimarySMTPAddress = "[email protected]"
    Members = @("[email protected]", "[email protected]", "[email protected]")
}
New-DistributionGroup @groupParams

Following this, the next imperative is creating the Application Access Policy using PowerShell:

$policyParams = @{
    AppId = $AppID
    PolicyScopeGroupId = "[email protected]"
    AccessRight = "RestrictAccess"
    Description = "Restrict this app to members of distribution group AAPAppRegSG."
}
New-ApplicationAccessPolicy @policyParams

By implementing these measures, App Registrations are refined to exclusively target designated resources—specifically, our superhero mailboxes. Verification of this tailored access can be conducted using the Test-ApplicationAccessPolicy command:

Test-ApplicationAccessPolicy -Identity "[email protected]" -AppId $AppID

In conclusion, the integration of Application Access Policies serves as a pivotal strategy for organizations aiming to strike a balance between harnessing the capabilities of App-Registrations and maintaining precise control over access to critical resources. This approach not only fortifies security but also ensures a seamless and focused utilization of PowerShell and the Microsoft Graph API within the Entra ID environment.

If you have any thoughts or feedback on this topic, feel free to share them with me on Twitter at Christian Ritter.

Best regards, Christian

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