Remediation

From Azure Portal

1. In the Azure portal, open the portal menu.
2. Select Microsoft Entra ID.
3. Under Manage, select External Identities.
4. Select External collaboration settings.
5. Under Guest invite settings, set Guest invite restrictions to either Only users assigned to specific admin roles can invite guest users or No one in the organization [...].
6. Click Save.

From PowerShell

Enter the following:

Connect-MgGraph
Update-MgPolicyAuthorizationPolicy -AllowInvitesFrom "adminsAndGuestInviters"

Alternatively, to set this to the most restrictive No one in the organization [...] enter the following:

Connect-MgGraph
Update-MgPolicyAuthorizationPolicy -AllowInvitesFrom "none"