General

This check verifies that SharePoint is set up so that guest users must log in with an email address that matches the address to which the invitation was sent.

Rationale

Invitation emails can be forwarded to other email addresses. Thus, an invitation could end up in the wrong hands without any control for your organization. Therefore, it's better to ensure that the invitation can only be accepted by someone who can authenticate with the original email address.

Attic Fix

For this check a Fix is available! It will be offered through a ticket in Attic, after which you can accept.

Manual Instruction

This setting can be changed using PowerShell. Set the following value:

isRequireAcceptingUserToMatchInvitedUserEnabled = true

More information

1. https://learn.microsoft.com/en-us/graph/api/resources/sharepointsettings?view=graph-rest-1.0