Guest Contributor Unknown External User - O365

Identifying Guest Contributor Unknown External User Collaborators

In the event that you come across a collaborator labeled "Guest Contributor Unknown external user" on a O365 Incident, it is important to identify and understand its origin. The issue arises when the sharing options on an file are set to “Public with link”. This is the default sharing option in many instances and, while it can be perfectly acceptable, it does assume one thing; that the user viewing the document is already signed in to Office 365. If the user is not already signed in to Office 365 then they won’t be asked to sign in. After all, the settings allow anyone to access the document with only the link.

What causes this and what can be done to address it?

A user might be signed into the Outlook app on their desktop but may not have signed in to Office 365 online services yet; In this latter case then the user will be taken in to the document without ever being asked to sign in and, even though they are part of the organization, they will show up as "Guest Contributor Unknown external user". You can enforce a login to Office 365 services by enabling the “Shared exclusively with internal users” option in a document, which will force the user to sign in so that Office 365 can verify that they are in your organization.