Friday, August 24, 2018

Using Guest Access in PowerShell to access Unified Groups and Teams in Office365

This is a continuation somewhat of my previous post on using a generic oAuth script to access EWS but in this post I'm going to look at how you can authenticate as a Guest user to a tenant you have been granted Guest access to and then access the resources like a Unified Office365 Group or Teams( because Teams uses Groups) via the Graph API.

To Access another tenant as a Guest (considering that Guest Access is enabled and you have accepted the invitation and logged in at least once with that user) you need to generate an Access Token against the Target tenants endpoints. To do this you can discover what a particular tenants Authentication and Token endpoints are by making a request like the following

$RequestURL = "{0}/.well-known/openid-configuration" -f $TargetDomain
$Response = Invoke-WebRequest -Uri  $RequestURL
$JsonResponse = ConvertFrom-Json  $Response.Content

This will return a result like the following

Out of this result the Authorization Endpoint and the Token_endpoint are what you need to make a successful authentication against the Target Tenant to get a guest access token.

For technical completeness the above step is not necessarily needed as the endpoint should also support you using the domain name as per . But like many things in IT there are many ways of doing the same things.

I've posted up a script that first does the above the request and then will authenticate against the Target Tenant and return an Access Token that can then be used to access the Graph. I've created a test cmdlet called Test-GuestAuth that will enumerate the unified Groups the guest is a member of and get the first two conversation items  in each of the unified Groups, I've put the code up for this here

The script is on GitHub here

To use this script use something like the following

Test-GuestAuth -UserName -TargetDomain

For the clientId you would need to use in oAuth that clientId has to have been consented to by the Target Tenant admins, in the above script I've used the default Office clientId which should be enough to get access to Group conversation etc.

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