ACS | YASP Blog

Introduction

A while back ago we already discussed how to consume SharePoint Online (SPO) REST in PowerShell. Here is a brief recap:

it was stated that Invoke-RestMethod cmdlet could not be utilized for performing REST requests since it does not support claims-based authentication
a custom Invoke-RestSPO function has been implemented that sends HTTPS requests to SPO RESTful web services. A SharePointOnlineCredentials class from SharePoint Client Component SDK was utilized for authentication which represents an object that provides credentials to access SharePoint Online resources.

This time I would like demonstrate another approach, in particular how PowerShell can gain authorization to SharePoint resources by passing an access token to SharePoint with each HTTP request. To issue an access token from Microsoft Azure Access Control Service (ACS) that allows the app access to the resources in the SharePoint tenancy we will implement the corresponding PowerShell function. Let’s get started.

Getting Access Token from Microsoft Azure Access Control Service

The Get-SPOAccessToken function demonstrates how to obtain the access token from a Microsoft Azure Access Control Service (ACS) account that is associated with the customer’s Microsoft Office 365 tenancy:

	<#
	.Synopsis
	Obtain an app-only access token from ACS.
	.DESCRIPTION
	Retrieves an app-only access token from ACS to call the specified principal
	at the specified targetHost. The targetHost must be registered for target principal. If specified realm is
	null, the "Realm" setting in web.config will be used instead
	.EXAMPLE
	Get-SPOAccessToken -Url "https://contoso.sharepoint.com/_api/web" -ClientId "" -ClientSecret ""
	#>
	Function Get-SPOAccessToken([string]$ClientId,[string]$ClientSecret,[Uri]$WebUri){
	$SharePointPrincipal = "00000003-0000-0ff1-ce00-000000000000"
	$realm = GetRealmFromTargetUrl –TargetApplicationUri $WebUri
	$accessToken = GetAppOnlyAccessToken –ClientId $ClientId –ClientSecret $ClientSecret –TargetPrincipalName $SharePointPrincipal –TargetHost $WebUri.Authority –TargetRealm $realm
	return $accessToken.access_token
	}

	function GetRealmFromTargetUrl([Uri]$TargetApplicationUri)
	{

	$url = $WebUrl + "/_vti_bin/client.svc"
	$headers = @{}
	$headers.Add('Authorization','Bearer')
	try {
	$response = Invoke-WebRequest –Uri $TargetApplicationUri –Headers $headers –Method Get
	}
	catch [Net.WebException] {
	$authResponseHeader = $_.Exception.Response.Headers["WWW-Authenticate"]
	#$bearerKey = "Bearer realm="
	$bearer = $authResponseHeader.Split(",")[0]
	$targetRealm = $bearer.Split("=")[1]
	return $targetRealm.Substring(1,$targetRealm.Length–2)
	}
	return $null
	}


	Function GetAppOnlyAccessToken([string]$ClientId,[string]$ClientSecret,[string]$TargetPrincipalName,[string]$TargetHost,[string]$TargetRealm)
	{
	$resource = GetFormattedPrincipal –PrincipalName $TargetPrincipalName –HostName $TargetHost –Realm $TargetRealm
	$ClientId = GetFormattedPrincipal –PrincipalName $ClientId –Realm $TargetRealm
	$contentType = 'application/x-www-form-urlencoded'
	$stsUrl = GetSecurityTokenServiceUrl –Realm $TargetRealm
	$oauth2Request = CreateAccessTokenRequestWithClientCredentials –ClientId $ClientId –ClientSecret $ClientSecret –Scope $resource
	$oauth2Response = Invoke-RestMethod –Method Post –Uri $stsUrl –ContentType $contentType –Body $oauth2Request
	return $oauth2Response
	}


	Function GetSecurityTokenServiceUrl([string]$Realm)
	{
	return "https://accounts.accesscontrol.windows.net/$Realm/tokens/OAuth/2"
	}


	Function CreateAccessTokenRequestWithClientCredentials([string]$ClientId,[string]$ClientSecret,[string]$Scope)
	{
	$oauth2Request = @{
	'grant_type' = 'client_credentials';
	'client_id' = $ClientId;
	'client_secret' = $ClientSecret;
	'scope' = $Scope;
	'resource' = $Scope
	}
	return $oauth2Request
	}

	function GetFormattedPrincipal([string]$PrincipalName, [string]$HostName, [string]$Realm)
	{
	if ($HostName)
	{
	return "$PrincipalName/$HostName@$Realm"
	}
	return "$PrincipalName@$Realm"
	}

view raw

Get-SPOAccessToken.ps1

hosted with ❤ by GitHub

Get-SPOAccessToken function is intended for requesting an access token from Azure ACS, it accepts Client Id and Client Secret parameters that are generated while App registration with Azure ACS (see “How to register App” for a more details).

Using Invoke-RestMethod in Office 365

Invoke-SPORestMethod function demonstrates how to include the access token to make a REST API call to SharePoint, passing the OAuth access token in the HTTP Authorization header:

	.\Get-SPOAccessToken.ps1

	<#
	.Synopsis
	Sends an HTTP or HTTPS request to a SharePoint Online REST-compliant web service.
	.DESCRIPTION
	This function sends an HTTP or HTTPS request to a Representational State
	Transfer (REST)-compliant ("RESTful") SharePoint Online web service.
	.EXAMPLE
	Invoke-SPORestMethod -Uri "https://contoso.sharepoint.com/_api/web" -AccessToken $accessToken
	#>
	Function Invoke-SPORestMethod()
	{
	Param(
	[Uri]$Uri,
	[Object]$Body,
	[Hashtable]$Headers,
	[Microsoft.PowerShell.Commands.WebRequestMethod]$Method = [Microsoft.PowerShell.Commands.WebRequestMethod]::Get,
	[string]$AccessToken
	)
	$contentType = 'application/json;odata=verbose'
	if(-not $Headers) {
	$Headers = @{}
	}
	$Headers["Accept"] = "application/json;odata=verbose"
	$Headers.Add('Authorization','Bearer ' + $AccessToken)
	Invoke-RestMethod –Method $Method –Uri $Uri –ContentType $contentType –Headers $Headers –Body $Body
	}

view raw

Invoke-SPORestMethod.ps1

hosted with ❤ by GitHub

Examples

The following example demonstrates how to retrieve List resource properties:

	param (
	$ClientId,
	$ClientSecret,
	$WebUri,
	$ListTitle
	)

	.\Get-SPOAccessToken.ps1




	Function Get-SPOList([Uri]$WebUri,[string]$ClientId,[string]$ClientSecret,[Uri]$ListTitle)
	{
	$accessToken = Get-SPOAccessToken –ClientId $ClientId –ClientSecret $ClientSecret –WebUri $WebUri
	$endpointUri = "$WebUri/_api/web/lists/getbytitle('$ListTitle')"
	$data = Invoke-SPORestMethod –Uri $endpointUri –AccessToken $accessToken
	return $data.d
	}


	Get-SPOList –WebUri $WebUri –ClientId $ClientId –ClientSecret $ClientSecret –ListTitle $ListTitle

view raw

Get-SPOList.ps1

hosted with ❤ by GitHub

But before running the specified script we need to perform one more step in order to grant permissions to the app principal otherwise the unauthorized error will occur as shown on picture below:

Navigate to http://<SharePointWebsite>/_layouts/15/AppInv.aspx
Look up the app based on the Client ID that you just generated and click Lookup, it will find the app principal. Then paste the AppPermissionRequests XML into the Permissions text box and click Create
Once you click Create, the Trust dialog will appear, click Trust

That’s it. Now, after executing the specified script, the output will look like shown below

How to register App

Below is provided a step by step instruction how to register an App, for a complete guide follow this article:

To create the app identity, navigate to http://<SharePointWebsite>/_layouts/15/AppRegNew.aspx on the tenancy or farm
Enter values for the form fields as shown below on picture

where
App ID: App ID, also known as client ID, is a GUID that can be generated (when you click Generate) or pasted into AppRegNew.aspx. The value must be unique for each app, and must be lower case
App Secret: The app secret, also known as the client secret, is an opaque string. It is generated on the AppRegNew.aspx page by using the Generate button. The following is an example of an app secret: Ywjaoz7DJBGhoLQ2t0IbVCA5pfqqI722ZIVt+ENLk0g=
Title: Choose your own user-friendly title; for example, PowerShell Console
App Domain: The host name of the remote component of the app for SharePoint
Redirect URI: The endpoint in your remote application or service to which ACS sends an authentication code
Click Create on the form. The page will reload and show a confirmation of the values you entered as shown on picture below
Save Client Id and Client Secret values. After that you could verify whether Get-SPOAccessToken function returns access token. The picture below shows the output after executing the command:
Get-SPOAccessToken -ClientId “1523cf05-b437-4e73-9ad1-a652da8f2ae5” -ClientSecret “Ywjaoz7DJBGhoLQ2t0IbVCA5pfqqI722ZIVt+ENLk0g=” -WebUri “https://contoso.sharepoint.com/”