Tuesday 29 January 2013

SharePoint 2013: Elevate User Access with App Only Policy

In SharePoint 2013 Apps, the authorization is handled by a 2 part mechanism. Along with the current user's permissions, the app permissions are also taken into account. User permissions and app permissions are two separate entities which dictate whether an app is allowed to perform a certain action.

When you build an app, you can specify that what all permissions are going to be required for the app to run successfully. You have to specify this in the App Manifest under the Permissions tab.

(Click Image to Zoom)

When a user installs an app, he can only grant permissions to the app up to the level which he has. So if a user has "Read" permissions on a list, he cannot grant the app "Full Control" on the same list.

There are basically 3 types of app authorizations or authorization policies in SharePoint 2013.

1) User Only Policy: In this policy, only the user permissions are taken into account. App permissions are ignored. It is checked whether the user has the permissions to perform the action and if yes, then he/she is allowed to perform the action.

2) App + User Policy: The user as well as the app permissions are taken into account. This is an AND operation. If both the user and the app have the permission to perform the action,only then the action is allowed.

3) App Only Policy: Only the permissions granted to the app are taken into account. User permissions are ignored. So even if the current user accessing the app does not have appropriate permission, the action is allowed provided the app has the permission to do so.

The App Only policy works by using a separate OAuth token which runs under the user "SHAREPOINT/App" much like SPSecurity.RunWithElevatedPriviledges() runs under the "SHAREPOINT\System" user (app pool account) in the Server Object Model.

Since OAuth is required to generate the app token, the App-Only policy can only be used in the Auto-hosted and Provider-Hosted apps. SharePoint-Hosted apps cannot use the app only policy.

To enable the App Only policy in your app, you have to add the "AllowAppOnlyPolicy" attribute to your "AppPermissions" element in the App Manifest:

Note: Although you can add this attribute to the App Manifest of the SharePoint-Hosted apps, it will be ignored as the App Only policy is not supported in SharePoint hosted apps.

Enough chit chat, lets see how we can actually utilize the App Only policy to elevate user permissions. I have written the following code in the code behind of the Default.aspx of an Auto-Hosted App:

As you can see, the ClientContext which is opened with the appOnlyAccessToken will run with the identity of the SHAREPOINT\App account. This is a very good practice to remember even when using RunWithElevatedPreviledges in the Server Object Model. You can switch on/switch off database calls with app only token. This means that if you need to make only one operation with the elevated token, then you can do that and get back to using the current user's token in one action.

Saturday 12 January 2013

Authenticating .NET CSOM in SharePoint Online

The process of authenticating the .NET Client Object Model in SharePoint Online has been greatly simplified in the 2013 version. Earlier (2010), you had to go through a lot of steps for doing the exact same thing. You  had to open a web browser instance, force the user to enter the credentials in the browser and then grab that  cookie from Internet Explorer and pass it to the .NET CSOM. If you are interested, the code of the old authentication method can be found here:

Fortunately in the 2013 version, the process is a lot simplified by the introduction of the SharePointOnlineCredentials class. This class is part of the Microsoft.SharePoint.Client.dll itself so you don't have to include an extra assembly in your application. All you have to do is pass an instance of the SharePointOnlineCredentials class to the ClientContext.Credentials property. The SharePointOnlineCredentials takes in two parameters which are your Login Email for your Office 365 SharePoint site and your password in the SecureString format.

You can grab all the required SharePoint 2013 client assemblies from here:

For this particular demo, you will need to reference the Microsoft.SharePoint.Client.dll and the Microsoft.SharePoint.Client.Runtime.dll assemblies in your project.

 The code is as follows: