Do you have a business requirement to block the download of specific files or file types from OneDrive? What about detailed auditing to understand what files are downloaded or viewed? Well, today is your lucky day – because this is all possible with Microsoft security technology and takes minutes to create. I’m going to walk you through how to do this, and in return, make you look like an IT Rockstar to your organization!
Note: There are other methods to restrict those files from being synchronized using the OneDrive desktop client, we won’t cover those today however (but are accessible in the SharePoint Online Admin Portal)
IMPORTANT: Nothing is 100% secure and it’s all about defense in depth. If you want that extra ply in the tinfoil hat, I highly recommend protecting and encrypting those files with Azure Information Protection as that extra layer of protection.
Also, it’s important to note,the method below at the time of this writing is in public preview.
My organization, an engineering firm, designs buildings for their commercial and government clients. These design plans often contain additional documentation that are in the form of a .PDF and sometimes photos in the form of a .JPEG (or .jpg).
These .PDF and .JPEG files are highly confidential and thus we want to make sure they never leave OneDrive in Office 365 and can only be viewed in a web browser. In other words, we need to block the ability for an end-user to download these two file types from OneDrive. So, how do we do this?
Azure Active Directory Conditional Access and Microsoft Cloud App Security Conditional Access App Control to the rescue! These two products are part of Microsoft 365 E5 or EMS E5 or my new favorite: Microsoft 365 E3 + Identity & Threat Protection. The two products that make up this solution are Azure Active Directory and Microsoft Cloud App Security.
Let’s take a look at how to do this!
Step 1: Create a Azure AD Conditional Access Policy
From within the Azure portal -> Azure Active Directory -> Conditional Access -> New Policy I am going to create a new policy. First, give it a name, “OneDrive Block JPEG and PDF”. Next, assign it to specific users or groups of users. For testing purposes I’m assigning to Adele Vance (IMPORANT: Don’t lock yourself out! Careful planning is required when assigning to all users).
Next, add Office 365 SharePoint Online as the application to be applied to:
Under Session, select Use Conditional Access App Control, then click Done.
Next, click Enable policy to enable the policy and click Create.
Step 2: Launch OneDrive (via portal.office.com)
Wait 15 minutes for the new Conditional Access policy to propagate. Next, open a new browsing session (inprivate or on another computer) and logon as the test user that was just assigned to. In my case, I am going to sign in to portal.office.com in an in-private session as Adele. Browse to OneDrive in the Office portal and open a file in the web browser. Sign out of this web browsing session when done.
Step 3: Configure Microsoft Cloud App Security
We now need to configure Microsoft Cloud App Security (CAS) and create the appropriate policies.
To start, validate that OneDrive is a connected application by browsing to http://portal.cloudappsecurity.com and navigating to Investigate -> Connected Apps. Notice OneDrive for Business will be listed and connected: (Yes, you can also connect CAS to G-Suite, Box, and other apps!)
Next, click on Conditional Access App Control apps and OneDrive for Business will also be displayed:
Step 4: Create the Session Policy in Microsoft Cloud App Security
Next, we need to create the policy that will provide the session control when Adele uses OneDrive in the Office 365 Portal. To do this navigate to Control -> Policies, click New Policy and select Session Policy.
Let’s give the policy a name and description:
Next, under Session control type select Control file download (with DLP). Under Activity source and activity filters configure configure them per the screenshot below
Scroll down (leave content inspection blank and don’t check the box) and under Actions select Block. OPTIONAL: Configure user email notification or customize block message. When finished at the bottom of the page click Create.
Step 5: Test the User Experience
Now it’s time to test and validate this is the behavior we want. Open a new web browsing session and login as the test user. In my case, I’m going to login to portal.office365.com using Adele Vance’s account in an in-private browser session.
Once signed in, navigate to OneDrive in the Office 365 Portal. When you click on OneDrive, notice the splash page indicating this site is being monitored!
Also, notice the address of the site. It’s being proxied through CAS.MS indicating this session is being controlled by Cloud App Security:
Click Continue to Microsoft OneDrive for Business
Notice I have two files, a .PDF and a .JPEG in the OneDrive folder:
Hover the cursor over the PDF and click the ellipses, and select Download
Notice, the file download is blocked with a splash message indicating it’s blocked!
Now, I know what you’re wondering, “Matt what’s that file it wants to save?” When I open that file, it’s just a warning:
From here, within the Cloud App Security Portal, I can audit the activity and receive additional details around this attempt:
Additional alerting can be generated, with an email or SMS notification sent. Imagine having CAS send an email to your ticket system so you can be notified of this violation? What about sending to your SIEM? Endless possibilities.
As you can see, with a bit of an open mind and creativity, possibilities to build true security solutions that lead to a real business outcome, is entirely possible. The total time spent creating this solution was 10 minutes. Don’t forget to test (which obviously will add to the 10 minutes) all the scenarios for this. Questions? Let me know in the comments below!
Enjoy and help us make this world more secure! –Matt Soseman