MCAS – Device Identity via Certificates and Progressive Web Apps

I have a customer scenario where we needed to explore leveraging certificates in order to identify corporate Windows 10 machines for the purposes of preventing corporate data from being downloaded from O365 services to non-corporate assets. There are a few different ways that this can be tackled, however, other routes proved to be dead ends for various technical reasons, so it was landed on leveraging device certificates via MCAS in order to control data spillage.

https://docs.microsoft.com/en-us/cloud-app-security/proxy-intro-aad#managed-device-identification

Utilizing device identity via MCAS with certificates does mean that your user traffic for the devices with certs (and without) will have to go through Conditional Access App Control for all the sessions (reverse proxy). It took some trial and error to get this to work. I do want to point out that this method is technically not supported for O365 services as proxying traffic for O365 can impact the user experience and impact the SLAs.

Scenario

  1. Hybrid Azure AD Joined machines will be allowed to access corporate resources unfettered
  2. Corporate devices without HAADJ will have a certificate deployed to them. The certificate will be used in a session control policy via MCAS to allow the device to download corporate data from O365
  3. Devices without certificates (corporate or otherwise) will be treated as untrusted. They will still be allowed to access corporate data (as allowed by other conditional access policies outside of the scope of this blog) but they will not be allowed to download data from O365

Hybrid Azure AD Join: https://docs.microsoft.com/en-us/azure/active-directory/devices/hybrid-azuread-join-managed-domains

HAADJ is a function of AAD and AAD Connect where machines are effectively synced from your on-prem AD into AAD. If a device has been synced, that information can be leveraged within conditional access policies as a piece of criteria during authentication. At this point, you can decide to request additional controls (such as MFA), whether to allow or block access, and even potentially drop the session into the MCAS reverse proxy for additional session control. In order to cover these scenarios for my lab, I configured two conditional access policies – the first policy enables the scenario I want to allow and the second policy blocks the rest.

Policy #1 – ITOp5 – RP – Block Download without Cert

My target user is ITOp5 in my lab. This is a standard user account that has been synced from the on-prem AD via AADC.

Apps – I went lazy for this and simply selected all apps

Conditions

Only targeting Windows since this was my client’s use case

Only allowing browser – the reverse proxy only works with the browser. Since that is the case, we are only going to allow browser based access.

For device state, we want to include all devices…

But exclude devices that are actually compliant (this is a 2-fer and how we catch all 3 scenarios with 2 conditional access policies)

In this case, I am only granting access, but additional controls and requirements could be implemented

Lastly, I am putting my user into Conditional Access App Control aka the reverse proxy if the above criteria is met

Policy #2 – ITOp5 – Block Non Browser on Non Compliant Devices

This policy is almost exactly the same with two exceptions. The first exception is Client apps – in the allow policy this is set to Browser. In the block, this is set to everything else

Lastly, rather than grant the policy is set to block

With these CAPs, folks coming in on a Windows machine either need to be marked compliant via Intune or have their devices HAADJ in order to have full access to all resources and to be able to use thick client apps. Everyone else is going to be routed into the reverse proxy. For the proxied users, if they have a certificate, they will be allowed to download data to their endpoint. If they do not have the certificate, the user will be forced to leverage the cloud-based tools within O365 to collaborate and work with corporate data.

MCAS – Session Control Policy

Configuration here is straight forward. It is a Control file download (with DLP) policy. For the criteria, it is going to specifically target my test user and block if the user does not have the certificate. Given that, the criteria is going to be targeted to the devices that do not have a valid certificate

For the file criteria I could get specific and try to stop content specifically with PII, financial data, HIPAA, etc. but I really want to stop all egress in order to prevent non corporate devices from coming under potential eDiscovery. With that, the criteria for what I am looking for is left blank to catch all content

Lastly, the policy is set to block with a custom block message. Notifications are optional

This is it! Other than getting certificates to the end points a HAADJ joined machine should be allowed to connect regardless of app. An unmanaged device should be routed into the reverse proxy and data egress should be blocked.

Verified this was the case in lab with this configuration and I have the behavior I want.

Device Certificate

For the root cert that goes into MCAS, it just needs to be the base64 encoded public cert (.CER) file that you import. Simply export, import and done.

https://docs.microsoft.com/en-us/cloud-app-security/proxy-intro-aad#client-certificate-authenticated-devices

The device identification mechanism can request authentication from relevant devices using client certificates. You can either use existing client certificates already deployed in your organization or roll out new client certificates to managed devices. You then use the presence of those certificates to set access and session policies.

Not a ton of detail – this suggests that a device certificate on the endpoint should be sufficient. There is a little more detail further in the article that suggests an SSL certificate. From my testing, I have found that the ultimate requirement for this certificate itself is that it has Client Authentication in the Extended Key Usage. Lastly, the certificate needs to be installed into the user’s personal store – the local machine store will not work.

Walking through the process using a MS PKI to deploy the cert – this is probably not the most efficient and is definitely not the route you would go to deploy certificates in bulk to end users. This is purely the process I used and I wanted to document it here to show how I get a cert manually into the end user store.

Certificate Authority Template

I duplicated the Web Server Template, added Server and Client Authentication in for Extended Key Usage. I then published the template for use within in CA.

End User Certificate Request

I am doing this manually with a certificate signing request. On the endpoint, create a request.ini with the following and run the certreq command

I took the output in the csr6.req file and pasted it into the CertSrv site requesting a certificate utilizing the custom Web Server template I created with Client and Server authentication in the EKU

I downloaded the .cer file, copied it to the target workstation and than ran the following to import the cert directly into the user’s personal store

At this point, we can validate that the browser is able to see the certificate by going into Settings -> Search for Certificates

I can also see the certificate if I go into the certificate manager and look in the user’s personal store

Now, when I browse to O365 I hit the login page, I provide my username and then password. Immediately post password, I get the following

I select my certificate and hit ok. This allows me into O365. At this point, I can browse anywhere in the cloud service and download files since I now have a valid certificate that I presented immediately post authentication to the reverse proxy. Given how the conditional access policies are configured, this prevents all non browser apps from accessing the services. This means that only machines that are HAADJ or Intune managed (and compliant) are allowed to use thick client applications. Machines outside of that are allowed to access O365 but are not allowed to download content unless they have the certificate. Even the machines with the certificate are forced into use the web apps for O365 as applications like Outlook, Teams, OneDrive sync, etc. are not allowed to connect directly to the service. These users with the cert would be able to download content directly to their machine and work on it offline but they would then have to re-upload the content via the browser when it came time.

Progressive Web Apps

PWAs introduce and interesting option for enabling productivity but still flowing the traffic through and controlling data egress via the reverse proxy. This is very slick and I use PWAs for sites/apps both for O365 and otherwise.

https://docs.microsoft.com/en-us/microsoft-edge/progressive-web-apps-edgehtml/get-started

That’s the dev resource for PWAs so what does this actually mean for O365? You can pin specific sites (like Outlook and Teams) as PWAs on a machine. They are still web apps, but they get better performance, they get extra caching and they all the nice little tweaks to make them _almost_ as good as the thick client. Keep in mind, the _almost_ is my opinion and it does depend on the app. For example, I use a music stream service on home machine. The app for the stream service is a resource hog. I pin the PWA for that app and it is significantly lighter on resources on my machine. I think each web app (both in and out of the MS ecosystem) warrants exploring.

Edge Chromium

Summary

Requirements for leveraging Device Certificate via MCAS to block downloads of corporate data to unmanaged devices

Device Certificate Requirement

  • Certificate has Client Authentication in the Extended Key Usage
  • Certificate is located in the user’s personal store

Conditional Access

  • Non browser apps are blocked for the targeted users – required or the session will not be routed into the reverse proxy and this negates the usage of the certificate in order to allow/block downloads
  • Browser access is set to be granted through Conditional Access App Control

MCAS Session Control Policy

  • Block downloads of all data to devices that are not tagged with having the certificate

Resultant Scenario

  • All users off network and/or in scope would be forced to use web-based clients for a connected experience. This means thick client apps such as Outlook, OneDrive Sync, Teams and even tools like Excel and Word will not work when connecting directly to cloud data
  • All users with a valid certificate would be able to download the data and work with tools like Word and Excel in an offline mode and then reupload the data into SharePoint or OneDrive
  • Progressive Web Apps provide a nice alternative to thick clients and/but they do not allow local disk access
  • End users would still have full access and be able to use the web-based collaboration tools within the cloud without allowing egress of data to their endpoints

Happy MCAS-ing!

Hack Job, MCAS

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