Most Microsoft-based Hybrid Identity implementations use Active Directory Federation Services (AD FS) Servers, Web Application Proxies and Azure AD Connect installations. In this series, labeled Hardening Hybrid Identity, we’re looking at hardening these implementations, using recommended practices.

In this part of the series, we’ll look at the required Hybrid Identity URLs that you want to add to the Trusted Sites list in Internet Explorer.

Note:
This is the second part for adding Microsoft Cloud URLs to Internet Explorer’s zone. In this part we look at the Trusted Sites zone. In the previous part we looked at the Local Intranet zone.

Note:
Adding URLs to the Trusted Sites zone for Internet Explorer, also applies to Microsoft Edge.

Why look at the Trusted Sites?

Hybrid Identity enables functionality for people using on-premises user accounts, leveraging Azure Active Directory as an additional identity platform. By default, Azure AD is the identity platform for Microsoft Cloud services, like Exchange Online, SharePoint Online and Azure.

By adding the URLs for these services to the Trusted Sites list, we enable a seamless user experience without browser prompts or hick-ups to these services.

Internet Explorer offers built-in zones. Per zone, Internet Explorer is allowed specific functionality. Restricted Sites is the most restricted zone and Internet Explorer deploys the maximum safeguards and fewer secure features (like Windows Integrated Authentication) are enabled.

The Trusted Sites zone, by default, offers a medium level of security.

Possible negative impact (What could go wrong?)

Internet Explorer’s zones are defined with specific default settings to lower the security features for websites added to these zones.

When you use a Group Policy object to add websites that don’t need the functionality of the Trusted Sites zone to the zone, the systems in scope for the Group Policy object are opened up to these websites. This may result in unwanted behavior of the browser such as browser hijacks, identity theft and remote code executions, for example when you mistype the URLs or when DNS is compromised.

While this does not represent a clear and immediate danger, it is a situation to avoid.

Getting ready

The best way to manage Internet Explorer zones is to use Group Policy.

To create a Group Policy object, manage settings for the Group Policy object and link it to an Organizational Unit, Active Directory site and/or Active Directory domain, log into a system with the Group Policy Management Console (GPMC) installed with an account that is either:

• A member of the Domain Admins group, or;
• The current owner of the Group Policy Object, and have the Link GPOs permission on the Organizational Unit(s), Site(s) and/or Domain(s) where the Group Policy Object is to be linked, or;
• Delegated the Edit Settings or Edit settings, delete and modify security permission on the GPO, and have the Link GPOs permission on the Organizational Unit(s), Site(s) and/or Domain(s) where the Group Policy Object is to be linked.

The URLs to add

You’ll want to add the following URLs to the Trusted Sites zone, depending on the way you’ve setup your Hybrid Identity implementation:

*.live.com

*.microsoft.com

*.microsoftonline.com

*.windows.net

ajax.aspnetcdn.com

microsoft.com

microsoftline.com

microsoftonline-p.net

onmicrosoft.com

The above URLs are used in Hybrid Identity environments. While they overlap with some of the URLs for the Local Intranet Zone, these URLs allow side services to work properly, too.

*.msappproxy.net

Web applications that you integrate with Azure Active Directory through the Azure AD Application Proxy are published using https://*.msappproxy.net URLs. Add the above wildcard URL to the Trusted Sites list, when you’ve deployed or are planning to deploy Azure AD App Proxy. If you use vanity names for Azure AD App Proxied applications, add these to the Trusted Sites list, as well.

Other Office 365 services

Most  Hybrid Identity implementations are used to allow access to Office 365 only. Last year, 65% of Hybrid Identity implementations are used to unlock access to one or more Office 365 services, like Exchange Online, SharePoint Online, OneDrive for Business and Teams, only. This blogpost focuses on the Hybrid Identity URLs, but you might want to add more Office 365 URLs and IP address ranges to the Trusted Sites list as you deploy, roll out and use Office 365 services. You can use this (mostly outdated) Windows PowerShell script to perform that action, if you need.

How to add the URLs to the Trusted Sites zone

To add the URLs to the Trusted Sites zone, perform these steps:

• Log into a system with the Group Policy Management Console (GPMC) installed.
• Open the Group Policy Management Console (gpmc.msc)
• In the left pane, navigate to the Group Policy objects node.
• Locate the Group Policy Object that you want to use and select it, or right-click the Group Policy Objects node and select New from the menu.
• Right-click the Group Policy object and select Edit… from the menu.
  The Group Policy Management Editor window appears.
• In the main pane of the Group Policy Management Editor window, expand the Computer Configuration node, then Policies, Administrative Templates, Windows Components, Internet Explorer, Internet Control Panel and then the Security Page node.

• In the main pane, double-click the Sites to Zone Assignment List setting.
• Enable the Group Policy setting by selecting the Enabled option in the top pane.
• Click the Show… button in the left pane.
  The Show Contents window appears.
• Add the above URLs to the Trusted Sites zone by entering the URL in the Value name column and the number 2 in the Value column for each of the URLs.
• Click OK when done.
• Close the Group Policy Editor window.
• In the left navigation pane of the Group Policy Management Console, navigate to the Organization Unit (OU) where you want to link the Group Policy object.
• Right-click the OU and select Link an existing GPO… from the menu.
• In the Select GPO window, select the GPO.
• Click OK to link the GPO.

Repeat the last three steps to link the GPO to all OUs that require it. Take Block Inheritance into account for OUs by linking the GPO specifically to include all people in scope.

Concluding

To enable functionality in a Hybrid Identity implementation, we need to open up the web browser to allow functionality for specific web addresses. By enabling the right URLs we minimize our efforts in enabling the functionality and also minimize the negative effect on browser security.

There is no need to add all the URLs to specific Internet Explorer zones, when you don’t need to functionality. However, do not forget to add the specific URLs when you enable specific functionality like the Azure AD Application Proxy and remove specific URLs when you move away from specific functionality.

Further reading

Office 365 URLs and IP address ranges
Group Policy – Internet Explorer Security Zones
Add Site to Local Intranet Zone Group Policy