This blog post is the third blog post of a series dedicated to Zero Trust security in Microsoft 365.

In the first two blog posts, we set the basics by going over the free features of Azure AD that can be implemented in an organization that starts its Zero Trust journey in Microsoft 365. We went over the Security Defaults, the per-user MFA settings and some Azure AD settings that allowed us to improve our default security posture when we create a Microsoft 365 environment.

Previous blog posts:

• Enforce Zero Trust in Microsoft 365 – Part 1: Setting the basics;
• Enforce Zero Trust in Microsoft 365 – Part 2: Protect against external users and applications.

Introduction

In this blog post, we will see what Azure AD Conditional Access is, how it can be used to further improve security and introduce its integration capabilities with other services.

As a reminder, our organization has just started with Microsoft 365. However, we have decided to go for Microsoft 365 for our production environment. Therefore, we want to have a look at a more advanced feature, Azure AD Conditional Access policies. This feature requires an Azure AD Premium P1 license which comes as a standalone license or which is also included in some Microsoft 365 licenses (Microsoft 365 E3/A3/G3/F1/F3, Enterprise Mobility & Security E3, Microsoft 365 Business Premium, and higher licenses). Note that one license should be assigned to each user in scope of any Conditional Access policies.

Azure AD Conditional Access allows to take identity-driven signals to make decisions and enforce policies. They can be seen as if-then statements. For instance, if a user wants to access SharePoint Online, which is a Microsoft cloud application that can be integrated in such policies, the user, more specifically, the user’s request, is required to meet specific requirements, defined in those policies. Let’s now see what the capabilities of those policies are.

Conditional Access

This part will be more theoretical to make sure everyone has the basics. Therefore, if you are already familiar to Azure AD Conditional Access Policies, you can directly jump to the next section for the implementation where we go over some prerequisites and important actions that need to be done to avoid troubles when setting up those policies based on our hands-on experience.

Conditional Access signals

As we have seen, signals will be considered to make a decision. It is possible to configure the following signals:

• User, group membership or workload identities (also known as service principals or managed identities in Azure): It is possible to target or exclude specific users, groups, or workload identities from a Conditional Access policy;
• Cloud apps or actions: Specific cloud applications such as Office 365, the Microsoft Azure Management, Microsoft Teams applications, etc. can be targeted by a policy. Moreover, specific user actions like registering security information (registering to MFA or Self-Service Password Reset) or joining devices can be included as well. Finally, authentication context can also be included. Authentication contexts are a bit different as they can be used to protect specific sensitive resources accessed by users or user actions in the environment. We will discuss authentication contexts in details in later blog post;
• Conditions: With an Azure AD Premium P1 license, specific conditions can be set. This includes:
  • The device platforms: Android, iPhone, Windows Phone, Windows, macOS and Linux;
  • The locations: Conditional Access works with Named Locations which can include country/countries or IP address(es) that can be seen as trusted or untrusted;
  • The client apps: client apps which support modern authentication: Browser and Mobile apps and desktop clients; and legacy authentication clients: Exchange ActiveSync clients and other clients;
  • Filter for devices: allows to target or exclude devices based on their attributes such as compliance status in the device management solution, if the device is managed in Microsoft Endpoint Manager or on-premises, or registered in Azure AD, as well as custom attributes that have been set on devices;
  • Note that these conditions need to be all matched for the policy to apply. If a condition such as the location is excluded and match an attempt to access an application, the policy will not apply. Finally, if multiple policies matched, they will all apply, and access controls will be combined (the most restrictive action will be applied in case of conflicts).

Conditional Access access controls

Then, we have the access controls which are divided into two main categories, the “grant” and the “session” controls. These access controls define the “then do this” part of the Conditional Access policy (if all conditions have matched as mentioned previously). They can be used to allow or block access, require MFA, require the device to be compliant or managed as well as other more specific controls.

Grant controls

• Block access: if all conditions have matched, then block access;
• Grant access: if all conditions have matched, then grant access and optionally apply one or more of the following controls:
  • No controls are checked: Single-Factor Authentication is allowed, and no other access controls are required;
  • Require Multi-Factor Authentications;
  • Require authentication strength: allows to specify which authentication method is required for accessing the application;
  • Require device to be marked as compliant: this control requires devices to be compliant in Intune. If the device is not compliant, the user will be prompted to make the device compliant;
  • Require Hybrid Azure AD joined devices: this control requires devices to be hybrid Azure AD joined meaning that devices must be joined from an on-premises Active Directory. This should be used if devices are properly managed on-premises with Group Policy Objects or Microsoft Endpoint Configuration Manager, formerly SCCM, for example;
  • Require approved client apps: approved client apps are defined by Microsoft and represent applications that supports modern authentication;
  • Require app protection policy: app protection policies can be configured in Microsoft Intune as part of Mobile Application Management. This control does not require mobile devices to be enrolled in Intune and therefore work with bring-your-own-device (BYOD) scenarios;
  • Require password change;
  • For multiple controls (when multiple of the aforementioned controls are selected):
    • Require all the selected controls;
    • Require one of the selected controls.

Session controls

• Use app enforced restrictions: app enforced restrictions require Azure AD to pass device information to the selected cloud app to know if a connection is from a compliant or domain-joined device to adapt the user experience. This control only works with Office 365, SharePoint Online and Exchange Online. We will see later how this control can be used;
• Use Conditional Access App Control: this is the topic of a later blog post, but it allows to enforce specific controls for different cloud apps with Microsoft Defender for Cloud Apps;
• Sign-in frequency: this control defines how often users are required to sign in again every (x hours or days). The default period is 90 days;
• Persistent browser session: when a persistent session is allowed, users remain signed in even after closing and reopening their browser window;
• Customize continuous access evaluation: continuous access evaluation (CAE) allows access tokens to be revoked based on specific critical events in near real time. This control can be used to disable CAE. Indeed, CAE is enabled by default in most cases (CAE migration);
• Disable resilience defaults: when enabled, which is the case by default, this setting allows to extend access to existing session while enforcing Conditional Access policies. If the policy can’t be evaluated, access is determined by resilience settings. On the other hand, if disabled, access is denied once the session expires;
• Require token protection for sign-in sessions: this new capability has been designed to reduce attacks using token theft (stealing a token, hijacking or replay attack) by creating a cryptographically secure tie between the token and the device it is issued to. At the time of writing, token protection is in preview and only supports desktop applications accessing Exchange Online and SharePoint Online on Windows devices. Other scenarios will be blocked. More information can be found here.

Conditional Access implementation

Before getting started with the implementation of Conditional Access policies, there are a few important considerations. Indeed, the following points might determine if our Zero Trust journey is a success or a failure in certain circumstances.

Per-user MFA settings

If you decided to go for the per-user MFA settings during the first blog post, you might consider the following:

• As mentioned before, Conditional Access policies can be used to enforce a sign-in frequency. However, this can also be achieved using the ‘remember multi-authentication’ setting. If both settings are configured, the sign-in frequency enforced on end users will be a mix of both configuration and will therefore lead to prompting users unexpectedly;
• If trusted IPs, which require an Azure AD Premium P1 license, have been configured in the per-user MFA settings, they will conflict with named locations in Azure AD Conditional Access. Named locations allow you to define locations based on countries or IP address ranges that can then be used to allow or block access in policies. Besides that, if possible, named locations should be used because they allow more fine-grained configurations as they do not automatically apply to all users and in all scenarios;
• Finally, before enforcing MFA with Conditional Access policies, all users should have their MFA status set to disabled.

Security Defaults

Moreover, if you opted for the Security Defaults, it needs to be disabled as they can’t be used together.

How and where to start?

Now that we have some concepts about Conditional Access and some considerations for the implementation, we can start with planning the implementation of our policies. First, we need to ensure that we know what we want to achieve and what the current situation is. In our case, we first want to enforce MFA for all users to prevent brute force and protect against simple phishing attacks.

However, there might be some user accounts used as services accounts in our environment, such as the on-premises directory synchronization account for hybrid deployments, which can’t perform multi-factor authentication. Therefore, we recommend identifying these accounts and excluding them from the Conditional Access policy. However, because MFA would not be enforced on these accounts, they are inherently less secure and prone to brute force attacks. For that purpose, Named Locations could be used to only allow these service accounts to login from a defined trusted location such as the on-premises network (this now requires an additional license for each workload identity that you want to protect: Microsoft Entra Workload Identities license). Except for the directory synchronization account, we do not recommend the use of user accounts as service accounts. Other solutions are provided by Microsoft to manage applications in Azure in a more secure way.

Our first policy could be configured as follows (note that using a naming convention for Conditional Access policies is a best practice as it eases management):

1. Assign the policy to all users (which includes all tenant members as well as external users) and exclude service accounts (emergency/break-the-glass accounts might also need to be excluded):

2. Enforce the policy for all cloud applications:

3. Require MFA and enforce a sign-in frequency of 7 days:

4. Configure the policy in report-only first

We always recommend configuring Conditional Access policies in report-only mode before enabling them. The report-only feature will generate logs the same way as if the policies were enabled. This will allow us to assess any potential impact on service accounts, on users, etc. After a few weeks, if no impact has been discovered, the policy can be switched to ‘On’. Note that there might be some cases where you may want to shorten or even skip this validation period.

These logs can be easily access in the ‘Insights and reporting‘ panel in Conditional Access:

Conclusion

In this third blog post, we learned about Conditional Access policies by going over a quick introduction on Conditional Access signals and access controls. Then, we went over some implementation considerations to make sure our Zero Trust journey is a success by preventing unexpected behaviors and any impact on end users. Finally, we implemented our very first Conditional Access policy to require Multi-Factor Authentication on all users except on selected service accounts (which is not the best approach as explained above).

If you are interested to know how NVISO can help you planning your Conditional Access policies deployment and/or support you during the implementation, feel free to reach out or to check our website.

In my next blog post, we will see which policies can be created to enforce additional access controls without requiring user devices to be managed in Intune to further protect our environment.

About the author

Guillaume Bossiroy

Guillaume is a Senior Security Consultant in the Cloud Security Team. His main focus is on Microsoft Azure and Microsoft 365 security where he has gained extensive knowledge during many engagements, from designing and implementing Azure AD Conditional Access policies to deploying Microsoft 365 Defender security products.

Additionally, Guillaume is also interested into DevSecOps and has obtained the GIAC Cloud Security Automation (GCSA) certification.