Microsoft Hardens Authenticator App to Prevent MFA Fatigue

Number Matching and Geographic Context Now GA for Authenticator App

Updated 20 February 2023

Eleven months after releasing the features to preview, Microsoft has made number matching and additional context generally available with the tag line of advanced Microsoft Authenticator security features. The capabilities are available for multi-factor authentication (MFA) flows now and will be available for passwordless flows soon.

In a nutshell, these features relieve the MFA fatigue some users experience when they process MFA challenges to sign into Microsoft 365 and other apps. Instead of blindly responding to a prompt or “simple approval” (which could be hijacked by an attacker – see MITRE Att&ck technique T1621), the user is forced to respond to the challenge by entering a matching number. This addresses the problem (as experienced in the recent Uber compromise) where an account holder responds to an MFA challenge without putting their brain in gear. MFA fatigue is a very real and current problem.

Additional context allows the Authenticator app to display information about the location of the sign-in and the app provoking the challenge. The extra information helps the user to understand if the sign-in that provoked the challenge is valid. Together, Microsoft says that number matching and additional context help organizations to “prevent accidental [user] approvals in Microsoft Authenticator.”

New UI in Azure AD Admin Center

Tenants can roll the features out to all users or a targeted group, Microsoft has refreshed the UI in Authentication Methods section under Security in the Azure AD admin center (Figure1). You can also configure the settings with a Graph API request, but I wouldn’t bother. The Azure AD admin center does everything you need.

Microsoft planned to implement number matching for all Authenticator users in February 2023 but rescheduled the deployment date to May 8, 2023. At that point, Microsoft will remove the UI to enable or disable this feature from the Azure AD admin center. and all authentication challenges using the Authenticator app will require users to respond to generated numbers rather than the traditional Deny/Approve choice. This is part of Microsoft’s ongoing campaign to increase security by default across Microsoft 365.

Responding to a Non-Fatigued MFA Challenge

I found that it took about ten minutes before Azure AD implemented the updated settings in its challenges. The number challenge uses the same UI as the preview (Figure 2).

Figure 3 shows how the Microsoft Authenticator app (for iOS) prompts the user to enter the number requested by Azure AD. You can also see the additional geographic (based on the IP address of the device used for the sign-in) and application context presented to allow the user to judge if the sign-in is legitimate.

Speaking of Authenticator on iOS, Microsoft says that the app now uses App Transport Security (ATS) for improved privacy and data integrity between Authenticator and web services like the Microsoft 365 apps.

They also say that Authenticator on Android allows users to search their accounts and that this capability is coming to iOS “soon.” I use Authenticator for multiple Microsoft 365 tenants, my Microsoft consumer account, and applications like Twitter and GitHub, so searching will be a nice addition.

Changes Improve Authenticator’s Resistance to Attack

Why is number matching and additional context important for Authenticator? At the TEC 2022 conference, Alex Weinert, Microsoft VP for Identity Security, appealed for Microsoft 365 tenants to deploy multi-factor authentication more broadly (i.e., to increase the overall level of protection from the current 26.84% of user accounts). MFA protects more administrator accounts (34.15%), but that’s hardly a reason to celebrate.

During his TEC session, Alex discussed the updates now available for Authenticator and stressed how these made the app less susceptible to attack and less likely for its users to succumb to the human weakness seen in MFA fatigue. I imagine that with these updates, Microsoft now regards Authenticator as having the same authentication strength as Windows Hello and FIDO-2 keys.

The nice thing about the cloud is that changes like this roll-out without any intervention required on the part of tenants. It’s entirely in your hands to decide whether to take advantage and make MFA challenges more resistant to attack. It makes sense to do so.

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