So my company decided to migrate office suite and email etc to Microsoft365. Whatever. But for 2FA login they decided to disable the option to choose “any authenticator” and force Microsoft Authenticator on the (private) phones of both employees and volunteers. Is there any valid reason why they would do this, like it’s demonstrably safer? Or is this a battle I can pick to shield myself a little from MS?
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How would MS Authenticator make it any better than TOTP?
To break TOTP, the attacker would need to:
a) be able to observe the initial exchange of the TOTP secrets. To do that, the attacker needs access to the victim’s computer (on user level) at that specific time they set up TOTP. TOTP is a TOFU concept and thus not designed to protect against that. However, if the attacker controls the victim’s computer at that time, the victim is screwed anyways even before setting up 2FA.
b) have access to the TOTP app’s secret storage and to the victim’s login credentials (e.g. by phishing). If the attacker can gain that level of access, they would also have access to the Microsoft Authenticator’s secret storage, so there is no benefit of the Microsoft app.
On the other hand, Microsoft Authenticator is a very huge app (>100MB is huge for an authenticator app, Aegis is just 6MB, FreeOTP+ 11MB), i.e. it brings a large attack surface, especially by connecting to the internet.
I don’t think Microsoft Authenticator brings security benefits over a clean and simple TOTP implementation.
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If it is just the location, then it could be spoofed.
If it is something that requires physical presence, then you need both devices to communicate with each other. If it is not done via QR code (like some online banking do), then both devices need to be connected, e.g. via WiFi or Bluetooth. In this case, if an attacker controls one of the devices (that’s the class of attacks 2FA should prevent you from), the attacker probably controls both devices. So what’s the point then?