Microsoft has announced that they’re continuing the path away from Legacy Authentication, with the decommission of legacy auth to EWS on Exchange Online on October 13th 2020. Instead of waiting for that looming date, there’s a bunch of security reasons to only have Modern Authentication for Microsoft 365.

I’ve already written up on Protect Your Office 365 Accounts By Disabling Basic Authentication and Blocking Legacy Authentication – Conditional Access vs Authentication Policies – but when I migrated from Authentication Policies to Conditional Access, I didn’t realise ActiveSync wasn’t included as part of blocking Legacy Authentication, even though it connects without MFA.

The guide from Microsoft on how to block Legacy Authentication doesn’t actually mention ActiveSync, so it’s easy to miss like I initially did! You’ll need to block ActiveSync altogether as far as I know, as it doesn’t support MFA.

Although I still think Conditional Access is easier to manage than Authentication Policies, there is one caveat; even with an ActiveSync block in place via Conditional Access, too many attempts by a user will lock their account briefly. This might cause problems or require work to get those users to clean up whatever device is trying to log in. With an Authentication Policy I don’t believe this happens because it’s blocked earlier in the sign-in process – you won’t see logs, and the account can’t get locked.

There is of course, a checkbox around ActiveSync, and a way to block it using Conditional Access, but I had mixed results in blocking it successfully until I did it exactly this way:

Create a new Conditional Access Policy and set these options:

In the Users and Groups section, you can narrow this down from ‘All Users’ for testing or for a gradual rollout.

The user experience is interesting on this one – they can still sort of authenticate, but instead of getting their emails, they will see a single email advising that their access has been blocked:

On top of this, you can use Azure AD to audit who might be using ActiveSync before you put any sort of block in place. As per usual, there’s a good Microsoft article on Discovering and blocking legacy authentication which can walk you through this, but in short:

Via the Azure Portal, go to Azure Active Directory > Users. Under Activity, go to Sign-ins. Click Add filters, and choose Client App > Tick the three ‘Exchange ActiveSync’ options and press ‘Apply’. You’ll see the last 7 days of sign in attempts using ActiveSync, which should give you an idea of how many users are using it, and who.

Blocking Legacy Authentication, plus blocking ActiveSync will give you a much more secure environment, protecting from account attacks.