I’ve already written a post on why Legacy Authentication (Basic) is bad, and Modern Authentication is good. At the time of writing, Authentication Policies were the way to go to block Legacy Authentication methods. Of course, things change and there’s now a better* option to look at – Conditional Access.
I’ve also covered Conditional Access before, and it’s really hard to fault the solution. There are now Baseline policies deployed by default (still in preview though) to Azure AD tenants with recommended best practices:
One of these is for blocking legacy authentication – but I’m not going to recommend you turn this on (at least for starters, it’s good at the end when you know you have full modern authentication support), as it’s a tenant wide setting that has no exceptions if you need to allow legacy authentication for an account (unlike Require MFA for admins, which does allow exceptions).
Instead, you can create your own policy that does the same. This means you can gradually roll it out, and put exceptions in place until you either work around them, or live with them. If you have a requirement for an account that requires legacy auth, then you need to consider how else you’ll protect that account – can you use other Conditional Access policies to restrict it to a certain region/locations, certain apps, platforms etc – lock it down as much as you can, and make sure the account has a long unique password.
The single important setting to block legacy auth via a Conditional Access Policy is blocking access to ‘Other clients’ via Client apps:
Microsoft have a full guide on how to set this up on docs.microsoft.com.
So, why is this better than using Authentication Policies? Two main reasons:
If an account has their access or signin blocked due to an Authentication Policy, it’s not logged. You can look at the user in Azure AD and check the sign-ins, but you won’t see anything. However, if it blocked via Conditional Access, you’ll have a nice log entry showing you it was blocked:
Side note: Although in this example I was logging in from Australia, I was trying to connect to Exchange Online via PowerShell. That seems to often be detected as being in the US, so be careful with region blocking.
The other reason is that Authentication Policies can take up to 4 (!) hours to apply, although it’s often more like an hour. That is a long time to wait, and you just have to keep waiting and trying until it works – except if you did it wrong, you won’t know and you’ll keep waiting. Or, if you need to unblock access while rolling out, it’s a long time to roll back.
Authentication Policies do have their place though, they give more granular control over what you want to block or not – say you know you want to block POP3 access company wide, but not IMAP – that’s possible in there, but not via Conditional Access.
Unless you have a good reason to use Authentication Policies, just use Conditional Access (and assuming you have Azure AD Premium P1 or P2 licensing to actually let you use Conditional Access, and if you are using Azure AD you should be on that licensing anyway). It’ll make your life easier!