Conditional Access policy: Block legacy authentication

1. Sign in to the Microsoft Entra admin center as at least a Conditional Access Administrator.
2. Browse to Protection > Conditional Access.
3. Select Create new policy.
4. Give your policy a name. We recommend that organizations create a meaningful standard for the names of their policies.
5. Under Assignments, select Users or workload identities.
   1. Under Include, select All users.
   2. Under Exclude, select Users and groups and choose any accounts that must maintain the ability to use legacy authentication. Exclude at least one account to prevent yourself from being locked out. If you don't exclude any account, you won't be able to create this policy.
6. Under Target resources > Cloud apps > Include, select All cloud apps.
7. Under Conditions > Client apps, set Configure to Yes.
   1. Check only the boxes Exchange ActiveSync clients and Other clients.
   2. Select Done.
8. Under Access controls > Grant, select Block access.
   1. Select Select.
9. Confirm your settings and set Enable policy to Report-only.
10. Select Create to create to enable your policy.

After administrators confirm the settings using report-only mode, they can move the Enable policy toggle from Report-only to On.