An Azure AD Conditional Access Policy for Guests that requires MFA for all applications.

Suggested name:
CA4{XX}-Guests-BaseProtection-AllApps-AllConditions-MFA

Assignments

Users

Users and groups  
Include users All guest and external users

Cloud apps or actions

Cloud apps  
Include All cloud apps

Access controls

Grant access  
Require Multifactor authentication

References

https://learn.microsoft.com/en-us/azure/architecture/guide/security/conditional-access-architecture https://learn.microsoft.com/en-us/azure/architecture/guide/security/conditional-access-framework https://learn.microsoft.com/en-us/azure/active-directory/conditional-access/concept-conditional-access-policy-common