How to: Block legacy authentication to Azure AD with Conditional Access

To give your users easy access to your cloud apps, Azure Active Directory (Azure AD) supports a broad variety of authentication protocols including legacy authentication. However, legacy authentication does not support multifactor authentication (MFA). MFA is in many environments a common requirement to address identity theft.

Note

Effective October 1, 2022, we will begin to permanently disable Basic Authentication for Exchange Online in all Microsoft 365 tenants regardless of usage, except for SMTP Authentication.

Alex Weinert, Director of Identity Security at Microsoft, in his March 12, 2020 blog post New tools to block legacy authentication in your organization emphasizes why organizations should block legacy authentication and what other tools Microsoft provides to accomplish this task:

For MFA to be effective, you also need to block legacy authentication. This is because legacy authentication protocols like POP, SMTP, IMAP, and MAPI can't enforce MFA, making them preferred entry points for adversaries attacking your organization...

...The numbers on legacy authentication from an analysis of Azure Active Directory (Azure AD) traffic are stark:

  • More than 99 percent of password spray attacks use legacy authentication protocols
  • More than 97 percent of credential stuffing attacks use legacy authentication
  • Azure AD accounts in organizations that have disabled legacy authentication experience 67 percent fewer compromises than those where legacy authentication is enabled

If your environment is ready to block legacy authentication to improve your tenant's protection, you can accomplish this goal with Conditional Access. This article explains how you can configure Conditional Access policies that block legacy authentication for all workloads within your tenant.

While rolling out legacy authentication blocking protection, we recommend a phased approach, rather than disabling it for all users all at once. Customers may choose to first begin disabling basic authentication on a per-protocol basis, by leveraging Exchange Online authentication policies, then (optionally) also blocking legacy authentication via Conditional Access policies when ready.

Customers without licenses that include Conditional Access can make use of security defaults to block legacy authentication.

Prerequisites

This article assumes that you are familiar with the basic concepts of Azure AD Conditional Access.

Note

Conditional Access policies are enforced after first-factor authentication is completed. Conditional Access isn't intended to be an organization's first line of defense for scenarios like denial-of-service (DoS) attacks, but it can use signals from these events to determine access.

Scenario description

Azure AD supports several of the most widely used authentication and authorization protocols including legacy authentication. Legacy authentication refers to basic authentication, a widely used industry-standard method for collecting user name and password information. Typically, legacy authentication clients can't enforce any type of second factor authentication. Examples of applications that commonly or only use legacy authentication are:

  • Microsoft Office 2013 or older.
  • Apps using mail protocols like POP, IMAP, and SMTP AUTH.

For more information about modern authentication support in Office, see How modern authentication works for Office client apps.

Single factor authentication (for example, username and password) is not enough these days. Passwords are bad as they are easy to guess and we (humans) are bad at choosing good passwords. Passwords are also vulnerable to various attacks, like phishing and password spray. One of the easiest things you can do to protect against password threats is to implement multifactor authentication (MFA). With MFA, even if an attacker gets in possession of a user's password, the password alone is not sufficient to successfully authenticate and access the data.

How can you prevent apps using legacy authentication from accessing your tenant's resources? The recommendation is to just block them with a Conditional Access policy. If necessary, you allow only certain users and specific network locations to use apps that are based on legacy authentication.

Conditional Access policies are enforced after the first-factor authentication has been completed. Therefore, Conditional Access is not intended as a first line defense for scenarios like denial-of-service (DoS) attacks, but can utilize signals from these events (for example, the sign-in risk level, location of the request, and so on) to determine access.

Implementation

This section explains how to configure a Conditional Access policy to block legacy authentication.

Messaging protocols that support legacy authentication

The following messaging protocols support legacy authentication:

  • Authenticated SMTP - Used to send authenticated email messages.
  • Autodiscover - Used by Outlook and EAS clients to find and connect to mailboxes in Exchange Online.
  • Exchange ActiveSync (EAS) - Used to connect to mailboxes in Exchange Online.
  • Exchange Online PowerShell - Used to connect to Exchange Online with remote PowerShell. If you block Basic authentication for Exchange Online PowerShell, you need to use the Exchange Online PowerShell Module to connect. For instructions, see Connect to Exchange Online PowerShell using multifactor authentication.
  • Exchange Web Services (EWS) - A programming interface that's used by Outlook, Outlook for Mac, and third-party apps.
  • IMAP4 - Used by IMAP email clients.
  • MAPI over HTTP (MAPI/HTTP) - Primary mailbox access protocol used by Outlook 2010 SP2 and later.
  • Offline Address Book (OAB) - A copy of address list collections that are downloaded and used by Outlook.
  • Outlook Anywhere (RPC over HTTP) - Legacy mailbox access protocol supported by all current Outlook versions.
  • POP3 - Used by POP email clients.
  • Reporting Web Services - Used to retrieve report data in Exchange Online.
  • Universal Outlook - Used by the Mail and Calendar app for Windows 10.
  • Other clients - Other protocols identified as utilizing legacy authentication.

For more information about these authentication protocols and services, see Sign-in activity reports in the Azure Active Directory portal.

Identify legacy authentication use

Before you can block legacy authentication in your directory, you need to first understand if your users have apps that use legacy authentication and how it affects your overall directory. Azure AD sign-in logs can be used to understand if you're using legacy authentication.

  1. Navigate to the Azure portal > Azure Active Directory > Sign-ins.
  2. Add the Client App column if it is not shown by clicking on Columns > Client App.
  3. Add filters > Client App > select all of the legacy authentication protocols. Select outside the filtering dialog box to apply your selections and close the dialog box.
  4. If you have activated the new sign-in activity reports preview, repeat the above steps also on the User sign-ins (non-interactive) tab.

Filtering will only show you sign-in attempts that were made by legacy authentication protocols. Clicking on each individual sign-in attempt will show you more details. The Client App field under the Basic Info tab will indicate which legacy authentication protocol was used.

These logs will indicate which users are still depending on legacy authentication and which applications are using legacy protocols to make authentication requests. For users that do not appear in these logs and are confirmed to not be using legacy authentication, implement a Conditional Access policy for these users only.

Block legacy authentication

There are two ways to use Conditional Access policies to block legacy authentication.

Directly blocking legacy authentication

The easiest way to block legacy authentication across your entire organization is by configuring a Conditional Access policy that applies specifically to legacy authentication clients and blocks access. When assigning users and applications to the policy, make sure to exclude users and service accounts that still need to sign in using legacy authentication. When choosing the cloud apps in which to apply this policy, select All cloud apps, targeted apps such as Office 365 (recommended) or at a minimum, Office 365 Exchange Online. Configure the client apps condition by selecting Exchange ActiveSync clients and Other clients. To block access for these client apps, configure the access controls to Block access.

Client apps condition configured to block legacy auth

Indirectly blocking legacy authentication

Even if your organization isn’t ready to block legacy authentication across the entire organization, you should ensure that sign-ins using legacy authentication aren’t bypassing policies that require grant controls such as requiring multifactor authentication or compliant/hybrid Azure AD joined devices. During authentication, legacy authentication clients do not support sending MFA, device compliance, or join state information to Azure AD. Therefore, apply policies with grant controls to all client applications so that legacy authentication based sign-ins that cannot satisfy the grant controls are blocked. With the general availability of the client apps condition in August 2020, newly created Conditional Access policies apply to all client apps by default.

Client apps condition default configuration

What you should know

It can take up to 24 hours for the Conditional Access policy to go into effect.

Blocking access using Other clients also blocks Exchange Online PowerShell and Dynamics 365 using basic auth.

Configuring a policy for Other clients blocks the entire organization from certain clients like SPConnect. This block happens because older clients authenticate in unexpected ways. The issue doesn't apply to major Office applications like the older Office clients.

You can select all available grant controls for the Other clients condition; however, the end-user experience is always the same - blocked access.

SharePoint Online

To block user access via legacy authentication to SharePoint Online, organizations must disable legacy authentication on SharePoint using the Set-SPOTenant PowerShell command and setting the -LegacyAuthProtocolsEnabled parameter to $false. More information about setting this parameter can be found in the SharePoint PowerShell reference document regarding Set-SPOTenant

Next steps