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Block legacy authentication with Conditional Access

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Microsoft recommends that organizations block authentication requests using legacy protocols that don't support multifactor authentication. Based on Microsoft's analysis more than 97 percent of credential stuffing attacks use legacy authentication and more than 99 percent of password spray attacks use legacy authentication protocols. These attacks would stop with basic authentication disabled or blocked.

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

User exclusions

Conditional Access policies are powerful tools, we recommend excluding the following accounts from your policies:

  • Emergency access or break-glass accounts to prevent lockout due to policy misconfiguration. In the unlikely scenario all administrators are locked out, your emergency-access administrative account can be used to log in and take steps to recover access.
  • Service accounts and Service principals, such as the Microsoft Entra Connect Sync Account. Service accounts are non-interactive accounts that aren't tied to any particular user. They're normally used by back-end services allowing programmatic access to applications, but are also used to sign in to systems for administrative purposes. Calls made by service principals won't be blocked by Conditional Access policies scoped to users. Use Conditional Access for workload identities to define policies targeting service principals.
    • If your organization has these accounts in use in scripts or code, consider replacing them with managed identities.

Template deployment

Organizations can choose to deploy this policy using the steps outlined below or using the Conditional Access templates.

Create a Conditional Access policy

The following steps help create a Conditional Access policy to block legacy authentication requests. This policy is put in to Report-only mode to start so administrators can determine the impact they have on existing users. When administrators are comfortable that the policy applies as they intend, they can switch to On or stage the deployment by adding specific groups and excluding others.

  1. Sign in to the Microsoft Entra admin center as at least a Conditional Access Administrator.
  2. Browse to Protection > Conditional Access > Policies.
  3. Select 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. Microsoft recommends you exclude at least one account to prevent yourself from being locked out due to misconfiguration.
  6. Under Target resources > Resources (formerly cloud apps) > Include, select All resources (formerly '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.

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.

Identify legacy authentication use

To understand if your users have client apps that use legacy authentication, administrators can check for indicators in the sign-in logs with the following steps:

  1. Sign in to the Microsoft Entra admin center as at least a Reports Reader.
  2. Browse to Identity > Monitoring & health > Sign-in logs.
  3. Add the Client App column if it isn't shown by clicking on Columns > Client App.
  4. Select Add filters > Client App > choose all of the legacy authentication protocols and select Apply.
  5. Also perform these steps on the User sign-ins (non-interactive) tab.

Filtering shows you sign-in attempts made by legacy authentication protocols. Clicking on each individual sign-in attempt shows you more details. The Client App field under the Basic Info tab indicates which legacy authentication protocol was used. These logs indicate users who are using clients that depend on legacy authentication.

Additionally, to help triage legacy authentication within your tenant use the Sign-ins using legacy authentication workbook.

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