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Conditional Access Policy - Block Legacy Authentication

Legacy protocols like POP, SMTP and IMAP don't support MFA and can be used to password spray attack your Office 365 mailboxes. You can disable legacy authentication and improve your Office 365 security using this Azure Conditional Access policy.

Blocking legacy authentication in Office 365

Applications that use legacy authentication do not support two factor authentication and so are vulnerable to password spray attacks. Some protocols that use basic authentication are: SMTP, IMAP, POP, Exchange Web Services (EWS), Exchange ActiveSync (EAS)

  • Blocking legacy authentication will block older clients that do not use modern authentication
  • You should add exceptions for any mailboxes that need to use legacy protocols otherwise they will stop working when the policy is applied
  • It can take up to 24 hours for the conditional access policy to take effect
  • If you have security defaults enabled then a policy to block legacy authentication will already be configured

Check if legacy authentication is enabled in Office 365

You can check the Azure AD sign-in logs to find clients that are still using legacy authentication

Azure Active Directory
Monitoring - Sign-ins

Use the Client app filter to look for legacy authentication clients - tick all the legacy clients

Select one of the sign-ins then click the Basic info tab

You can see the Client app is IMAP

Test if IMAP and SMTP are enabled using Outlook

Let's do a quick test to check if IMAP and SMTP are enabled for a mailbox. This way we can repeat the test after creating the Conditional Access Policy to confirm that IMAP and SMTP are disabled.

Create a new Outlook profile using these settings to connect to the mailbox using IMAP and SMTP

Office 365 IMAP Settings
Incoming mail server (IMAP) outlook.office365.com
IMAP port 993
IMAP security SSL
Office 365 SMTP Settings

Outgoing mail server (SMTP) smtp.office365.com
SMTP port 587
SMTP security TLS
Outgoing server (SMTP) requires authentication

We've successfully connected to the mailbox using IMAP and SMTP

Conditional Access Policy to block legacy authentication

Sign into the Azure Portal

Microsoft Azure Portal

Create a new policy

Azure Active Directory
Security - Conditional Access - New Policy

Name Block Legacy Authentication

Users and groups
Include: All Users
Exclude: your admin account

Cloud apps or actions

Include: All cloud apps

Client apps
Configure Yes

Legacy authentication clients
Tick Exchange ActiveSync clients, Other clients

Access controls - Grant
Block Access

Enable the policy in Report-only mode

Monitor Azure Active Directory sign-ins

Enable the policy in report-only mode so you can monitor the Azure AD sign-in logs and check that the policy is working as expected.

In this example we can see the IMAP client connection

The report-only tab allows us to check what would happen if the policy was enabled

Report-only - more - show details

Client app IMAP
Grant controls block

This example shows that the IMAP client access has been blocked

Once you're happy that the policy is working OK, you can enable it

Test if IMAP and SMTP are now disabled using Outlook

Let's try re-connecting to the mailbox using IMAP and SMTP with the Outlook profile we created earlier, this will confirm that the conditional access policy is now working and IMAP and SMTP are blocked

Test Account Settings

Connecting to the mailbox using IMAP now doesn't work - we get repeatedly prompted for a username and password

Failed to connect to the mailbox using IMAP and SMTP

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Saturday, 23 September 2023
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