Prevent Data Leakage using Exchange Online Transport Rules and Raise the Office365 Secure Score

Over the last month, two clients contacted me requesting immediate support to mitigate and stop the data leakage caused by compromised credentials of specific users in Office365 and Exchange Online. The use of Multi-Factor Authentication has become a best practice and could have prevented this situation. However, there are other security controls that we recommend putting in place to prevent data leakage from Office365.

The bad actors’ “modus operandi” it is usually the same. After compromising the account, hackers access the compromised mailbox using OWA or perhaps Outlook, and setup multiple mailboxes rules. Some of these rules are to exfiltrate data of the organization such as auto-forwarding or send phishing emails to saved contacts and expand their malicious activity. Usually, the last mailbox rules we have identified is to automatically delete their activity from the “Sent Items” folder so it will be unnoticeable by the user.

One of the ways we can help stop data exfiltration from client created rules is using Exchange Transport Rules. Implementing a Transport Rule based around the following can stop emails that are set to be Auto-Forwarded to an external address.

In summary you create a rule based on the following logic.

  • IF The Sender is located ‘Inside the organization’
  • AND IF The Recipient is located ‘Outside the organization’
  • AND IF The message type is ‘Auto-Forward’
  • THEN Reject the message with the explanation ‘External Email Forwarding via Client Rules is not permitted’.

Transport Rule

This will stop delivery of the Auto-Forward message and issue an NDR message to the sender. Exceptions can be also created if needed.

Similarly, this will help you to increase your Office365 Secure Score which now has a new security control called ‘Client Rules Forwarding Blocks’ that implements a Transport Rule to help mitigate client created rules that Auto-Forward to external addresses.

Azure Authentication, Identity, and Access Management Best Practices (The Series)

Over the years, I have seen how authentication processes have been evolving into what it is and what we know today. My clients already in the cloud or looking into adopting the cloud are constantly asking me about the “Best Practices” to secure their Microsoft cloud adoption. This is why I have decided to write this blog post series of Microsoft Cloud authentication, Identity and Access Management.

Network firewalls used to be considered the perimeter defense of the network. But with cloud services, that perimeters keep getting more porous, and that perimeter defense can’t be as effective as it was before the explosion of BYOD devices and cloud applications.

Today, enterprises are starting to understand that identity needs to be the primary perimeter for security. This is a shift from the traditional focus on network security. Azure Active Directory (Azure AD) is the Azure solution for identity and access management. Azure AD is a multitenant, cloud-based directory and identity management service from Microsoft. It combines core directory services, application access management, and identity protection into a single solution.

Best Practice

  1. Centralized Identity Management (Best Practice 1/10)
  2. Enable single sign-on for the Microsoft Cloud (Best Practice 2/10)
  3. Enable Conditional Access (Best Practice 3/10)
  4. Enable self-service Password reset with Azure AD Premium (Best Practice 4/10)
  5. Enable Conditional Access for cloud access. (Best Practice 5/10)
  6. Enforce multi-factor verification for users (Best Practice 6/10)
  7. Use role-based access control (Best Practice 7/10)
  8. Turn on Azure AD Privileged Identity Management (Best Practice 8/10)
  9. Define at least two emergency access accounts. (Best Practice 9/10)
  10. Ensure all critical admin roles have a separate account for administrative tasks (Best Practice 10/10)