The Threat
eSentire Threat Intelligence team is aware of recent attempts to target C-level and finance employees across multiple eSentire customers. These multistage attacks rely on Business Email Compromise (BEC) in order to impersonate users and facilitate wire fraud.
Employees, particularly those handling payments, are encouraged to review the recent eSentire advisory on Office 365 related phishing and remain vigilant of email-based threats [1]. Network administrators are advised to review the below guidance on limiting the impact of BEC through detective and preventative controls in Office 365.
What we’re doing about it
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The eSentire Threat Intelligence team is monitoring this threat for additional information
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A historical search on indicators observed during the campaign has been performed for all esENDPOINT clients and affected clients have been notified
What you should do about it
All Employees
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Review emails for suspicious indicators (such as unknown sender/suspicious URLs) prior to opening links or attachments or actioning payment requests [2]
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Deny and report suspicious two-step verification requests (see Additional Guidance below)
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Review attack scenarios in FinCEN Advisory on BEC scams [3]
Employees Responsible for Financial Transactions
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Ensure that robust authentication policies for transferring funds are in place
Network/Email Administrators
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Ensure a strong Multi-Factor Authentication (MFA) system is in place [4]
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Ensure employees are properly trained in securely using the MFA solution
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Consider blocking emails containing blob[.]core[.]windows[.]net or azurewebsites[.]net phishing links [1]
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Block the creation of mail forwarding rules to external addresses in Office 365 (see Additional Guidance below)
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Apply detection for suspicious mail forwarding rules in Office 365 (see Additional Guidance below)
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Consider implementing Azure Active Directory Identity Protection to block unusual logins (see Additional Guidance below)
Additional information
eSentire is aware of recent customer incidents involving BEC schemes. Employees responsible for financial transactions were targeted with credential phishing. Attempts were then made to gather payment related emails and initiate fraudulent financial transactions.
Recent customer incident: Finance employee compromised
In this incident, credentials for a finance employee were used to obtain payment related emails for the target organization.
This organization did have Azure MFA enabled; however, the attacker initiated the second authentication step using the phone call option shown here:
Figure 1: Screengrab Showing Alternative Identity Verification Steps
The victim received the call via their work phone and followed the voice prompts to approve the identity verification request. The phone call was not reported by the employee as it was not deemed suspicious.
Once access to the victim’s account was obtained, mail forwarding rules were created by the attacker to reroute payment related emails to an external address:
Figure 2: Mail Rule Created by Attacker. Forwards Payment Related Emails to External Address.
Subsequent email exfiltration was flagged by Office 365 and remediation steps were taken.
Additional Guidance for Employees
Suspicious MFA Requests
As demonstrated above, attackers can initiate a push notification in the Microsoft Authenticator app or phone call in hopes that the target will blindly approve it.
Neither method provides sufficient context (see Figure 3 for push notification example), and users are advised to deny and report the request if they did not initiate it themselves or if they are unsure.
Figure 3: MFA Push Notification Example in Microsoft Authenticator
Additional Guidance for Office 365 Administrators
Blocking creation of mail forwarding rules to external addresses in Office 365
If auto-forwarding emails to accounts outside of your organization is not a business requirement, administrators can block the creation of email forwarding rules to external domains. For step by step instructions on how to disable auto-forwarded email rules, see the official Microsoft Office Support documentation [5].
Detection of suspicious mail forwarding rules in Office 365
Office 365 offers administrators an Auto-Forward Messages Report which can be used to identify suspicious auto-forward rules. The dashboard illustrates new domain forwarding rules along with the user accounts that implemented the rule. See the official Microsoft guidance for detail instructions [6].
Figure 4: Office 365 Mail Flow Dashboard: Image taken from https://docs.microsoft.com/en-us/office365/securitycompliance/mfi-auto-forwarded-messages-report
Azure Active Directory Identity Protection
Azure Active Directory Identity Protection is used to identify compromised accounts through anomaly detection; such as login location or unknown device. This feature requires an Azure AD Premium P2 license. [7]
References:
[1] https://esentire.com/security-advisories/office-365-phishing-follow-up
[2] https://www.esentire.com/blog/can-your-employees-identify-a-phishing-attack/
[3] https://www.fincen.gov/resources/advisories/fincen-advisory-fin-2016-a003
[4] https://auth0.com/blog/different-ways-to-implement-multifactor/
[5] https://support.office.com/en-us/article/stop-auto-forwarding-emails-in-microsoft-365-f9d693ba-5c78-47c0-b156-8e461e062aa7
[6] https://docs.microsoft.com/en-us/office365/securitycompliance/mfi-auto-forwarded-messages-report
[7] https://docs.microsoft.com/en-us/azure/active-directory/identity-protection/overview
