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THE THREAT eSentire has identified publicly available Proof-of-Concept (PoC) exploit code, for the critical Microsoft Exchange vulnerability CVE-2021-42321. CVE-2021-42321 was announced as part of Microsoft’s November Patch Tuesday release. Exploitation would allow a remote threat actor, with previous authentication, to execute code on vulnerable servers. Prior to the patch release, Microsoft…
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Security advisories — Aug 29, 2019

BEC Scams Targeting VIPs & Finance Employees

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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

What you should do about it

All Employees

Employees Responsible for Financial Transactions

Network/Email Administrators

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:

[image src="/assets/93107a2e85/img1.jpg" id="2400" width="400" height="215" class="center ss-htmleditorfield-file image" title="img1"]

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:

[image src="/assets/cd01b28e1c/img2.jpg" id="2401" width="400" height="103" class="center ss-htmleditorfield-file image" title="img2"]

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.

[image src="/assets/7644626e8a/img3.jpg" id="2402" width="251" height="418" class="center ss-htmleditorfield-file image" title="img3"]

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 [6].

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 [7].

[image src="/assets/097accaa45/img4.jpg" id="2403" width="400" height="333" class="center ss-htmleditorfield-file image" title="img4"]

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. [8]

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://cba.ca/protecting-your-business-from-business-email-compromise-fraud

[5] https://auth0.com/blog/different-ways-to-implement-multifactor/

[6] https://support.office.com/en-us/article/stop-auto-forwarding-emails-in-microsoft-365-f9d693ba-5c78-47c0-b156-8e461e062aa7

[7] https://docs.microsoft.com/en-us/office365/securitycompliance/mfi-auto-forwarded-messages-report

[8] https://docs.microsoft.com/en-us/azure/active-directory/identity-protection/overview

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