Investigating Suspicious Inbox Rules in Potential BEC Attacks

Understanding Inbox Rules in BEC Attacks

Business Email Compromise (BEC) is a sophisticated form of cybercrime that preys on businesses by exploiting their email systems. One significant tactic employed by cybercriminals in BEC attacks is the use of suspicious inbox rules. Inbox rules can automate how emails are organized, but when misconfigured or used maliciously, these rules can redirect sensitive information or hinder detection of illicit communications.

Triggering Suspicious Mailbox Rule Alerts

In many organizations, alerts regarding suspicious mailbox rules may be triggered by unexpected patterns, such as rules being created or modified without proper authorization. Common triggers include the movement of emails to unusual folders, marking messages as read automatically, or even hiding emails from the user’s view. Cybersecurity teams must be vigilant when such alerts surface, as these can indicate potential compromises that require immediate investigation.

Reviewing Inbox Rules and Sign-in Activity

Utilizing PowerShell, IT teams can execute commands to review inbox rules set by users. Commands such as Get-InboxRule help unveil alterations to inbox configurations. Investigators should meticulously search for actions that are indicative of malevolent intent, such as forwarding rules directing messages to unknown external addresses.

Furthermore, scrutinizing sign-in activity is equally important. Checking for unusual IP addresses, geographical locations, and unfamiliar devices can provide vital context in understanding whether a user account has been compromised. Seen together, these elements create a clearer picture of potential BEC attempts.

A Real-World Investigation Example

A notable case involved an organization that reported suspicious inbox rules where emails were automatically forwarded to an external address. However, upon further investigation using PowerShell, the email forwarding was found to be set to a legitimate account owned by an employee who had forgotten to disable the rule. Despite these suspicious findings, no confirmed compromise of the organization's email system was established. This case illustrates the importance of regularly reviewing inbox configurations and alerting staff regarding safe email practices.

Conclusion: The Importance of Monitoring Inbox Rules

In conclusion, monitoring and investigating inbox rules plays a crucial role in identifying potential BEC attacks. Cybercriminals often leverage these rules as a persistence method to remain undetected while they steal sensitive information. Regular scrutiny of inbox configurations, sign-in activity, and implementing education for employees on BEC threats significantly enhance an organization's cybersecurity posture.

Tools & Commands Used

During this investigation, the following tools and commands were used to validate mailbox activity and identify suspicious inbox rules:

### Exchange Online PowerShell :Used to review mailbox rules, including hidden rules that may not be visible in the Outlook client.

• Connect-ExchangeOnline

• Get-InboxRule -Mailbox user@domain.com

• Get-InboxRule -Mailbox user@domain.com -IncludeHidden

• Get-Mailbox user@domain.com | Select-Object ForwardingSMTPAddress, ForwardingAddress, DeliverToMailboxAndForward