IBM X-Force Incident Response and Intelligence Services (IRIS) assesses that threat groups of likely Nigerian origin are engaged in a widespread credential harvesting, phishing and social engineering campaign designed to steal financial assets. Beginning in the fall of 2017, X-Force IRIS experienced a significant increase in clients reporting instances of fraud or attempted fraud via wire transfer payments. These threat groups successfully used business email compromise (BEC) scams to convince accounts payable personnel at some Fortune 500 companies to initiate fraudulent wire transfers into attacker-controlled accounts, resulting in the theft of millions of dollars.
Business email compromise scams involve taking over or impersonating a trusted user’s email account to target companies that conduct international wire transfers with the goal of diverting payments to an attacker-controlled account.

These attacks are almost entirely based on phishing and social engineering, and are thus attractive to cybercriminals due to their relative simplicity. In most cases, BEC scams involve little to no technical knowledge, malware or special tools.

A recent report by Trend Micro predicted that BEC attacks will comprise over $9 billion in losses in 2018, up from $5.3 billion at the end of 2016. According to the FBI, BEC scams have been reported in every U.S. state and across 131 nations, and have resulted in high-profile arrests.
The following tactics were common to the attacks examined by X-Force IRIS researchers:

Phishing emails were sent either directly from or spoofed to appear to be from known contacts in the target employee’s address book.

Attackers mimicked previous conversations or inserted themselves into current conversations between business email users.

Attackers masqueraded as a known contact from a known vendor or associated company and requested that wire payments be sent to an “updated” bank account number or beneficiary.

Attackers created mail filters to ensure that communications were conducted only between the attacker and victim and, in some cases, to monitor a compromised user’s inbox.

In cases in which additional approval or paperwork was needed, the attackers found and filled out appropriate forms and spoofed supervisor emails to get required approvals.

Without the use of any malware, and with legitimate stakeholders performing the actual transactions, traditional detection tools and spam filters failed to identify evidence of a compromise.
The BEC scams identified by IBM incident responders consist of two separate but connected goals. The first is to harvest mass amounts of business user credentials, and the second is to use these credentials to impersonate their rightful owners and ultimately trick employees into diverting fund transfers to bank accounts the attackers control.

To achieve the first goal, the attackers used credential sets they had already compromised to send a mass phishing email to the user’s internal and external contacts. The phish was often sent to several hundred contacts at a time and was engineered to look legitimate to the spammed contacts.
To accomplish the second goal, the attackers focused on stolen credentials from companies that use single-factor authentication and an email web portal. For example, companies that only require a username and password for employees to access their Microsoft Office 365 accounts were compromised. Using email web portals ensured the attackers’ ability to complete these attacks online and without compromising the victim’s corporate network. The attackers specifically targeted personnel involved in the organization’s accounts payable departments to ensure that the victim had access to the company’s bank accounts.

Before engaging with any employee, the attackers likely undertook a reconnaissance phase, looking through activity within the user’s email folders in search of subjects and opportunities to exploit and, eventually, creating or inserting themselves into relevant conversations.
Since the attackers conducted correspondence from a victim user’s email, they created email rules to keep the victim unaware of the compromise. In cases in which the attackers impersonated the user, the attackers auto-deleted all emails delivered from within the user’s company. They likely did this to prevent the user from seeing any fraudulent correspondence or unusual messages in his or her inbox. Additionally, the attacker auto-forwarded email responses to a different email to read the responses without logging in to the compromised account.

Separately, when attackers used stolen credentials to send mass phishing emails, they simultaneously set up an email rule to filter all responses to the phish, undelivered messages, or messages containing words such as “hacked” or “email” to the user’s RSS feeds folder and marked them as read.

Source: IBM X-Force IRIS Uncovers Active Business Email Compromise Campaign Targeting Fortune 500 Companies