New Silent Starling Attack Group Puts Spin on BEC

A new West African cybercriminal group is targeting vendors with a technique called "vendor email compromise," which it has used to successfully compromise more than 700 employee email accounts at more than 500 companies in at least 14 countries, Agari researchers report.

The Agari Cyber Intelligence Division (ACID) began watching Silent Starling in July. Researchers say the group has three primary members in Nigeria, though the group is believed to be larger. Activity started in 2015 and mostly consisted of romance scams and check fraud before pivoting to business email compromise (BEC) in 2016 and vendor email compromise (VEC) in 2018.

BEC is an increasingly common enterprise threat, costing organizations up to $300 million each month, the US Treasury Department reports. Payment invoice scams, which made up nearly half of all fraudulent transactions in 2018, cost businesses more than $1.5 billion that year. This is likely to grow as criminals gain access to, and abuse, legitimate email accounts, Agari reports.

VEC puts a new spin on an old threat. In traditional BEC scams, attackers pretend to be high-ranking employees to request wire transfers from the finance department. Silent Starling, the first group spotted using VEC, instead targets vendors with a low-and-slow approach that ultimately leads to a higher payout.

ACID was researching BEC attacks in July when one suspicious email requested a wire transfer. The team engaged with the attacker, requesting bank account numbers to complete the transfer and continuously saying the bank rejected them. For months the attacker sent numbers for different mule accounts used to launder money, eventually giving 13 to ACID.

Researchers sent the mule account data to law enforcement and financial partners but continued to investigate Silent Starling, discovering a sophisticated operation that takes far more time, patience, and research to pull off. A new report published today details the findings.

Inside A VEC Attack
It starts with an email, says Crane Hassold, senior director of threat research at Agari. Silent Starling typically compromises an employee account with a OneDrive or DocuSign-spoofed phishing attack. When they obtain credentials, the attackers set up forwarding rules so the employee's emails are forwarded to an inbox they control. Then they sit, wait, and observe.

"A number of the accounts they were able to compromise were employees in accounts receivable, CFOs, [and] office managers involved with day-to-day financial transactions," he says. OneDrive is by far the most common phishing page, intended to lift enterprise credentials that are generally worth more because they can be exploited in several different ways, he adds.

To maximize its effectiveness, Silent Starling needs to understand the workflow of its target. Attackers analyze emails to personalize the messages they send to the organization's employees, customers, and partners. This is what sets VEC scams apart: By collecting intel on transactions, conversations, and exchanges, attackers can learn how people communicate and how a vendor structures its invoicing process. As a result, they're well-equipped to create emails that are "virtually undetectable," researchers say in their report – even more so as their fraudulent emails are sent from a legitimate account.

When the opportunity arrives, attackers uses this intelligence to slip into the transaction process: They send a fake invoice to the target vendor's client, updating the customer with new banking details. The client, who doesn't notice the email is fake, then sends money directly to the attackers.

Small Targets, Big Payoff
What's interesting about Silent Starling's approach is it targets smaller victims, Hassold explains. Its biggest target was a US-based company with only a couple hundred employees; its smallest had only a couple. These vendors are a jumping-off point so the group can take money from its real targets: the much-larger organizations paying smaller firms to perform a specific service.

Still, the group doesn't have specific organizations in mind. It seems the initial credential phishing is only "lightly targeted," with attackers sending many messages to compromise email accounts and looking through those emails to identify employees in the finance department. In one instance, Silent Starling compromised email accounts of 39 people at a single company. Since late 2018 it has received copies of more than 20,000 emails from compromised inboxes.

"Vendor email compromise is going to be a big threat in the next 12 to 18 months," Hassold says. In a basic CEO spoofing attack, an operator may be able to net $10,000 to $15,000 before raising a red flag. A VEC attack enables criminals to slip right into the vendor payment process.

"Because there are mimicking real payments of hundreds of thousands of dollars, the payoff can be significantly higher," he explains. "It's very different and much more sophisticated."

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