A new email Trojan campaign spotted by security researchers has added another twist in evasive attacker behavior: Researchers with GreatHorn report that the waves of attacks they've observed since yesterday afternoon are rapidly randomizing the email content characteristics of their lures.

"Masquerading as a confirmation on a paid invoice, the attack is sophisticated in that it lacks the consistency of a typical volumetric attack, making it more challenging for email security tools to identify and block," says E.J. Whaley, solutions engineer at GreatHorn. 

Attackers have long leaned on metamorphic and polymorphic malware techniques to make swift changes to the code in order to evade detection. It was only a matter of time before they started taking that philosophy to the scam delivery vehicles. This method is in contrast to what Whaley says is typical in most Trojan phishing email lures, where they'll stick to a single pattern with "slight customizations." Instead, these attacks are switching up subject lines, email content, email addresses, display name spoofs, and destination URLs. 

"Body content generally follows a pattern that confirms the receipt of a payment for an invoice but uses slightly different language to evade capture," Whaley says.

While the subject lines vary, they all seem to cluster around references to receipts or invoices. Sometimes the attacks are very targeted — using spoofed names of a fellow employee of the target — and sometimes they use random names, sent from numerous compromised email accounts. The bulk of the compromise accounts are coming from South American companies. 

The attackers are trying to trick users into downloading a Word template with a VBA downloader trojan embedded within. GreatHorn says the attack was detected in about one in 10 accounts in its user base.

Paid invoice attacks like these are increasingly at the top of phishers’ playbooks. Last year six of 10 phishing messages used some variation of "invoice" as its subject.

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