A phishing campaign is using payroll-themed emails to trick users into inadvertently infecting their machines with TrickBot.

At the beginning of November, Palo Alto Networks’ Unit 42 research team identified a phishing campaign sending out attack emails whose subject lines referred to payroll and annual bonuses. These emails didn’t arrive with an attachment. Instead, they included links to what appeared to be a Google Docs document. That file, in turn, contained links to malicious files hosted on Google Drive that acted as simple downloaders of TrickBot. Upon execution, the malware established persistence on the infected machine by creating a scheduled task that ran at user login.

As noted by Unit 42, this phishing campaign was unique in that malicious actors used SendGrid, a legitimate email delivery service (EDS), to send out the initial attack emails. They had also used SendGrid to conceal the malicious Google Drive links contained in the Google Docs document.

A Busy Year for TrickBot

TrickBot has certainly been up to some tricks this year. Back in April 2019, for instance, Cybereason observed attackers using the malware in tandem with the Emotet Trojan to deliver samples of the Ryuk ransomware family. That was just a few months before researchers at Deep Instinct discovered TrickBooster, a module that lets TrickBot harvest email credentials and contacts from its victims for the purpose of abusing their inboxes to send out malspam. In August 2019, IBM X-Force confirmed that it had come across a fileless version of TrickBot that did not save its typical modules and configurations to disk on infected Windows machines.

How to Defend Against Malicious Email Campaigns

Security professionals can help defend their organizations against phishing campaigns such as the one described above by using ahead-of-threat detection to monitor for suspicious domains before they become active in a malspam operation. Teams should also leverage artificial intelligence (AI)-powered solutions to help defend against TrickBot and other constantly evolving threats.