Following a 4-month vacation, Emotet’s operators are back at it, borrowing a recently introduced spear-phishing technique to deliver their malware: hijacking legitimate email conversations.
Also referred to as Geodo, Emotet emerged as a banking Trojan, but has evolved into stealing other types of sensitive information and into becoming a downloader for other malware families, such as the TrickBot Trojan and Ryuk ransomware.
This year, Emotet has been absent from the threat landscape since early June, but the activity surrounding it has recommenced on September 16, multiple security organizations are reporting.
The new campaign appears widespread, targeting users all around Europe, but also those in the United States. Hundreds of thousands of messages were sent as part of this distribution effort.
In the early hours of Monday, malicious emails delivering Emotet featured templates in German, Polish, and Italian, Malwarebytes says. Since then, the attacks have expanded to include Austria, Switzerland, Spain, the United Kingdom, and the United States.
One of the most notable characteristics of the new campaign is the reuse of stolen email content to trick recipients into opening attached or linked-to Word documents containing malicious macros designed to fetch and execute Emotet.
“Once they have swiped a victim’s email, Emotet constructs new attack messages in reply to some of that victim’s unread email messages, quoting the bodies of real messages in the threads,” Cisco Talos notes.
This hijacking of legitimate email threads ensures higher success rates, as the recipient is more likely to open an attachment received as a reply in an ongoing conversation.
The technique involves not only taking over existing email conversations, but also using real subject headers and email contents, thus adding to the legitimacy of the messages and making the malicious email more difficult for anti-spam systems to filter.
Additionally, Emotet has been swiping victims’ credentials for sending outbound email and has also distributed the information to other bots in its network, which then used the credentials to send Emotet attack messages.
In April 2019, Emotet was using stolen email conversations in only 8.5% of the infection attempts. This week, stolen email threads appeared in nearly one quarter of Emotet’s outbound emails.
The malware operators also appear to have a considerably large database of potential recipients to draw from, with 97.5% of Emotet’s recipients in April 2019 receiving only a single malicious message.
“Although we frequently observe threat actors take breaks to retool, change payloads, or even take holidays, the breaks are not usually this long, particularly for malware that had become so prominent in the threat landscape. We observed traffic to the command and control infrastructure recently and expected that campaigns would resume shortly,” Sherrod DeGrippo, Senior Director of Threat Research and Detection at Proofpoint, told SecurityWeek in an emailed statement.