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If you've noticed an uptick of spam that addresses you by name or quotes real emails you've sent or received in the past, you can probably blame Emotet. It's one of the world's most costly and destructive botnets—and it just returned from a four-month hiatus.
Emotet started out as a means for spreading a bank-fraud trojan, but over the years it morphed into a platform-for-hire that also spreads the increasingly powerful TrickBot trojan and Ryuk ransomware, both of which burrow deep into infected networks to maximize the damage they do. A post published on Tuesday by researchers from Cisco's Talos security team helps explain how Emotet continues to threaten so many of its targets.
Spam sent by Emotet often appears to come from a person the target has corresponded with in the past and quotes the bodies of previous email threads the two have participated in. Emotet gets this information by raiding the contact lists and email inboxes of infected computers. The botnet then sends a follow-up email to one or more of the same participants and quotes the body of the previous email. It then adds a malicious attachment. The result: malicious messages that are hard for both humans and spam filters to detect.
"It's easy to see how someone expecting an email as part of an ongoing conversation could fall for something like this, and it is part of the reason that Emotet has been so effective at spreading itself via email," Talos researchers wrote in the post. "By taking over existing email conversations and including real Subject headers and email contents, the messages become that much more randomized and more difficult for anti-spam systems to filter."
Using the Emotet spam message shown above, which incorporates a previous conversation between two aides to the mayor of a US city, here's how the ruse works, according to Talos:
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