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World’s Most Destructive Botnet Returns With Stolen Passwords and Email In Tow

An anonymous reader quotes a report from Ars Technica: 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. 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. The use of previously sent emails isn’t new, since Emotet did the same thing before it went silent in early June. But with its return this week, the botnet is relying on the trick much more. About 25% of spam messages Emotet sent this week include previously sent emails, compared with about 8% of spam messages sent in April. “To make sending the spam easier, Emotet also steals the usernames and passwords for outgoing email servers,” the report adds. “Those passwords are then turned over to infected machines that Emotet control servers have designated as spam emitters. The Talos researchers found almost 203,000 unique pairs that were collected over a 10-month period.”

Malwarebytes says Emotet has brought back another tactic where it refers to targets by name in subject lines. “Once opened, the documents attached to the emails claim that, effective September 20, 2019, users can only read the contents after they have agreed to a licensing agreement for Microsoft Word,” reports Ars Technica. “And to do that, according to a post from security firm Cofense, users must click on an Enable Content button that turns on macros in Word.”

“After Office macros are enabled, Emotet executables are downloaded from one of five different payload locations,” Cofense researchers Alan Rainer and Max Gannon wrote. “When run, these executables launch a service that looks for other computers on the network. Emotet then downloads an updated binary and proceeds to fetch TrickBot if a (currently undetermined) criteria of geographical location and organization are met.”

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