Emotet Returns to Hit 100K Mailboxes Per Day

Just in time for the Christmas holiday, Emotet is sending the gift of Trickbot.

After a lull of nearly two months, the Emotet botnet has returned with updated payloads and a campaign that is hitting 100,000 targets per day.

Emotet started life as a banking trojan in 2014 and has continually evolved to become a full-service threat-delivery mechanism. It can install a collection of malware on victim machines, including information stealers, email harvesters, self-propagation mechanisms and ransomware. It was last seen in volume in October, targeting volunteers for the Democratic National Committee (DNC); and before that, it became active in July after a five-month hiatus, dropping the Trickbot trojan. Before that, in February, it was seen in a campaign that sent SMS messages purporting to be from victims’ banks.

“The Emotet botnet is one of the most prolific senders of malicious emails when it is active, but it regularly goes dormant for weeks or months at a time,” said Brad Haas, researcher at Cofense, in a Tuesday blog. “This year, one such hiatus lasted from February through mid-July, the longest break Cofense has seen in the last few years. Since then, they observed regular Emotet activity through the end of October, but nothing from that point until today.”

The botnet is also staying true to form in terms of payloads, researchers said. “In October the most common secondary payloads were TrickBot, Qakbot and ZLoader; today we observed TrickBot,” according to Haas.

The TrickBot malware is a well-known and sophisticated trojan first developed in 2016 as a banking malware – like Emotet, it has a history of transforming itself and adding new features to evade detection or advance its infection capabilities. Users infected with the TrickBot trojan will see their device become part of a botnet that attackers use to load second-stage malware – researchers called it an “ideal dropper for almost any additional malware payload.”

Typical consequences of TrickBot infections are bank-account takeover, high-value wire fraud and ransomware attacks. It most recently implemented functionality designed to inspect the UEFI/BIOS firmware of targeted systems. It has made a serious resurgence following an October takedown of the malware’s infrastructure by Microsoft and others.

Several security firms spotted the latest campaign, with Proofpoint noting via Twitter, “We’re seeing 100k+ messages in English, German, Spanish, Italian and more. Lures use thread hijacking with Word attachments, pw-protected zips and URLs.”

Thread hijacking is a trick Emotet added in the fall, flagged by researchers at Palo Alto Networks. The operators will insert themselves into an existing email conversation, replying to a real email that’s sent from a target. The recipient has no reason to think the email is malicious.

Sherrod DeGrippo, senior director of threat research and detection at Proofpoint, told Threatpost that the campaign this week is pretty standard fare for Emotet.

“Our team is still reviewing the new samples and thus far we’ve only found minor changes. For example, the Emotet binary is now being served as a DLL instead of an .exe,” DeGrippo said. “We typically observe hundreds of thousands of emails per day when Emotet is operating. This campaign is on par for them. As these campaigns are ongoing, we are doing totals on a rolling basis. Volumes in these campaigns are similar to other campaigns in the past, generally around 100,000 to 500,000 per day.”

She added that the most interesting thing about the campaign is the timing.

“We typically see Emotet cease operations on December 24 through early January,” she noted. “If they continue that pattern, this recent activity would be incredibly short and unusual for them.”

Malwarebytes researchers meanwhile noted that the threat actors are alternating between different phishing lures in order to social-engineer users into enabling macros – including COVID-19 themes. The researchers also observed the Emotet gang loading its payload with a fake error message.

Haas’ Cofense team observed the same activity, noting that it marks an evolution for the Emotet gang.

“The new Emotet maldoc includes a noticeable change, likely meant to keep victims from noticing they’ve just been infected,” he said. “The document still contains malicious macro code to install Emotet, and still claims to be a “protected” document that requires users to enable macros in order to open it. The old version would not give any visible response after macros were enabled, which may make the victim suspicious. The new version creates a dialog box saying that “Word experienced an error trying to open the file.” This gives the user an explanation why they don’t see the expected content, and makes it more likely that they will ignore the entire incident while Emotet runs in the background.”

DeGrippo told Threatpost that an initial look at the emails indicates that some of the hijacked threads ask recipients to open a .zip attachment and provide a password for access.

The malware’s resurgence, though lacking in any dramatic developments from previous activity, should be watched by administrators, researchers said.

“Emotet is most feared for its alliances with other criminals, especially those in the ransomware business. The Emotet – TrickBot – Ryuk triad wreaked havoc around Christmas time in 2018,” according to Malwarebytes. “While some threat actors observe holidays, it is also a golden opportunity to launch new attacks when many companies have limited staff available. This year is even more critical in light of the pandemic and the recent SolarWinds debacle. We urge organizations to be particularly vigilant and continue to take steps to secure their networks, especially around security policies and access control.”

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