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A new phishing campaign is being conducted using the TrickBot botnet to deliver the Bazar backdoor and Buer loader malware. The campaign was detected by researchers at Area 1 Security and has been running since early October.
The Bazar backdoor is used to gain persistent access to victims’ networks, while the Buer loader is used to download additional malicious payloads. Previously, Buer has been used to deliver ransomware payloads such as Ryuk and tools such as CobaltStrike.
Area 1 Security researchers detected two email lures in this campaign. One is a fake notification about termination of employment and the other a fake customer compliant. The employment termination email appears to have been sent by an authority figure in the head office of the company being targeted and states that the individual has been terminated. Further information on the termination and payout are provided in a document that appears to be hosted on Google Docs.
If the link is clicked, the user will be directed to a Google Doc decoy preview page and is advised to click another link if they are not redirected. That link directs them to a URL where a file download is initiated. The user will be presented with a security warning asking if they want to run the file. Doing so launches a PE32+ executable on Windows systems and triggers a sequence of events that results in the download of either the Buer loader or the Bazar backdoor. Constant Contact links are also being used in this campaign.
The use of cloud services for hosting malicious documents is now commonplace. It is a tactic used to bypass security solutions that scan attached files for malicious code such as macros. By linking to legitimate cloud services, some security solutions will fail to detect the link as malicious and will deliver the emails to users’ inboxes. Should the links in the emails be classified as malicious by URL scanning security solutions, the attackers can simply switch to different URLs.
Last month Microsoft announced a takedown operation that saw it take control of the infrastructure used by the operators of TrickBot. This major operation was only temporarily effective at disrupting the botnet infrastructure. Microsoft said the takedown operation was only likely to be temporary, as the TrickBot operators would likely rebuild their operation on different infrastructure.
Area 1 Security researchers note that this campaign resumed after just two days after the takedown of the botnet and, this time around, the TrickBot gang is using sinkhole resistant EmerDNS TLDs, which make any further takedown attempts difficult.