Hackers Hijack Email Reply Chains on Unpatched Exchange Servers to Spread Malware

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A new email phishing campaign has been spotted leveraging the tactic of conversation hijacking to deliver the IcedID info-stealing malware onto infected machines by making use of unpatched and publicly-exposed Microsoft Exchange servers.

"The emails use a social engineering technique of conversation hijacking (also known as thread hijacking)," Israeli company Intezer said in a report shared with The Hacker News. "A forged reply to a previous stolen email is being used as a way to convince the recipient to open the attachment. This is notable because it increases the credibility of the phishing email and may cause a high infection rate."

The latest wave of attacks, detected in mid-March 2022, is said to have targeted organizations within energy, healthcare, law, and pharmaceutical sectors.
"The payload has also moved away from using Office documents to the use of ISO files with a Windows LNK file and a DLL file," researchers Joakim Kennedy and Ryan Robinson said. "The use of ISO files allows the threat actor to bypass the Mark-of-the-Web controls, resulting in execution of the malware without warning to the user."

The idea is to send fraudulent replies to an already existing email thread plundered from the victim's account by using the compromised individual's email address to make the phishing emails appear more legitimate.

"The use of conversation hijacking is a powerful social engineering technique that can increase the rate of a successful phishing attempt," the researchers concluded. "By using this approach, the email appears more legitimate and is transported through the normal channels which can also include security products."