Researchers at Intezer warn that attackers are hijacking email conversations to distribute the IcedID banking Trojan. This technique makes the phishing emails appear more legitimate and helps them bypass security filters.

“In the new IcedID campaign we have discovered a further evolution of the threat actors’ technique,” the researchers write. “The threat actor now uses compromised Microsoft Exchange servers to send the phishing emails from the account that they stole from. 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. 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. With regards to targeting, we have seen organizations within energy, healthcare, law, and pharmaceutical sectors.”

The researchers explain that conversation hijacking is “a powerful social engineering technique” because the phishing email appears to be coming from a trusted contact.

“The attack-chain starts with a phishing email,” the researchers write. “The email includes a message about some important document and has a password protected ‘zip’ archive file attached. The password to the archive is given in the email body…. What makes the phishing email more convincing is that it’s using conversation hijacking (thread hijacking). A forged reply to a previous stolen email is being used. Additionally, the email has also been sent from the email account from whom the email was stolen from.”

While this tactic isn’t unique to this threat actor, the researchers note that this development shows that the attackers are continuing to improve their operations.

“In the current mid-March campaign, we have discovered reuse of the same stolen conversation now being sent from the email address that received the latest email,” Intezer says. “Back in January when this conversation was also used, the “FROM” address was “webmaster@[REDACTED].com” with the name of the recipient of the last email in the conversation. By using this approach, the email appears more legitimate and is transported through the normal channels which can also include security products.”

New-school security awareness training can enable your employees to thwart social engineering attacks.