According to a report released Monday by Kaspersky Lab, Darkhotel is not known to have been a client of the Italian Hacking Team spyware company, but took advantage of the zero-day exploit after it was leaked last month.
That's not the group's only use of zero-day exploits. According to Kaspersky, Darkhotel has been investing "significant money" in several zero-days.
Over the past year, the group has also extended its geographical reach around the world, and is targeting new victims from North and South Korea, Russia, Japan, Bangladesh, Thailand, India, Mozambique and Germany, said Kurt Baumgartner, principal security researcher at Kaspersky Lab.
"Some of the targets are diplomatic or have strategic commercial interests," he said.
This is in addition to the group's existing focus on top executives from the U.S. and Asia in the electronics, finance, pharmaceutical, automotive, chemical, and defense industries.
The group continues to make use of stolen certificates and extremely targeted and long-term phishing techniques. For example, some targets can be hit by attacks several months apart.
Its defensive techniques have also been improved, Baumgartner said.
"Darkhotel now tends to hide its code behind layers of encryption and appear to be using SSH on victim hosts," he said. "It is likely that it has slowly adapted to attacking better-defended environments. And not only are its obfuscation techniques becoming stronger, but its anti-detection technology list is growing."
The latest version of its downloader can now identify and bypass antivirus software from 27 vendors.
Baumgartner declined to comment on Darkhotel's national origins, or name individual companies that have been targeted.
He recommended that companies train employes to be aware of spearphishing techniques.
He also suggested that employees should be familiar with the right-to-left-override method of faking file names. This is a technique in which hackers use Unicode characters to change the direction in which text is written, normally used for Arabic and Hebrew text. But the bad guys can also use this technique to change file names so that, say, an executable can have a more innocent-seeming extension like PDF or JPG.
When the user clicks on what they think is a JPG image file, the executable code runs, instead, and one of the things it does is save an actual image file and open it with MSPaint. Then, while the user is distracted by the picture, it installs malware downloader code.
"Organizations should pay careful attention to unusual netflow and of course deploying anti-malware capabilities that can identify and prevent zero-day exploit activity is very helpful," he added.