Researchers have discovered a previously unknown custom downloader family that reputed Vietnamese APT group OceanLotus has been using since at least early 2018 to infect victims with payloads such as Cobalt Strike Beacon.

The ongoing campaign's targets are either based in Vietnam or speak Vietnamese, which is in keeping with the m.o. of OceanLotus, which is known to conduct cyber espionage against foreign governments and dissidents of interest to the Southeast Asian nation.

Palo Alto Networks' Unit 42 division said in a company blog post today that its researchers observed a "high number of samples," of the DLL downloader – enough to determine that the attackers operated during typical Vietnamese working hours.

"OceanLotus has been an active threat actor group for a number of years and remains one of the most sophisticated threat actors in the APAC region," the blog post states. "As we have seen with the new KerrDown downloader being used in their recent campaigns, the group continues to build and employ new tools and techniques in their overall operations and playbooks."

Kerrdown infections occur one of two known methods, both of which involve phishing emails with content or file names written in Vietnamese. The first delivery leverages Microsoft Office documents with malicious macros while the second relies on RAR archives containing a legitimate program with DLL side-loading capabilities.

One observed sample of KerrDown was determined to download – in memory only – a variant of Cobalt Strike Beacon, a beaconing feature used by pentesters as part of the Cobalt Strike threat emulation software package. Although Cobalt strike is sold commercially, it is often misused for malicious purposes, Palo Alto notes.