North Korean hackers have adopted a new trick to evade Microsoft Windows’ built-in defenses and spread powerful malware, security researchers said Tuesday, successfully utilizing the technique against at least one international firm.

According to a report from Russian cybersecurity firm Kaspersky, a North Korean threat group tracked as BlueNoroff has begun burying malicious scripts inside of optical disk image (.iso) and virtual hard disk (.vhd) files. The technique prevents Windows from flagging the file as originating from the internet, a security mechanism that usually provides warning pop-ups to the user.

“This technique only works for