Hackers believed to be working for the North Korean government have upped their game with a recently discovered Mac trojan that uses in-memory execution to remain stealthy.

In-memory execution, also known as fileless infection, never writes anything to a computer hard drive. Instead, it loads malicious code directly into memory and executes it from there. The technique is an effective way to evade antivirus protection because there’s no file to be analyzed or flagged as suspicious.