Asus’ software update system was hacked and used to distribute malware to about 1 million Windows computers, according to the cybersecurity firm Kaspersky Lab. The malware was disguised as a “critical” software update, distributed from Asus’ servers, and signed using a real Asus certificate that made it appear to be valid. Details of the hack were first revealed by Motherboard, and Kaspersky plans to release more details at an upcoming conference.

It’s not clear what the hackers were after. However, the hackers did seem to target specific Asus customers: the malware included special instructions for 600 systems, to be identified by specific MAC addresses. Once one of those systems was detected, the update would then install more malicious programs to further compromise the system.

Kaspersky named the attack “ShadowHammer.” This kind of targeting is often associated with espionage attacks by nation states, most notably Stuxnet, which spread widely but did little to no harm on most infected systems.