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.

Asus has yet to comment on Kaspersky’s findings

It doesn’t appear that Asus has contacted customers or taken action to stop the malware. In an email to The Verge, Asus said it would issue an official statement on the malware tomorrow afternoon. Asus apparently denied that the malware had come from its servers after being contacted by Kaspersky, then it stopped responding, according to Motherboard.

While the malware could have been distributed to 1 million computers, Kaspersky tells Motherboard that the total PCs that installed it is estimated to be in the “hundreds of thousands.” Kaspersky says 57,000 people using its security software had the malware installed, and Symantec told Motherboard that it identified 13,000 customers with the malware.

Hacking a company’s update system allows malicious actors to breach computers on a wide scale. It hasn’t been done frequently, but the fact that it can be done at all is a huge risk. Work is being done to develop more secure update systems, but for now, companies largely rely on their own solutions.

Update March 25th, 12:30PM ET: Asus says it will issue a statement tomorrow afternoon.