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Firewalls made by Zyxel are being wrangled into a destructive botnet, which is taking control of them by exploiting a recently patched vulnerability with a severity rating of 9.8 out of a possible 10.

“At this stage if you have a vulnerable device exposed, assume compromise,” officials from Shadowserver, an organization that monitors Internet threats in real time, warned four days ago. The officials said the exploits are coming from a botnet that’s similar to Mirai, which harnesses the collective bandwidth of thousands of compromised Internet devices to knock sites offline with distributed denial-of-service attacks.

According to data from Shadowserver collected over the past 10 days, 25 of the top 62 Internet-connected devices waging “downstream attacks”—meaning attempting to hack other Internet-connected devices—were made by Zyxel as measured by IP addresses.