Zyxel warns of bad signature update causing firewall boot loops

Zyxel reports that a faulty Application Signature Update pushed on Jan 24–25 caused USG FLEX and ATP Series firewalls (ZLD, with active security licenses) to experience reboot loops, high CPU, ZySH daemon errors, and login failures; Nebula-managed devices and USG FLEX H (uOS) are unaffected. Recovery requires on-site console access via RS232 to back up configurations, load a special firmware, and then restore settings via the web GUI; Zyxel provided step-by-step guidance and scheduled support sessions, emphasizing this is not a CVE or security issue.