This is bad:
More than 100,000 Zyxel firewalls, VPN gateways, and access point controllers contain a hardcoded admin-level backdoor account that can grant attackers root access to devices via either the SSH interface or the web administration panel.
[…]
Installing patches removes the backdoor account, which, according to Eye Control researchers, uses the “zyfwp” username and the “PrOw!aN_fXp” password.
“The plaintext password was visible in one of the binaries on the system,” the Dutch researchers said in a report published before the Christmas 2020 holiday.
Source: schneier.com