Zyxel products have a hardcoded root user you can access from internet

TL;DR: If you have a Zyxel USG, ATP, VPN, ZyWALL or USG FLEX you should update to the latest firmware version today. You can find the full list of affected devices here and the Zyxel advisory here.

Zyxel is a popular brand for firewalls that are marketed towards small and medium businesses. Their Unified Security Gateway (USG) product line is often used as a firewall or VPN gateway. As a lot of us are working from home, VPN-capable devices have been quite selling well lately.

When doing some research (rooting) on my Zyxel USG40, I was surprised to find a user account ‘zyfwp’ with a password hash in the latest firmware version (4.60 patch 0). The plaintext password was visible in one of the binaries on the system. I was even more surprised that this account seemed to work on both the SSH and web interface.

$ ssh zyfwp@
Password: Pr*******Xp
Router> show users current
No: 1
  Name: zyfwp
  Type: admin

The user is not visible in the interface and its password cannot be changed. I checked the previous firmware version (4.39) and although the user was present, it did not have a password. It seemed the vulnerability had been introduced in the latest firmware version. Even though older versions do not have this vulnerability, they do have others (such as this buffer overflow) so you should still update.

As SSL VPN on these devices operates on the same port as the web interface, a lot of users have exposed port 443 of these devices to the internet. Using publicly available data from Project Sonar, I was able to identify about 3.000 Zyxel USG/ATP/VPN devices in the Netherlands. Globally, more than 100.000 devices have exposed their web interface to the internet.

Source: Undocumented user account in Zyxel products (CVE-2020-29583) – EYE

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