The Zyxel Multy X (AC3000 Tri-Band WiFi System) device doesn't use a suitable mechanism to protect the UART. After an attacker dismantles the device and uses a USB-to-UART cable to connect the device, he can use the 1234 password for the root account to login to the system. Furthermore, an attacker can start the device's TELNET service as a backdoor.

References

• https://nvd.nist.gov/vuln/detail/CVE-2018-9149
• https://www.slideshare.net/secret/qrHwDOJ71eLg7f