The ZyXEL Communications Corporation first introduced ZyNOS in 1998.
Web and/or command line interface (CLI) depending on the device. Web access is accomplished by connecting an ethernet cable between a PC and an open port on the device and entering the IP address of the device into the Web browser. An RS-232 serial console port is provided on some devices for CLI access, which is accomplished by using SSH or telnet.
CLI command types
Listed below are the categories that the CLI commands are grouped by.
- basic settings
- advanced application
- IP application
As of March 2014[update], Secunia reports seven advisories and six vulnerabilities on ZyNOS version 3.x. Five advisories are unpatched; Secunia rates the most severe unpatched advisory as less critical.
As of January 2015[update], a DNS vulnerability has been found in certain ZyNOS firmware versions. The versions that are affected have not been narrowed down. The attack can be done from a remote location regardless if the user interface is accessible from the outside of a LAN. 
- Mickey Tseng. "ZyNOS General FAQ". Zyxeltech.de. Retrieved 2014-03-07.
- [dead link]
- [dead link]
- "ZyBook2.book" (PDF). Retrieved 2014-03-07.
- "Ethernet Switch Reference Guide V3.90 (Nov 2008)" (PDF). Retrieved 2014-03-07.
- Mickey Tseng. "ZyNOS CI Command List". Zyxeltech.de. Retrieved 2014-03-07.
- "ZyXEL ZyNOS 4.x". Secunia. Retrieved 2014-03-07.
- "ZyXEL ZyNOS 3.x". Secunia. Retrieved 2014-03-07.
- "DNS hijacking flaw affects D-Link DSL router, possibly other devices". Lucian Constantin. Retrieved 2015-01-30.