It was found that in a certain release of the OS used on Zyxel network security devices an undocumented access was introduced.
This could be displayed as an unplanned error by the vendor but it arises the question if their products can be trusted at all to be run in any critical infrastructure.
source: https://www.eye.security/blog/undocumented-user-account-in-zyxel-products-cve-2020-29583
archived source: https://wellthissucks.xyz/2020/12/23/zyxel-undocumented-user-account-archived-source/