The issue is related to a command injection vulnerability in the configuration parser of the affected Zyxel devices. This vulnerability can be exploited by an unauthenticated, LAN-based attacker to execute some OS commands by using a crafted GRE configuration when the cloud management mode is enabled. The exploitation occurs during an attempted `ip addr` command, and the order of operations is important for successful exploitation. It is estimated that out of approximately 7,600 devices using vulnerable firmware, around 607 were using the vulnerable configuration.