This vulnerability is a command line injection in the Juplink RX4-1500 router (version 1.0.3). The vulnerability is triggered in their custom telnet daemon, and the endpoint to enable telnet has since been removed.
The vulnerability is triggered by simply running the kill command in their custom telnet interface. The vulnerability is due to unsanitized user input being executed inadvertently. The router manufacturers did not intend for their custom telnet management interface to run arbitrary system commands.
Here is an example of a maliciously crafted command: kill 9999; ls
For more information and vulnerability analysis, please see my report on the Juplink RX4-1500.