(bad code)
If the JTAG interface on this device is not hidden by the manufacturer, the interface may be identified using tools such as JTAGulator. If it is hidden but not disabled, it can be exposed by physically wiring to the board.
By issuing a "halt" command before the OS starts, the unauthorized user pauses the watchdog timer and prevents the router from restarting (once the watchdog timer would have expired). Having paused the router, an unauthorized user is able to execute code and inspect and modify data in the device, even extracting all of the router's firmware. This allows the user to examine the router and potentially exploit it.
JTAG is useful to chip and device manufacturers during design, testing, and production and is included in nearly every product. Without proper authentication and authorization, the interface may allow tampering with a product.