Don't worry: today remains V-Day and not Z-Day. On Monday, hackers were responsible for broadcasting bogus emergency messages warning TV viewers of an imminent zombie invasion. It's a series of intrusions that underscore the vulnerability of the nation's public warning system.

"Civil authorities in your area have reported that the bodies of the dead are rising from the grave and attacking the living," stated one warning broadcast over KRTV in Great Falls, Montana, according to Reuters. It went on to warn viewers not "to approach or apprehend these bodies as they are extremely dangerous."

Investigators have yet to determine the cause of the hacks, which were also perpetrated on Emergency Alert System devices used by stations in Michigan, California, Tennessee, and New Mexico. But Mike Davis, a hardware security expert and principal research scientist at security firm IOActive, told reporters he recently found a variety of weaknesses in some of the machines used to receive emergency messages and then automatically interrupt regular programming to broadcast them over the air. Weaknesses included devices that still used default passwords that are listed in user manuals hosted online and authentication bypass vulnerabilities that allow hackers to log in even when they don't have a password.

"There is some really, really, terrible software on the other side of that box," Davis told Threatpost.

While a bogus message about zombies seems little more than a harmless prank, the intrusion had potentially much more serious consequences. ABC10 and its sister station CW 5 disconnected their networks from the EAS system to prevent further intrusions. The move could have prevented authentic warnings from reaching viewers had an emergency broadcast been dispatched during that time. More importantly, the hoaxes underscore the vulnerability of a national alert system that's mandatory for all wireless, cable, and satellite TV systems. It was originally designed to give the US president the ability to address the public during times of emergency.

The vulnerability of the system is hardly unusual. Last week, Ars reported on critical weaknesses in widely used devices sold by Honeywell that allow hackers to commandeer heating systems, elevators, and other industrial equipment in large buildings. The defects in emergency alert devices are more of the same.