After hackers played several high-profile pranks with traffic signs, including warning San Francisco drivers of a Godzilla attack, the US government advised operators of electronic highway signs to take "defensive measures" to tighten security.
Last month, signs on San Francisco's Van Ness Avenue were photographed flashing "Godzilla Attack! Turn Back" and highway signs across North Carolina were altered last week to read "Hack by Sun Hacker".
The Department of Homeland Security's Industrial Control Systems Cyber Emergency Response Team, or ICS-CERT, has advised cities, highway operators and other customers of digital-sign maker Daktronics Inc to take "defensive measures" to minimise the possibility of similar attacks.
It said information had been posted on the internet advising hackers how to access those systems using default passwords in the company's software.
"ICS-CERT recommends entities review sign messaging, update access credentials and harden communication paths to the signs," the agency said.