A video billboard displayed an image of a naked man on a busy intersection in Atlanta, Georgia after it was hacked by a group calling itself the Assange Shuffle Collective.
The billboard, in Buckhead, displayed the 'obscene and intimate' (although unidentifiable) image of the man, instead of going through its regular cycle of advertisements.
Passers-by called the police but the hackers, who also tried to change electronic signs outside McDonald's, have threatened to do it again.
The incident took place just days after security researcher Dan Tentler had warned Yesco, the company operating the billboard and thousands of others like it.
But despite his efforts, his findings were dismissed. "Not interested but thank you for the follow-up," the company told via email.
The FBI and Homeland Security are investigating the incident.
'Happy afternoon entertainment'
The Assange Shuffle Collective claimed responsibility for the stunt after a photo of the sign was posted to Reddit, calling it "happy afternoon entertainment," and adding: "Ironically, we didn't realise that Buckhead was an incredibly affluent neighborhood, which makes the whole thing terrifically good fun. Burn the rich."
The group, which claimed the hack "actually took some work on our part," said that Buckhead was "far from the only place that had some gaping holes exposed".
Although revealing that the billboards and their connections to the internet "are secured" the group wanted "to cause some chaos, and not much gets between us and chaos." It also posted what is believed to be a screenshot of another billboard's status page, including what images it is programmed to display, which had been changed to the same as shown in Buckland.
'Eat the wealthy'
The group also revealed that it was going to hack into a number of McDonald's noticeboards, changing their message from "Now Hiring" to "Eat The Wealthy," but they "got bored of waiting for the file format conversions to happen. That idea may still rise again, though, we'll see."
Tentler tweeted to explain how it was easy to find hundreds of similar billboards online which were all vulnerable in the same way; he added that in cases such as these the attack is less of a hack and more a case of digital trespass, because the passwords are easily guessable. "Logging in with default creds [credentials] is like finding an unlocked car with the keys still in the ignition," he said.