The collider is a massive underground particle accelerator designed to conduct particle physics experiments. It lies beneath the border between France and Switzerland.
On Wednesday, just as the collider was about to be tested for the first time, a group calling itself the "Greek Security Team" hacked into a computer connected to the collider and defaced a CERN Web page with a message in Greek.
According to the Daily Telegraph, the hackers derided CERN's IT security staff as "a bunch of schoolkids." They professed to have no plan to disrupt collider experiments; they said they wanted to highlight the lab's security problems.
"We're pulling your pants down because we don't want to see you running around naked looking to hide yourselves when the panic comes," the hackers wrote, according to the Daily Telegraph's translation of the Greek defacement.
CERN didn't immediately respond to a request for comment.
A CERN spokesman told the BBC that no damage was done, but that the incident revealed the need for stronger security.
Though the hacked page has since been removed, the Daily Telegraph has preserved a screenshot.
An unnamed CERN scientist told the Daily Telegraph that the hackers were "one step away" from a computer than controlled one of the 12,500-ton magnets in the collider. Had they managed to get into that machine, they could have turned parts of the collider off.