Facebook is battling widespread spam attack

A widespread spam attack on Facebook has caused violent and pornographic images to be posted on some users' profile pages, representing one of the worst security breaches in the young website's history and raising concerns about its vulnerability to hackers.

The company, which acknowledged the problem Monday, said it was working to shut down the accounts responsible for the attack.

The disturbing pictures surfaced as the company tries to quell concerns about user safety and privacy. Facebook is reportedly near a settlement with the Federal Trade Commission over complaints about the way it stores and shares user data. Experts said that while this latest attack didn't appear to compromise users' data, it was a serious security breach.

"Protecting the people who use Facebook from spam and malicious content is a top priority for us, and we are always working to improve our systems to isolate and remove material that violates our terms," Facebook spokesman Andrew Noyes said in a statement. "Our efforts have drastically limited the damage caused by this attack, and we are now in the process of investigating to identify those responsible."

According to Facebook, users were somehow tricked into copying and pasting malicious code into their browser bars. Hackers then gained access to their profiles and could post whatever they wished, and any of the user's Facebook friends could see the images.

Chester Wisniewski, a security researcher at Sophos, said similar schemes in the past have lured users in with promises of free or discounted products.

It was unclear Tuesday who was responsible. Groups of hackers have threatened to put out a virus to "take down Facebook" over their concerns with the way it handles user privacy.