Security firm Sophos warns Facebook users that the social-networking site has been hit by nasty porn storm and has been flooded with objectionable material for the past 24 hours.

“The content, which includes explicit hardcore porn images, photoshopped photos of celebrities such as Justin Bieber in sexual situations, pictures of extreme violence and even a photograph of an abused dog, have been distributed via the site—seemingly without the knowledge of users.”

“For the last 24 hours, many people have reported seeing highly-offensive images on their Facebook news feeds,” said Graham Cluley, a senior technology consultant at antivirus vendor Sophos, in an interview early Tuesday.

Facebook confirmed the attack and said it had “dramatically limited the damage” and was on the trail of those responsible.

“We experienced a coordinated spam attack that exploited a browser vulnerability,” Facebook spokesman Andrew Noyes said in a statement emailed to Reuters. “Our efforts have drastically limited the damage caused by this attack, and we are now in the process of investigating to identify those responsible.”

The fact that these photos spread for as long as 48 hours unchecked shows how much Facebook relies on individual users to flag inappropriate content. So stop commenting on the images and start flagging them.