Earlier Tuesday, Graham Cluley, a consultant with Web security firm Sophos, said that "explicit and violent" images had been flooding the News Feeds of Facebook users for the past 24 hours or so.
Cluley wrote on the Sophos blog that the images included hardcore porn; photoshopped images of celebrities, including teen pop star Justin Bieber, in sexual positions; "extreme violence;" and at least one image of an abused dog.
"What's clear," Cluley wrote, "is that mischief-makers are upsetting many Facebook users and making the social networking site far from a family-friendly place."
Several CNN.com staffers reported seeing some of the images by Tuesday morning.
Facebook's Wolens said that users were tricked into pasting malicious script into their browser URL bars, causing them to unknowingly share the offensive content.
He said no data or account information was compromised during the attack.
The blog AllFacebook reported that the social-media giant had been quietly taking down the images for most of the day Tuesday.
Writer Jackie Cohen said a request for comment on the images merely got a reply thanking her for "flagging" the images.
"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: people were commenting on the images more than flagging them," she wrote.
Users were, understandably, distraught.
"Seeing a dead dog on my Facebook news feed ........ Officially deactivating it," said one Twitter user in a post collected by Sophos.
"I saw a dead dog, Justin Bieber [performing a sex act] and a naked grandma," said another. "Time to delete facebook."
The Facebook statement said the site has built a mechanism to shut down pages sharing the links and contacted people affected by the attack with information about how to protect themselves.
