( CNN ) -- Facebook says a hack that exploited some Web browsers was responsible for a flood of porn , violent images and other graphic content that spread across the site over the past couple of days . Spokesman Frederic Wolens said Facebook 's security team had been working to identify the cause of the spam and that , by Tuesday afternoon , `` we have eliminated most of the spam caused by this attack . '' `` We are now working to improve our systems to better defend against similar attacks in the future , '' Wolens said in an e-mail . 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 . The site advised users never to copy and paste unknown code into their browser bars , always use up-to-date browsers and use the `` Report '' links on Facebook to report suspicious activity when they see it . Facebook did not say anything about who may have been behind the attacks . With questions still abounding , speculation on the Web turned -- as it often does in online hacking cases -- the controversial `` hacktivist '' collective Anonymous . A group claiming allegiance to Anonymous announced it was going to make November 5 `` Kill Facebook Day . '' That day came and went with little noticeable activity . But last week , an Anonymous-affiliated group announced in a YouTube video that it had created the `` Fawkes virus , '' a sophisticated tool that would attack Facebook . A handful of Twitter feeds widely acknowledged as being run by Anonymous members had made no mention of the Facebook posts Tuesday morning . At least two members had previously distanced themselves from Operation Facebook , saying it was doomed to fail and that Anonymous is not a cohesive group with unanimously approved goals . `` Using a simple Facebook account , the worm can be carried into other accounts with little or no interaction , '' an automated voice says in the video posted on the account `` AnonSecurity157 . '' `` We did not expect the intensity with which this would spread . '' The video claims the worm can be controlled remotely and that once it 's fully understood it `` will use this to its advantage against corruption . ''
NEW : Facebook hack exploited browsers to spread porn , violent images . NEW : The site says most of the offensive material has been removed . Members of hacker collective Anonymous claim they created a virus to attack Facebook . Facebook : No data or account information was compromised in the attack .