For the past several days, many Facebook users have been seeing adult images in their news feeds that feature pornography, violence and animal abuse \u2014 the result of what security analysts say is a mystery attack.Some of the images include a Photoshopped picture of Justin Bieber in a pornographic situation and a bloodied, dead dog.Graham Cluley, Senior Technology Consultant with Sophos, weighed in on the trend in his Naked Security blog Tuesday morning. According to Cluley, the content is often showing up on feeds without the knowledge of the member that appears to be responsible for it.[Also see: Security firm to Facebook: Clean up your act]"It isn't presently clear precisely how the offending content has been spread \u2014 whether users are falling for a clickjacking scheme, are being tagged in content without their knowledge, have poorly chosen privacy settings, have been tricked into installing malicious code, or have fallen victim to another vulnerability inside Facebook itself," said Cluley in his blog post. "What's clear, however, is that mischief-makers are upsetting many Facebook users and making the social networking site far from a family-friendly place."Several other news outlets, however, claim the offensive content is the result of a clickjacking exploit that originates with a malicious link that users click on hoping to see a Kim Kardashian video. After clicking on the link, all friends in the user's network get spammed with the disturbing images.[Also see: Social media risks: The basics]News of the hack is being widely discussed on Twitter, with many questioning whether the attack is the work of hacktivist group Anonymous, which had made threats several months ago to take down the social network on November 5th. With that date passed, some are wondering if the porn hack is a late attempt to make a statement.No group has officially taken the blame for the malicious activity yet and Facebook says it is investigating."It's precisely this kind of problem which is likely to drive people away from the site," said Cluley. "Facebook needs to get a handle on this problem quickly, and prevent it from happening on such a scale again."