San Francisco - A software "worm" evidently unleashed by hackers burrowed through Tumblr on Monday, defacing pages at the popular blogging platform with a vitriolic, expletive-laden message.A hacker group going by the acronym "GNAA" claimed responsibility for the diatribe, which dismissed most of what bloggers shared online as worthless."We are aware that there is a viral post circulating on Tumblr," the US-based company said in a terse message posted at Twitter."We are working to resolve the issue as swiftly as possible."Tumblr reports that it is home to 83.2 million blogs that jointly attract about 18.2 billion page views monthly. High-profile Tumblr blogs defaced by the worm were said to include those of CNET and The Verge.