The publisher of the uTorrent file-sharing program has admitted to suffering a major security breach that allowed attackers to substitute downloads of its client for malware pushing fake antivirus software.Anyone who downloaded its uTorrent program between 4.20am Pacific time (12.20pm BST) and just after 6am on 13 September will have been downloading the Security Shield scareware program, which pesters the user to pay for protection against non-existent threats it claims to have detected.Originally, the company believed that both BitTorrent and the cut-down uTorrent clients had been affected, but a later clarification said that the former was not now thought to be involved."Clarification: This only affects users who downloaded software specifically from utorrent.com between the hours above this morning. Users who previously downloaded our software are not affected," said a company blog."After further analysis, we don't believe BitTorrent.com or the BitTorrent Mainline\/Chrysalis clients were part of the incident."Not everyone is totally convinced by this statement, starting with said Paul Ducklin of Sophos.."Confusingly, the BitTorrent blog has recently been updated to claim that the software available from the www.bittorrent.com URI was not affected, implying that only those who downloaded UTorrent during the infection window would be at risk," he wrote in a comment article."Since the two sites share the same network infrastructure - both resolve to the same IP number in Limelight Networks' cloud - you might want to ignore that blog update and assume that any recent downloads from Bittorrent Inc. were dodgy and give yourself a thorough anti-malware checkover," said Ducklin.Although the fake antivirus concerned should be relatively easy to block for an up-to-date antivirus program, users might be tempted to ignore warnings as false positives. Once installed, it is also possible that the Trojan could call further malware.The company hasn't said how its security was breached, merely that one of its web servers was involved.The involvement of BitTorrent is ironic given that the network itself has in the past been used to distribute malware, usually in the form of programs masquerading as genuine but pirated software.