Winners of 10,000 MP3 players given away in a Japanese fast-food promotion won more than they bargained for, as their prizes contained a worm.

If they connected their new music players to their computers to fill them with songs then the worm, WORM_QQPASS.ADH, could infect the computers and steal personal data.

McDonald's Japan warned prize winners about the infected players on Friday. It offered software provided by Trend Micro that can remove the worm from infected computers, and invited winners to return their MP3 players for a free replacement.

The worm spreads from the MP3 player to other removable drives on computers running Windows and attempts to disable some antivirus applications. If it finds the QQ Instant Messenger application running, it attempts to steal account login information and other chat details, and transmit them by email.

McDonald's sent the prizes to 10,000 winners in late September.