Amazon is not just nudging shoppers to be careful this holiday season, it is issuing a direct warning that millions of its customers are being targeted by increasingly sophisticated scams. The company is flagging a surge in fake messages, bogus websites and account takeover attempts that piggyback on its popularity and the crush of seasonal deals. At stake is the security of more than 300 m active users worldwide and the money flowing through what has become the default storefront for much of the planet.