Microsoft said that a third-party app was responsible for a glitch which saw handsets transmitting large amounts of data without users being aware, although it has not publicly stated which app it was. The company told SeattlePI that the app “is configured in a manner that potentially causes larger than expected data downloads,” and that it is in contact with the supplier to “assist them in making the necessary fixes.” Microsoft has only recently begun working with larger numbers of third party developers, having initially worked with a limited number of partners to bring “flagship” apps to the platform. With apps needing to pass through the vendor’s approvals process, this means that, somewhat surprisingly, the glitch was not picked up by either the developers or Microsoft.

According to recent reports, WP7 devices have been sending 30MB–50MB of data  per day when in an idle state, significantly eating in to the data allowances of customers without unmetered tariffs. Among the possibilities suggested was that devices were sending performance and usage data to Microsoft without user intervention, or that devices were using 3G connections when WiFi is available, leading to higher than expected data consumption.

While Microsoft has not named the offending app, several suggestions were made by users, including Facebook and Yahoo – with both being high-profile apps, this may explain why Microsoft has opted for discretion. Microsoft says that a “small (low single-digit) percentage” of customers had reported a problem.