The "privacy leak" was spotted by Symantec, which said that the latest version of Norton Mobile Security, which includes a new Mobile Insight tool that dynamically assesses Android apps for potential security or privacy violations, had flagged Facebook's Android app.
"The first time you launch the Facebook application, even before logging in, your phone number will be sent over the Internet to Facebook servers," according to a Symantec blog post. "You do not need to provide your phone number, log in, initiate a specific action, or even need a Facebook account for this to happen."
According to Google Play, Facebook's Android app has been installed on between 100 million and 500 million devices. Symantec said that "a significant portion of those devices are likely affected."
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Numerous Android apps slurp excessive data for various reasons -- perhaps owing to developers' coding errors, to support "find my friend" features, or to allow developers or advertising networks to better track individual users.
But Facebook spokesman Derick Mains, confirming Symantec's bug report, said it had resulted from an inadvertent coding error. "The Android beta we released last week includes the fix," he said via email. "We did not use or process these numbers in any way, and have already deleted them from our servers."