One weakness of Android in practice is that the mobile carriers who provide nearly everyone with their phones are slow to provide updates to the operating system, if they ever do it at all. This leaves the vast majority of Android users running old versions of the OS.
A new project funded by DARPA, the Defense Advanced Research Projects Agency, installs an app on phones which monitors versions of software and reports it back to the servers of Duo Security, the company implementing the system.
According to Google's own stats (based on users of the Google Play store), only 10.7% of Android users are running the current version of the OS.
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