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Xamarin for Android prior to version 5.1 allows to replace internal DLL files inside the APK with files on SD card which are not in a secure storage.
Malicious application without any special permissions could drop backdoored DLL files into

/storage/sdcard0/Android/data/app_id/files/.__override__/

and the victim application would use files from SD.
Not just the main application library could be hijacked, but also Xamarin's System.dll and Mono.Android.dll, which are shipped in all Xamarin for Android
applications.

Developers should rebuild their applications using Xamarin for Android 5.1 or newer in the release mode.

This vulnerability was found by accident, which allowed me to eat for free for a month.

Timeline:
    03.04.2015 Vulnerability is found
    07.04.2015 Message sent to Xamarin
    08.04.2015 Xamarin acknowledged the vulnerability
    29.04.2015 Fixed stable version released

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