Exploit:Android/Masterkey identifies code designed to exploit a known vulnerability in the way the Android operation system verifies the authenticity of an app.
Once the scan is complete, the F-Secure security product will ask if you want to uninstall the file, move it to the quarantine or keep it installed on your device.
A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:
Check for the latest database updates
First check if your F-Secure security program is using the latest updates, then try scanning the file again.
Submit a sample
After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.
Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.
Exclude a file from further scanning
If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.
Note: You need administrative rights to change the settings.
From the F-Secure Mobile Threat Report Q3 2013:
In July [2013], security researchers publicly announced the discovery of a vulnerability in cryptographic signature verification for Android apps that, if exploited, would allow an attacker to modify a legitimate app’s code without affecting its cryptographic signature - essentially keeping the tampering from being detected during verification. Shortly after the announcement, researchers were able to find samples of such modified apps being distributed.
Google was reportedly notified of the "Masterkey" issue earlier in the year, and at the time of the announcement had fixed the issue in the Android open source codebase. Users would however still need to wait for a firmware update from their device manufacturer in order to receive the patched code.