Exploit:Android/Masterkey identifies code designed to exploit a known vulnerability in the way the Android operation system verifies the authenticity of an app.
The vulnerability leveraged by the exploit is application or platform specific; in other words, a specific program (or even a specific version of a particular program) must be installed in order for the exploit to be effective.
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From the F-Secure Mobile Threat Report Q3 2013:
In July , 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 appÃ¢â‚¬â„¢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.