Google has released patch for the master key vulnerability to third party Android Mobiles, but google's Nexus devices still remain vulnerable against mater key exploits. Android user are advised not to download apps from 3rd party app markets. You can use the python version of the scanner (download link) to check for the exploits that exploit this vulnerability.
A simple web search will give details about this vulnerability.
Download Python version of the scanner;
MasterKey Exploit Scanner
I recommend you to develop Android version of this simple tool to use in your Android mobiles.
