Need Xposed framework to work: http://forum.xda-developers.com/showthread.php?t=1574401
If you are unaware of the "Master-Key security vulnerability", please read the report:
http://bluebox.com/corporate-blog/bluebox-uncovers-android-master-key/
This patch is based on the official patch codebase from Google, adapted as Xposed module.
To test whether you are actually protected by this patch, try installing this proof-of-concept demo apk:
https://github.com/Fuzion24/AndroidMasterKeys/blob/master/example/apks/modded.apk?raw=true
If it says "not installed", congratulations, you are now immune to Master-Key vulnerability!
NOTE: BlueboxScanner may still report your device as "Unpatched/vulnerable". Just relax, its a false report. Do a real test with the above PoC demo.
Source code: https://gist.github.com/oasisfeng/5993367