Challenge: Some new reindeers joined Santa’s sledding crew. To easily remember their names, Santa wrote them down in his app. Can you find out who are the new crew members? note: Flag format is still xmas{flag}

Attached file: Reindeers.apk

Oh, APK again, lets attack from the same angle. Starting again with http://www.javadecompilers.com/ to get whatever source we can an look at the files. First thing that pops out is that there is a multi arch lib folder, containing libfirmware.so. Looking at the main java file, we can also see that it uses some JNI calls.

After creating a new project in Android Studio and adding a few lines, the code looks like this:

I did look at the contents of the .so file and tried to disassemble it, but that seemed to complicated at the time. Instead just running the code, produced the following output:

Flag: xmas{CharL13m4cdEnn13}

What did I learn: Dynamic is simpler than static… Usually.