(Practical) Android Malware Analysis


BSides London  - 3rd June 2015



Paul AMAR / Etienne Stalmans

# who



Paul [at] SensePost [.dot.] com     

@PaulWebSec / GitHub: PaulSec


Etienne [at] SensePost [.dot.] com

@kamp_staaldraad

why do we care?



Android Growth - Fortinet results  (2014 report)

What?



Static Analysis (Androguard, Dex2Jar, apktool, ...)    

Dynamic Analysis (DroidBox using the emulator)     


5 pracs to practice what we covered in the slides


how?


Need Vmware Player:                                            

> Create a new Machine                                    

> Linux, Ubuntu                                                        

> Import the Hard-drive with the .vmdk

Launch it, creds: student/mobile                


Tools available at ~/Tools                                       

Samples available at ~/Desktop/samples/

VMware perf



Enable Virtualization performance

Androguard



Disassembles/Decompiles Android apps                     

Different tools: androlyze, androdis, androauto, ...



https://github.com/androguard/androguard

Androguard 101


Let's analyze the APK!


Get in the folder:

$ cd ~/Tools/androguard


Run Androlyze using shell mode:
$ python ./androlyze.py -s

Androguard 102


In the shell, load the APK:

sample = APK('/path/to/file.apk')


And start investigating:
sample.get_permissions()
sample.get_activities()        
sample.show()                        
....                                                 

Androguard 103


d = dvm.DalvikVMFormat(a.get_dex())               


for _class in d.get_classes():                                         
    print _class.get_name()                               

for method in _class.get_methods():

print method.get_name()

Retrieves classes/methods from the loaded APK

but.. apk, apk..


an .APK is basically a ZIP archive.
Has been signed, and compiled     


Unzip it using:

$ unzip /path/to/file.apk

what's in there?


META-INF: meta info directory
lib: directory containing compiled code
res: resources directory
assets: application assets directory
AndroidManifest.xml: additional manifest file describing name, version, access rights and referenced library files for the app
classes.dex: the main Dalvik Executable file
resources.arsc: precompiled resources e.g. binary XML

APKtool


Disassembles/rebuilds resources to JAR/APK


$ apktool d /path/to/file.apk -o out/

Disassembles the APK

$ apktool b foo/
Builds foo folder into foo/dist/foo.apk file

https://ibotpeaches.github.io/Apktool/

usually, *phun* is in


*.dex files. 


DEX = Dalvik EXecutable file         

code that runs in the Dalvik VM


We need to convert it to a JAR archive

dex2jar



A set of tool to work with Android .dex and java .class files

Read/write the Dalvik Executable (.dex) file, Disassemble .dex to smali files, Convert .dex file to .class files (zipped as jar)


https://github.com/pxb1988/dex2jar

dex2jar 101



Convert .dex to a .jar
$ /path/to/d2j-dex2jar.sh /path/to/file.dex

JAR archive will be in d2j's folder.

JD-GUI



At that point, you can use any Java decompiler.


java -jar jd-gui-1.1.0.jar


http://jd.benow.ca/                                                      

https://github.com/java-decompiler/jd-gui




TADAM!

feeling a bit lazy?



https://gist.github.com/PaulSec/39245428eb74577c5234

but wait, there's more..


jadx -  tools to produce Java source code from Android Dex and Apk files
https://github.com/skylot/jadx

jadx 101 (RLY?)



$ jadx /path/to/file.apk        


$jadx-gui /path/to/file.apk


Done.

So, now...


Got the source code, might contain hundred of classes.

Thousand lines of code.                                                                         


Where/What to look for?


observations


Often..


  • Using HTTP to communicate (and/or SMS)
  • no SSL certificate for the panels
  • IMEI used as the victim's identifier
  • Encrypting using AES
  • Encoding data in Base64 (still..)

Low hanging fruit


Save the source files (*.java) in:

eg. /tmp/sample_test


And search for specific terms:

$ cd /tmp/sample_test
$ grep -r -i 'cipher' .        
$ grep -r -i 'http://' .        
$ grep -r -i 'base64' .     
....                                       

then,



Analyze how the communication  works                

Retrieve encryption keys                                                    

See how the app interacts on the filesystem        

Dynamic analysis



Basically, testing/evaluating the application by running it


In this workshop, two ways:

  1. Directly from the emulator
  2. Using DroidBox

adb


Stands for: Android Debug Bridge

Command line tool to communicate with

emulator or connected devices



https://developer.android.com/tools/help/adb.html

https://github.com/maldroid/adb_cheatsheet              

adb 101


List the devices already connected

$ adb devices


Install .APK
$ adb install /path/to/file.apk

Push/Pull file from device
$ adb push/pull <local> <remote>

Droidbox


Runs app in the emulator            
 Logs everything that happens..

.. and retrieves bunch of information:

  • Incoming/outgoing network data
  • File read and write operations
  • Sent SMS and phone calls, ...


Droidbox


Create a new AVD (Android Virtual Device),

Nexus 4, Android version 4.2.1

$ android


Start the emulator

$ ./startemu.sh <AVD name>


And install/launch the app
$ ./droidbox.sh /path/to/file.apk

burp suite setup


In order to monitor the HTTP(s) traffic

Configure a proxy on your phone:            


  • Wifi, Press <Network  Name>
  • Modify network
  • Show advanced options
  • Insert the proxy details


And launch the app.                                   




pracs




The VM comes up with 5 APKs.                                    
Check the file samples.txt and start in this order

~/Desktop/samples/samples.txt

Each prac should take around 20/30 mins.          

funny samples (1/4)




Ciphered strings using AES

sha256:c0cb135eef45bb8e411d47904ce638531d53473729c7752dc43c6d55d5ed86f8

FUNNY samples (2/4)

String obfuscation - XOR

sha256:99c4d780c0143af20191d6ffb0cc206605e397330ddd6a84185df1d112c1e963


Funny samples (3/4)



Interesting persistence technique

sha256:f75500da9728d95e33e40f9a1d8bf29959d5aa89827aeabfb3aaaa02a488dd39



FUNNY SAMPLES (4/4)



recent PornDroid sample (May 2015)

sha256:d5f29750a8cb158d9b89a1e02e8addc5e410d1ddc48e660589144ade47f794c5

Last sample?



bot.apk - iBanking sample                       

Specially crafted for BSides London

Retrieve phone numbers, C&C URL

wrap-up


This is just an introduction.                            

Lot of research regarding obfuscation:


Dex Education - Practicing Safe Dex | Black Hat 2012

DEF CON 22 - Tim Strazzere and Jon Sawyer - Android Hacker Protection Level 0

how to keep informed?



VirusTotal (#android #malware)                    

Contagio mini-dump (+ mailing)                      

amtrckr.info (Android Malware Tracker)      

AndroMalShare (202.117.54.231:8080)




questions?

BSides 2015 - Android Malware Analysis

By paul38

BSides 2015 - Android Malware Analysis

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