Hacking for Fun and Profit (Mostly for Fun). AnDevCon Boston

HACKINGAPKS FOR FUN
AND FOR PROFIT
(MOSTLYFOR FUN)
DAVIDTEITELBAUM
MAY2013
@davtbaum

2 © 2013 Apkudo LLC. www.apkudo.com
OBJECTIVES
Androidappdisassembly
Fundamentalsofcodeinjection
Smali/BaksmaliandreadingDalvikbytecode
Bestpracticesinhardeningyourapp
Expect to learn:

ROADMAP
PART I - CLASS PART II – DEMO/HACK
Approachtohacking
Tools–apktool,baksmali,smali
TheAPK
Allthingsbytecode
Snapchatdeepdive
Appdisassemblyandanalysis
Codeinjection
Recap

PART I - CLASS

1. UnzipAPK and disassemble classes.dex (baksmali)
2. Analyze – what is the application doing?
3. Inject byte code into the application to modify execution
4. Reassemble classes.dex (smali) and rezip/signAPK
APK HACKING
Approach
Disassemble
(baksmali)
.smali
Static analysis
Reassemble
(smali)
Code injection

CODE INJECTION
 Write patches in Java, compile, then use the
Smali/Baksmali tools to disassemble into Dalvik byte code
 Stick to public static methods in Dalvik byte code which
have no register dependencies.
 Let the compiler do the work - this hack was achieved
with only one line of code injection!
Best Practices:

TOOLS
 Access to a terminal environment (preferably Linux or Mac
osx)
 Android SDK
 keytool and jarsigner
 Smali/Baksmali - http://code.google.com/p/smali/
 Apktool - http://code.google.com/p/android-apktool/
 Editor of choice (emacs!)
You’ll need…

SMALI/BAKSMALI
 Baksmali disassembles Dalvik executable (.dex) into
readable Dalvik byte code (.smali)
 Smali re-assembles .smali files back into .dex Dalvik
executable
 Gives developers the ability to modify execution of anAPK
without having access to source code
Dalvik Assembler/
Disassembler

APKTOOL
 Wraps smali/baksmali andAndroid asset packaging tool
(aapt)
 Decodes resources and decompresses xml
 Great for manifest introspection
 Buggy :/
All in one reverser

THE APK
A container for your app
 Zipped file formatted based on JAR
META-INF/
AndroidManifest.xml
classes.dex
lib/
res/
resources.arsc

EXAMPLES
$unzipfoobar.apk–dfoobar
$cd./foobar
$ls
AndroidManifest.xml META-INF classes.dex res
resources.arsc lib
$baksmali–a10–d~/boot_class_pathclasses.dex
baksmali
API level boot class path dex file

EXAMPLES
$ls
AndroidManifest.xml META-INF classes.dex res
resources.arsc lib
out
$smali –a10./out–oclasses.dex
$zip–r~/hacked.apk./*
smali
API level output dex file
recursive

EXAMPLES
$apktooldfoobar.apk foobar
$cd./foobar
$ls
AndroidManifest.xml apktool.yml assets res smali
$cd../
$apktoolb./foobar
apktool
decode out directory
build

EXAMPLES
$keytool-genkeypair-v -aliasdefault–keystore
~/.keystore–storepasspassword
$jarsigner–keystore~/.keystore ./foobar.apk
default
keytool and jarsigner
alias

SMALI FILES
class representation in byte code
.class public Lcom/apkudo/util/Serializer;
.super Ljava/lang/Object;
.source "Serializer.java”
# static fields
.field public static final TAG:Ljava/lang/String; = "ApkudoUtils”
# direct methods
.method public constructor <init>()V
.registers 1
.prologue
.line 5
invoke-direct {p0}, Ljava/lang/Object;-><init>()V
return-void
.end method
Class information
Static fields
Methods
Direct
Virtual

SYNTAX
V void
Z boolean
B byte
S short
C char
F float
I int
J long
D double
[ array
types .method private doSomething()V
64 bit – special instructions

SYNTAX
• full name space slash separated
• prefixed with L
• suffixed with ;
Lcom/apkudo/util/Serializer;classes
const-string v0, "ApkudoUtils"
new-instance v1, Ljava/lang/StringBuilder;
invoke-direct {v1}, Ljava/lang/StringBuilder;-><init>()V
const-string v2, "docId: ["
invoke-virtual {v1, v2}, Ljava/lang/StringBuilder;-
>append(Ljava/lang/String;)Ljava/lang/StringBuilder;
move-result-object v1

SYNTAX
 Method definitions
 .method <[keyword]> <name>(<[param]>)<return type>
 Method invocations
 invoke-static – any method that is static
 invoke-virtual– any method that isn‟t private, static, or
final
 invoke-direct – any non-static direct method
 invoke-super – any superclass's virtual method
 Invoke-interface– any interface method
 Virtual methods require their class instance as a parameter!
.method private doSomething()Vmethods

SYNTAX
.method private doSomething()Vmethods
.method private delayedAnimationFrame(J)Z
.registers 8
.parameter "currentTime”
keyword method name parameters/return
# Static invocation
invoke-static {p2}, Landroid/text/TextUtils;->isEmpty(Ljava/lang/CharSequence;)Z
# Virtual invocation
invoke-virtual {v0, v1}, Lcom/google/android/finsky/FinskyApp;-
>drainAllRequests(I)V

SYNTAX
 All registers are 32 bits
 Declaration
 .registers – total number of registers
 .locals – total minus method parameter registers
 Naming scheme
 Pregisters – parameter registers
 implicit p0 = „this‟instance (non-static)
 V registers – local registers
 Pregisters are always at the end of the register list
.locals 16
.registers 18
Registers

SYNTAX
.method public onCreate()V
.registers 7
...
Register Example
v0 First local register
v1 Second local register
v2 …
v3 …
v4 …
v5 …
v6 p0 First param – ‘this’
p0 == v6

SYNTAX
.method public doIt(Ljava/lang/String;II)V
.registers 7
Register Example 2
v2 …
v3 p0 ‘this’
v4 p1 String
v5 p2 int
v6 p3 int
p3 == v6
p2 == v5
p1 == v4
p0 == v3

SYNTAX
.method public doIt(JI)V
.registers 7
# hint, j == long
Register Example 3
v2
v3
v4
v5
v6
Third local register
p0 ‘this’ instance
p1 long
p2 long
p3 int
v3 - is it…
A) Fourth local register?
B) This instance?
C) Long?
D) Int?
v4 - is it…
B) This instance?
C) Long?
D) Int?
v5 - is it…
B) This instance?
C) Long?
D) Int?
v6 - is it…
B) This instance?
C) Long?
D) Int?

SYNTAX
.method public static doIt(IJ)V
.registers 7
Register Example 4
v2
v3
v4
v5
v6
Third local register
Fourth local register
p0 Int
p1 Long
p2 Long
v3 - is it…
B) This instance?
C) Long?
D) Int?
v4 - is it…
B) This instance?
C) Long?
D) Int?
v5 - is it…
B) This instance?
C) Long?
D) Int?
v6 - is it…
B) This instance?
C) Long?
D) Int?

SYNTAX
 jumps
 goto <offset>
jumping
.method public doIt(JI)V
.registers 7
...
goto :goto_31
...
:goto_31
return-void

SYNTAX
 Conditionals
 If-eq
 If-ne
 If-le
 If-lt
 If-ge
 If-gt
 Add z for zero
 If-eqz
 If-nez
conditionals
method public foobar()V
.registers 2
const/4 v0, 0x0
if-eqz v0, :cond_6
return-void
:cond_6
# Do something
.end method

PUTTING IT ALL
TOGETHER
Example - Java
package com.google.android.finsky;
import android.app.Application;
import android.accounts.Account;
public class FinskyApp() extends Application {
Account mCurrentAccount;
public String getCurrentAccountName() {
if (mCurrentAccount != null) {
return mCurrentAccount.name;
} else {
return null;
}
}
}

PUTTING IT ALL
TOGETHER
Same example - smali
.method public getCurrentAccountName()Ljava/lang/String;
.registers 2
.prologue
.line 617
iget-object v0, p0, Lcom/google/android/finsky/FinskyApp;->mCurrentAccount:Landroid/accounts/Account;
if-nez v0, :cond_6
const/4 v0, 0x0
:goto_5
return-object v0
:cond_6
iget-object v0, v0, Landroid/accounts/Account;->name:Ljava/lang/String;
goto :goto_5
.end method
v1 p0 ‘this’ instance
Getting this field! of type …
into this reg

ONE FINAL
STEP
Obfuscation!
• Renames classes, class members and and method
• Preserves OS entry points and java namespace classes
• Slows down the static analysis process
• Not a silver bullet, but an easy first line of defense
iget-object v0, p0, Lcom/a/a/g;->a:Lcom/a/a/f;
invoke-static {v0}, Lcom/a/a/f;->a(Lcom/a/a/f;)Landroid/webkit/WebView;

PART II - DEMO
https://github.com/davtbaum/adc-demo

HACKING
SNAPCHAT

1. Picture messenger with a catch…
2. Self-destructive pictures!
3. Pictures only last up to 10 seconds, ensures the receiver cannot
save them
4. Alerts the sender if the receiver tries to take a screenshot
5. Net-worth $70M – over 20M snaps sent a day!1
WHAT IS
SNAPCHAT?
Real-time picture messenger
1. http://techcrunch.com/2012/12/12/sources-snapchat-raising-north-of-10m-at-around-70m-valuation-led-by-benchmarks-mitch-lasky/

SNAPCHAT
IN ACTION

1. UnzipAPK and disassemble classes.dex
2. Analyze for target resource (snapchat pictureAKA„snap‟)
3. Inject code to store or transmit resource
4. Reassemble classes.dex and rezip/resignAPK
HACKING
SNAPCHAT
Approach
Disassemble
(baksmali)
.smali
Static analysis/
Code Injection
Reassemble
(smali)

TOOLS
 Access to a terminal environment (preferably Linux or Mac
osx)
 Android SDK
 keytool and jarsigner
 Smali/Baksmali - http://code.google.com/p/smali/
 Apktool - http://code.google.com/p/android-apktool/
 Editor of choice (emacs!)
You’ll need…

STEP 1
 Query device for list of applications and associated file paths
 adbshellpm listpackages–f
 (optional)|grep–si“snapchat”
 Pull the files
 adbpull<file>~/snapchat/snapchat.apk
GET THE APP

STEP 2
 Extract classes.dexand remove keys
 unzipsnapchat.apk
 rm–r ./META-INF
 Disassemble:
 baksmali-a 10–d<framework_path> ./classes.dex
 -a=api-level
 -d=bootclasspathdir
 „adbpull/system/framework/ ./framework‟
DECOMPRESS AND
DISASSEMBLE

STEP 3
 apktool dump and inspectAndroidManifest.xml
for activities
 apktooldsnapchat.apk
 emacsAndroidManifest.xml
 Find the resource
 Use tools
 uiautomator to retrieve view hierarchy
(buggy)
 adbshelldumpsyswindow|grep–si
“mCurrentFocus”
 Resolve resource in code
ANDROID FORENSICS

STEP 3
 Resource located! Now we need to retrieve it…
 Don‟t write everything in byte code- build an application
that contains the resource retrieval code.
 Disassemble donor application and copy appropriate
methods into target app
 Easy enough, right?
RESOURCE RETRIEVAL
Java
resource
retrieval
code
Build Bytecode

DONOR APP
RESOURCE RETRIEVAL
package com.apkudo.util;
import android.app.Activity;
import android.graphics.Bitmap;
import java.io.FileOutputStream;
Import android.os.Bundle;
public class HackUtils extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
public void saveSnap(Bitmap bmp) {
try {
FileOutputStream out = new FileOutputStream(“/sdcard/test.png”);
bmp.compress(Bitmap.CompressFormat.PNG, 90, out);
} catch (Exception e) {
e.printStackTrace();
}
}
}

STEP 4
CODE INJECTION
 .method private showImage()V
 Isolate Bitmap
 Pass into resource retrieval method
invoke-virtual{v1,v2},Lcom/snapchat/android/model/ReceivedSnap;-
>getImageBitmap(Landroid/content/Context;)Landroid/graphics/Bitmap;
move-result-objectv0
#Patches
invoke-static{v0},Lcom/apkudo/util/HackUtils;->saveSnap(Landroid/graphics/Bitmap;)V
#EndofPatches

STEP 5
 Re-assemble
 smali–a10./out–oclasses.dex
 Compress
 zip–z0–r../snapchat.apk./*
 SignAPK
 jarsigner-verbose -keystore my-release-key.keystore
./snapchat.apkalias_name
REBUILD APK

STEP 6
 Install
 adb install –r ../snapchat.apk
 Run the app!
INSTALLAND EXECUTE

RECAP
 Obfuscate?
 Very simple to navigate using method name
 E.g. “showSnap()”.
 Push images to native layer
 OpenGL?
 Native code is much harder to reverse.
 Dynamic signature verification?
 There is no silver bullet!
ROOM FOR IMPROVEMENTS

Thankyou.
DAVID@ .COM@davtbaum

Hacking for Fun and Profit (Mostly for Fun). AnDevCon Boston

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