ANDROID APPS SECURITY Mobile Dev Day - Genova Marco Grassi @marcograss [email protected] Mobile Security Researcher @ viaForensics 1 

$ whoami • R&D Team Member @ viaForensics • Part of my job is to attack and break mobile apps 2 

3 Black Box Approach = We can use the app, Dynamic Analysis, Inspection + Reverse Engineering, Static Analysis (Mainly) APK 

AGENDA • Reverse Engineering and Obfuscation • Tampering Detection • Logging • File Storage • Secure Network Communications • IPC Attack Surface • RAM memory attacks • More Advanced Material : Runtime Manipulation • Extra: Creating Cheats for Android Games : ) 4 REAL WORLD EXAMPLES 

SANTOKU LINUX https://santoku-linux.com/ 5 

PULLING THE APK FROM THE DEVICE 6 • Often the APKs are downloaded from Google Play on the device, how can we extract them? Some solutions: 1. adb backup -apk com.mypackage (Works on Android 4.0 and newer) 2. Use a backup application (ASTRO File manager, Titanium Backup…) 3. adb shell , cd /data/app/, find your apk, then you can pull it with adb pull /data/app/ mypackage.apk (requires a adb root shell on the device) 

REVERSE ENGINEERING FREE TOOLS • apktool and smali/baksmali It will provide us a disassembled representation of the Dalvik bytecode, so sort “low level”, with registers, but very understandable because of bytecode metadata. Very useful to disable tampering protections, the code can be modified and the application can be recompiled and resigned. 7 

DISASSEMBLED SMALI CODE 8 

REVERSE ENGINEERING FREE TOOLS • dex2jar + Java decompiler (jd-gui, jad …) dex2jar will convert the .dex file to a .jar containing Java code We can then use the freely available Java decompilers and obtain back a Java representation of the code. Very readable if no obfuscation is in place. 9 

DECOMPILED JAVA CODE 10 JD-GUI Procyon CFR jadx(directly from dex) 

REVERSE ENGINEERING PRO TOOLS • JEB Decompiler Renaming feature, very handy with obfuscated applications Python and Java APIs Native Dalvik decompiler, it does not pass through Java byte code, decompilation is usually much better 11 

REVERSE ENGINEERING PRO TOOLS • IDA + Hex Rays Decompiler De facto the best interactive disassembler and decompiler on the market. Impressive set of APIs, you can write modules or scripts for everything. 12 

REVERSE ENGINEERING PRO TOOLS • Hopper Disassembler Very nice disassembler and decompiler with a killer price. 13 

OBFUSCATION PROGUARD • Free • Integrated into the build environment • NOT Android specific • http://developer.android.com/tools/ help/proguard.html 14 

DECOMPILED CODE WITH PROGUARD 15 

OBFUSCATION DEXGUARD • Commercial product from ProGuard author. • Android specific • Native support to string and code encryption and tamper detection • Very easy to use, with a config file like ProGuard 16 

DECOMPILED CODE WITH DEXGUARD 17 

TAMPERING DETECTION 18 • Check at runtime if the application has been modified in any way or if the signature is changed. • It can be done with the PackageManager class, retrieving the Signature[] array of your app and comparing with known values. If an attacker resigned the app, the signing key will be different. • Do the checks in multiple code points and use obfuscation, to avoid that it can be easily bypassed. • If your app ships only through Google Play, check with the APIs that it has been installed from Google Play and not from Unknown Sources. • If something is wrong, close the application without leaking informations where the protection code is, to make attacker’s life harder. 

DEFEATING TAMPERING DETECTION WHY OBFUSCATION IS FUNDAMENTAL 19 Why spend hours on implementing if our application has been modified, if there is a single point of failure? ! If the attacker can easily find the code, it can modify the application and disable it. 

LOGGING • Remove Logcat logging from your production builds. • It can be done with few lines in Proguard and Dexguard, they remove all the calls to Log.d, Log.e etc in the build process • It’s very easy for third party malware or an attacker to access the Logs on Android. 20 

FILE STORAGE EXTERNAL STORAGE • Try to avoid storing your data in the shared storage, almost any application can read it. (In 4.4 a small protection at permission level was added android.permission.READ_EXTERNAL _STORAGE, usually users does not check permissions too much anyway… Don’t rely on this.) 21 My Personal Data stored in a Evernote Note, publicly readable by anyone. CASE STUDY 

FILE STORAGE PRIVATE APP FOLDER • Encrypt your preferences/files • With root access they can be modified, avoid store sensitive data at all if possible • With a backup, they can be retrieved from the device usually • The private folder can be found on the device at path /data/data/yourpackage 22 That’s right.. It’s my User and my 36 character Password in PLAIN TEXT CASE STUDY 

FILE STORAGE SQLITE DATABASES 23 shell@hammerhead:/ $ pm list packages | grep easy package:com.handyapps.easymoney shell@hammerhead:/ $ exit $ adb backup -apk com.handyapps.easymoney unpack the backup with https://github.com/nelenkov/android-backup-extractor PASSCODE IN PLAIN TEXT RETRIEVED. FAIL! CASE STUDY 

SQLCIPHER 24 http://sqlcipher.net/sqlcipher-for-android/ Very easy to use encrypted SQLite database. Don’t store the key with the safe. The user must provide the password to access the content if possible. 

#1 RULE: YOU DO NOT IMPLEMENT YOUR OWN CRYPTOGRAPHY #2 Rule: You do NOT implement your own Cryptography 25 

SECURE NETWORK COMMUNICATIONS • It’s your responsibility to protect data in transit! • Don’t transmit sensitive information without SSL/TLS • Implement if possibile Certificate Pinning, in this way your communications will be more resistant to MITM attacks, for example if a malicious certificate is pushed into the device, or if an attacker can impersonate your web service with a trusted certificate. 26 

IPC ATTACK SURFACE THE ANDROID MANIFEST 27 • Avoid the flag android:debuggable=true in production, an attacker can attach with a debugger and execute arbitrary code in your app. • Double check your exported components. Export a component to other processes only if it’s strictly necessary and at least protect the component with a permission. Android has some permissive defaults, some components are exported even if they are not declared exported=true, check the documentation. • If you export a content provider or another component that grants access to data and accepts untrusted output, be careful on the input to avoid sql injections and path traversal attacks. 

IPC ATTACK SURFACE EXAMPLE: SCREEN BYPASS 28 McAfee Antivirus & Security ! Now patched It was possible to bypass the activation and use for free some functionalities. ! $ am start -a android.intent.action.MAIN -n com.wsandroid.suite/ com.mcafee.main.MfeMain ! Credits: Sebastián Guerrero, @0xroot CASE STUDY 

RAM MEMORY ATTACKS • An attacker can retrieve and inspect the ram memory used by our application and search for sensitive informations. • Avoid storing such sensitive informations inside instance or static variables. 29 

RAM MEMORY ATTACKS • An easiest way to get an incomplete (VM only) chunk of live memory from our application is to use the “Dump HPROF” functionality in the monitor tool, with a debuggable application or a device with the flag ro.debuggable=1 30 

APPENDIX Extras with more advanced material 31 

RUNTIME MANIPULATION Why modify the code of the application recompiling it when we can modify the code at runtime, without alerting the basic tampering detection? 32 

RUNTIME MANIPULATION 33 We can change the behaviour of the applications and the system without touching any APK and we can enable/disable plugins with ease. ! We must have a rooted phone and install a framework that will modify some low level components of the Android OS, to make our life easier. 

MOST POPULAR FRAMEWORKS • Cydia Substrate • Xposed Framework • (New) Frida 34 http://www.cydiasubstrate.com/ http://repo.xposed.info/ http://frida.re/ 

CANDY! Reverse Engineering is fun! 35 

LET’S USE RUNTIME MANIPULATION TO CHEAT IN ANDROID GAMES! 36 

AGIMAT • Simple cheat engine/app for Android using runtime manipulation • When more games are supported and if there is interest, it will be open sourced (no time) 37 

SUPER HEXAGON Addictive but difficult game for Android 38 

39 It’s difficult? Let’s slow down the game with Reverse Engineering and runtime manipulation! 

VIDEO DEMO 40 

SECURITY IS A PROCESS. 41 

SECURE MOBILE DEVELOPMENT BEST PRACTICES AVOIDING COMMON PROBLEMS AND CREATING MORE SECURE APPS FOR IOS AND ANDROID 42 http://bit.ly/L1fBeT 

43 bit.ly/1doIWa7 OWASP Mobile Security Project 

Great book to start with Secure Android Development, written by my friend @scottyab 44 

More advanced book on Android Hacking, by @pof and @jduck and others 45 

GET CERTIFIED bit.ly/1lwIGjl 46 

WE ARE HIRING! 47 

48 @0xroot, @abelenko, @ahoog42, Brendan , @Fuzion24, @insitusec, @giantpune, @jduck @JMDlux, @kevinswartz_1, @kstrzemp, @mattdorn, @pof, @rozelaudric, @scottyab, Terence , @thomas_cannon, @tom_anderson2, @viaforensics, @vialated and many others… 

@marcograss [email protected] 49 EOF 

HOW CAN WE DEVELOP A PLUGIN AND WHAT WE CAN DO WITH IT? 50 

1PASSWORD READER • Password wallet application for Android, a companion application of the Mac/Windows client, to be able to share our passwords between our PC and the mobile device, leveraging Dropbox or the Shared Storage. 51 CASE STUDY 

BE CAREFUL WITH BROADCASTED INTENTS 52 Vulnerable unprotected Broadcast Receiver to make the app timeout, with a Broadcasted Intent (Dangerous!) CASE STUDY 

LET’S INSTALL SOME MALWARE 53 CASE STUDY 

RESULTS 54 The Malware catch the Broadcast Intent before of the wallet. It suppress it, so the Wallet never get the Intent and never go to timeout its session. ! What we learned: The system often is not trusted when doing IPC with Intents, and in any case we must protect the exposed parts of our application, auditing and remediating. CASE STUDY 

1PASSWORD: WHY SHARED STORAGE AND DROPBOX? • This choices are forced for technical limitation in the sharing process between the PC and the device. • Without root permissions, the user can only write in the shared folder, or the application can use third party services, such file sharing API by Dropbox, to share the wallet file. 55 CASE STUDY 

FIRST LOOK • The 1Password wallet is totally unobfuscated, so an attacker can easily understand the logic of the application and the weak points. • First weak spot: LOGS, the application disabled in productions the logging of the user credentials and other internal information to the Logcat, but the logs are only disabled, the code that logs at the critical points (even the user password) it’s in there. 56 CASE STUDY 

HELLO WORLD: WHAT CODE CHANGE? LET’S ENABLE LOGGING 57 CASE STUDY 

REPLACED METHODS 58 CASE STUDY Xposed Framework Plugin to re enable logging in this app 

RESULTS 59 12-03 22:49:24.614: I/Xposed(3402): logMsg - === BEGIN validate password: testing=== 12-03 22:49:24.614: I/Xposed(3402): logMsg - BEGIN decryptWithPBKDEF2 encrypted len=1056 password=testing iterations:71428 12-03 22:49:27.606: I/Xposed(3402): logMsg - derivedKeysLen=32baseKey.len=16 ivec=16 12-03 22:49:27.606: I/Xposed(3402): logMsg - END decryptWithPBKDEF2: result.len=1024 12-03 22:49:27.616: I/Xposed(3402): logMsg - SL5 key validation OK 12-03 22:49:27.616: I/Xposed(3402): logMsg - BEGIN decryptWithPBKDEF2 encrypted len=1056 password=testing iterations:71428 12-03 22:49:30.449: I/Xposed(3402): logMsg - derivedKeysLen=32baseKey.len=16 ivec=16 12-03 22:49:30.459: I/Xposed(3402): logMsg - END decryptWithPBKDEF2: result.len=1024 12-03 22:49:30.459: I/Xposed(3402): logMsg - SL3 key validation OK 12-03 22:49:30.459: I/Xposed(3402): logMsg - === END validate password CASE STUDY