Android Application Security For Developers (ADD 2014)

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Application security ANDROID FOR DEVELOPERS FILIP MAELBRANCKE

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Security = managing risk ASSET VULNERABILITY THREAT

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Security = managing risk ASSET VULNERABILITY THREAT

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All in one device Increases threat proBability ! • GPS • Contacts • Camera • Email (work) • Wallet

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Always out Vulnerability / Exploitability ! • Stolen • Forgotten • Lost !

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Everyone uses it Vulnerability / Exploitability ! • Weak pins • Use of open public WiFi !

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Everyone uses it

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Android security

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Android security model ! ! ! ! ! ! ! ! ! ! Game X ! ! ! Game Y ! System ! ! ! ! ! ! ! ! ! ! ! ! ! ! Contacts ! ! ! Email ! ! ! Google Play ! Verify app signature ! ! App sandbox ! ! Permissions application isolation

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typical mobile app MOBILE APPLICATION UI LOCAL STORAGE REMOTING LAYER REMOTE API COMMUNICATION CHANNEL

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Security APP DATA NETWORK SERVICES

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Securing the app JAVA CLASS DEX

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reverse engineer

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OBTAIN APK FROM DEVICE adb backup -apk be.myapp ADB BACKUP app Titanium, Astro, Helium adb shell pm list packages -f adb pull /data/app/be.myapp-1.apk

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APK structure APK = zip APK AndroidManifest classes.dex Resources

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reverse engineer TOOLS ! • Apktool • Dex2jar • Apk to Java !

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reverse engineer smali / baksmali APKTOOL Low level disassembled Dalvik bytecode CODE code can be modiﬁed recompile / resign

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reverse engineer apktool d myapp.apk

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reverse engineer

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reverse engineer code

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reverse engineer convert .dex ﬁle to a .jar with java bytecode DEX2JAR dex -> java java decompiler CODE very readable

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reverse engineer

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reverse engineer Jeb Decompiler PAID dex -> java native dalvik decompiler

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reverse engineer

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Obfuscation

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Proguard obfuscate optimize Shrink

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proguard obfuscation

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proguard

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proguard configuration

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proguard Beware!

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proguard loggingwrapper

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proguard configuration

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proguard BEtter

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other techniques If possible, run code at server! server String encryption Hide sensitive strings eg “Secure” Native code Java Native Interface reflection Proxy Introduces indirection Class encryption Use DexGuard

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dexguard Same conﬁg proguard++ Commercial Good value for the money Tamper checks

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dexguard

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proguard tips Test! release build Mapping.txt Save! Crash? Supported on Crashlytics, Crittercism, ...

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TAMPER DETECTION

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Environment 1.installer 2.debugger / 3.BINARY Validation Tamper detection / protection

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INSTALLER PLAY STORE INSTALLER

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debugger Debugger check

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debugger Debugger check

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emulator EMULATOR check

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SIGNING KEY Valid signing key ! • SHA1 of signing cert • Embed • Check with runtime signature !

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SIGNING KEY Valid signing key

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rooted device root detection ! • Check typical apps / ﬁles • Check keys • /system r/w !

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tamper detection tips + use obfuscation! multiple checks Tampering detected Close application Don’t leak where the protection code is

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local Data protection Avoid it if you can Avoid External storage Avoid external storage for sensitive information For critical info set android:saveEnabled="false" Backup set android:allowBackup=false proper permissions MODE_PRIVATE with ﬁles

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local Data protection getWindow().setFlags(LayoutParams.FLAG _SECURE, LayoutParams.FLAG_SECURE); avoid screen shots LOGOUT on inactivity if usability allows and clear the cached information

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keylogger

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ANDROID NOT ENOUGH? rooted devices Internal Storage Full disk crypto brute forcing

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encryption

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JCA APP JCA (Java Cryptography Architecture) Provider Provider Message Digest Key Generation Digital Signature ...

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JCA Bouncy Castle Android OpenSSL APP JCA (Java Cryptography Architecture) Harmony

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bouncy castle Android = subset of upstream release cut-down CONSISTENT Consistent crypto across Android versions MINIMAL change github.com/rtyley/spongycastle Spongy castle Repackage of Bouncy Castle for Android

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encryption libs SQLCipher sqlcipher.net ! • Modiﬁed version of SQLite • AES-256 encryption • Drop-in replacement ! iocipher guardianproject.info/code/ iocipher ! Virtual encrypted disk

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key management Store along with the data (ﬁle private to the app) Store Embed Embed in source code (obfuscated ?) EASY TO EXTRACT

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key management don’t store Don’t store the key on the device Have it entered each time necessary Store In systems service SOLUTIONS

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key derivation Long random strings of bits encryption keys people vs keys Users are familiar with passwords Crypto algo PBKDF2WithHmacSHA1 password based encryption Generate strong crypto keys based on humanly-manageable passwords

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proper key derivation Using a salt protects from table- assisted / pre-computed dictionary attacks SALT key stretching Repeat the key derivation operation multiple times to produce the ﬁnal key Slows down brute force attacks

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key derivation https://github.com/nelenkov/android-pbe http://nelenkov.blogspot.jp/2012/04/using-password-based-encryption-on.html Nikolay Elenkov

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KEYCHain? Keystore provider ! • Since Android 4.3 • Can be hardware- backed https://github.com/nelenkov/android-keystore Nikolay Elenkov

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network

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Secure communication channel use https Use SSL / TLS ! • Confidentiality • Authentication ! VALIDATION Hostname verification ! Certificate pinning

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secure communication channel hostname verification

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SSL certificates CA issued, Android recognized CA issued self-signed certificates behaviour change custom TrustManager

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self-signed cert

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anti pattern don’t trust all!

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self-signed cert Certificate Custom trustmanager NO man-in-the-middle attacks import in your app

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Certificate authorities

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Trustmanager StrongTrustManager ! • Validate whole certiﬁcate chain • Debian certiﬁcate store !

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certificate pinning with expected certiﬁcate / public key Associate host hashing anonymize certiﬁcate / public key

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certificate pinning echo | openssl s_client -connect host:443 2>&1 | sed -ne '/-BEGIN CERTIFICATE-/,/- END CERTIFICATE-/p' > mycertiﬁcate.pem get certificate (openssl) embed in application /res/raw Custom Based on keystore Load into keystore SSL context Init SSL context with TrustManager https://developer.android.com/training/articles/security-ssl.html

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Securing services Controls ! • Kill switch for speciﬁc functionality • Server downtime communication • Mandatory update mechanism !

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securing services Backend REST and APIs can have similar vulnerabilities to web applications mitigate follow OWASP top 10

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Effective security Using CryptoLint, we performed a study on cryptographic implementations in 11,748 Android applications. Overall we ﬁnd that 10,327 programs – 88% in total – use cryptography inappropriately. The raw s c a l e o f m i s u s e i n d i c a t e s a w i d e s p r e a d misunderstanding of how to properly use cryptography in Android development. “ ”

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effective security hardcoded passphrases manually seeded SecureRandom insufﬁcient key generation iterations hardcoded salts non-random initialization vectors

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security testing Static analysis Manual code review design review Analysis Static Dynamic Penetration testing

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suggested reading Android Security Cookbook  Keith Makan / Scott Alexander-Bown (9781782167167) Android Security Internals   Nikolay Elenkov (9781593275815) Android Hacker’s Handbook  Joshua J. Drake et al. (9781118608647) Application Security for the Android platform  Jeff Six (9781449315078)  

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suggested reading developer.android.com  https://developer.android.com/training/articles/security-tips.html   https://source.android.com/devices/tech/security/ OWASP  https://www.owasp.org/index.php/OWASP_Mobile_Security_Project Google+ community   Android security discussions Blogs  http://nelenkov.blogspot.com.tr/… 

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Filip maelbrancke TWITTER: @fmaelbrancke EMAIL: filip@maelbrancke.net THANK YOU EMAIL: info@appfoundry.be consultant @ AppFoundry