Android Application Security, The Right Way

In this talk you will discover the typical attack surfaces of an Android application. We cover the importance of code protection, implementing secure coding practices, strong crypto implementations, executing in a secure environment and hardening network communications. You will walk away with best practices and common pitfalls to create secure applications.

Transcript

1. Securing Android Applications, The Right Way Dario Incalza dario.incalza@guardsquare.com
2. @h4oxer • Pre-sales Engineer at GuardSquare • I have opinions @h4oxer • Like breaking applications • Like securing applications $ whoami
3. @h4oxer • Company behind ProGuard and DexGuard • ProGuard is part of the Android SDK • HQ’s in Leuven, Belgium • @GuardSquare • www.guardsquare.com GuardSquare
4. Outline
5. @h4oxer • Attack Surfaces of Apps • Best Practices for Securing Applications • Cryptography • Code Protection • Secure Communications • Secure Execution Environment • Recap Outline
6. Attack Surfaces
7. Attack Surfaces Application Communication Execution Environment Reverse Engineering Piracy Trojan Injection Credential Theft Man-in-the- Middle Weak Protocols Debug Analysis Emulator Analysis Hooking Frameworks Rooted Environment Local Data Information Theft Privacy Leaks
8. @h4oxer • Static Analysis • APKTool, Smali/Baksmali, BytecodeViewer,
 JEB ($), IDA Pro ($$) … • Network Analysis • mitmproxy, charles, burpsuite, wireshark Some Tools
9. @h4oxer Some More Tools • Dynamic Analysis • Emulators: Android Emulator, Genymotion • Hooking Frameworks: • xPosed • Cydia Substrate (old) • Frida (uses JS) • Standard Tools: ptrace, JDB, GDB
10. Best Practices for Securing Applications
11. @h4oxer Best Practices • Use secure best coding practices • Protect, obfuscate and encrypt your application code • Harden your communication • Take into account the execution environment
13. Cryptography
14. @h4oxer Problems • How to store sensitive information on the device? • How to send sensitive information over the wire? • How to securely generate crypto keys? • How to manage crypto keys?
15. @h4oxer Crypto 101 • Symmetric Crypto = one key for encryption/decryption • AES, 3DES, Blowfish, many more • Public-key Crypto = private and public key • Encrypt with private key, decrypt with public key = digital signatures • Encrypt with public key, decrypt with private key = confidentiality • RSA, ElGamal, ECC, many more
16. @h4oxer Generating Secure Keys • Generate symmetric keys on the device for user-data • A 256 bit AES key derived from a password public byte[] getEncryptionKey(char[] strongPassword){ int iterationCount = 10000; int keyLength = 256; int saltLength = keyLength / 8; // same size as key output SecureRandom random = new SecureRandom(); byte[] salt = new byte[saltLength]; random.nextBytes(salt); KeySpec keySpec = new PBEKeySpec(strongPassword, salt, iterationCount, keyLength); SecretKeyFactory keyFactory = SecretKeyFactory .getInstance(“PBKDF2WithHmacSHA1"); return keyFactory.generateSecret(keySpec).getEncoded(); }
17. @h4oxer Securely Manage Crypto Keys 1. Ask for user password and do not store keys on the device, use PBKDF2 2. Generate keys and store in KeyStore • Vulnerable on rooted devices (hard) 3. Generate keys and store in SharedPreferences • Vulnerable on rooted devices (easy) 4. Use hardcoded key in application code • One key, reverse engineering, key leaked, big problem 5. Store generated key in /sdcard/ • Readable by all apps, stop
18. @h4oxer Cryptography • DON’Ts • Hardcoded Crypto Keys • Save Crypto Keys in /sdcard/ • Log sensitive information • Use AES in ECB mode • Use DES, MD5, it’s broken/weak • Implement DIY crypto • Do not use String objects for sensitive information • Not fixing the SecureRandom vulnerability < Jelly Bean
19. Crypto Libraries
20. @h4oxer • Wrapper for SharedPreferences • Uses AES-128 in CBC • Option for user supplied password • https://github.com/ scottyab/secure- preferences SecurePreferences SharedPreferences prefs = new SecurePreferences( context, ”userpassword”, ”prefs.xml” );
21. @h4oxer • Virtual Encrypted Disk, encrypted file storage • Clone of standard java.io.* • Three important methods • VirtualFileSystem.get() • VirtualFileSystem.mount(dbFile, password) • VirtualFileSystem.unmount() • https://guardianproject.info/code/iocipher/ IOCipher
22. @h4oxer IOCipher byte[] key = getEncryptionKey(password) VirtualFileSystem vfs = VirtualFileSystem.get(); String path = getDir("vfs", MODE_PRIVATE).getAbsolutePath() + “/container.enc” vfs.createNewContainer(path, key); vfs.mount(path, key); //Start using info.guardianproject.iocipher.* API
23. @h4oxer • Uses OpenSSL library • Standard AES-GCM • Small size, fast performance • Built and used by Facebook • http://facebook.github.io/conceal/ Conceal
24. @h4oxer Conceal Example KeyChain keyChain = new SharedPrefsBackedKeyChain(context,CryptoConfig.KEY_256)); Crypto crypto = AndroidConceal.get().createDefaultCrypto(keyChain); if (!crypto.isAvailable()) { return; } OutputStream fileStream = new BufferedOutputStream( new FileOutputStream(file)); OutputStream outputStream = crypto.getCipherOutputStream( fileStream, Entity.create("entity_id")); outputStream.write(plainText); outputStream.close();
25. @h4oxer Keep in mind…
26. Code Protection
27. @h4oxer Problems • How to make reverse engineering harder? • How to protect your code against extraction? • How to protect API keys? • How to hide cryptographic operations?
28. @h4oxer • Name obfuscation • String encryption • Class encryption • Resources, asset and native library encryption • Control flow and arithmetic obfuscation • Hide calls through reflection Code Protection
29. @h4oxer public String encryptSensitiveMessage() { String nuclearLaunchCode = "abc123"; String encryptionKey = “secretkey"; return CryptoEngine.encrypt(nuclearLaunchCode, encryptionKey); } For Example
30. @h4oxer public String encryptSensitiveMessage() { String nuclearLaunchCode = "abc123"; String encryptionKey = "secretkey"; Class clazz = Class.forName("CryptoEngine"); Method meth = clazz.getMethod(“encrypt”, String.class, String.class); return (String) meth.invoke(null, nuclearLaunchCode, encryptionKey); } Layer 1 - API Call Hiding
31. @h4oxer public String encryptSensitiveMessage() { String nuclearLaunchCode = Base64.decode("YWJjMTIz"); String encryptionKey = Base64.decode("c2VjcmV0a2V5"); Class clazz = Class.forName(Base64.decode("Q3J5cHRvRW5naW5l")); Method meth = clazz.getMethod(Base64.decode("ZW5jcnlwdA=="), String.class,String.class); return (String) meth.invoke(null,nuclearLaunchCode,encryptionKey); } Layer 2 - String Obfuscation
32. @h4oxer public String a() { String a = e.f("YWJjMTIz"); String b = e.f("c2VjcmV0a2V5"); Class c = Class.forName(e.f("Q3J5cHRvRW5naW5l")); Method d = c.getMethod(e.f(“ZW5jcnlwdA=="), String.class, String.class); return (String) d.invoke(null, a, b); } Layer 3 - Name Obfuscation
33. @h4oxer Apply Automatically • ProGuard • Open-source • Name obfuscation and optimisation • DexGuard • More advanced • Big brother of ProGuard • Backward compatible with ProGuard
34. Securing Communications
35. @h4oxer Problems • $ emulator -avd Nexus_5X_API_22 -http-proxy http://localhost:3030 • $ mitmproxy -p 3030 • Install mitmproxy certificate on emulator
36. @h4oxer Problems
37. @h4oxer • A certificate = cryptographically signed identification information • Certificates are issued by Certificate Authorities (CAs) • Your Android device trusts a number of CAs • SSL validation = check if certificate of server is issued by trusted CA SSL 101
38. @h4oxer SSL Validation Client Server Can you identify yourself? Sure, I am google.com, here is my certificate! 1. Client checks which CA issued the certificate 2. Do I trust the CA? • Yes, validation is done, connection is trusted. • No A. Is the certificate self signed? Validation failed. B. Is the certificate issued by another CA? Goto 2.
39. @h4oxer Thread - MiTM Attack Client Server Identity? Here is my certificate! • Attacker needs to get a trusted certificate • Hacked CAs: DigiNotar (2011) & Comodo (2011) • Or install his own certificate as trusted • Traffic can be read/altered by MitM MitM Identity? Here is my certificate!
40. @h4oxer Protect Against MitM • Android applications by default trust system CA store • SSL or Certificate Pinning • Option 1: pin on public keys • Option 2: provide your own trust store or certs
41. @h4oxer SSL Pinning - OkHttp • https://github.com/square/okhttp OkHttpClient client = new OkHttpClient.Builder() .certificatePinner(new CertificatePinner.Builder() .add("publicobject.com", "sha256/ afwiKY3RxoMmLkuRW1l7QsPZTJPwDS2pdDROQjXw8ig=") .build()) .build();
42. @h4oxer • Improves network security • Uses best practices for TLS/SSL • Custom certificate stores from Mozilla and Debian • TOR integration == cool! • https://github.com/ guardianproject/NetCipher NetCipher HTTP Client API NetCipher HttpUrlConnection StrongConnectionBuilder OkHttp3 StrongOkHttpClientBuilder Volley StrongVolleyQueueBuilder Apache StrongHttpClientBuilder StrongOkHttpClientBuilder builder = StrongOkHttpClientBuilder. forMaxSecurity(this)
43. Secure Execution Environment
44. @h4oxer • Static code protection leads to dynamic attacks • Three main attack techniques • Dynamic code injection a.k.a hooking • Attaching debuggers • Memory dumping Problems
45. @h4oxer • Tools: XPosed, Frida, Cydia Substrate • Requires rooted device • Places hooks • E.g., before encryption calls, after decryption calls Dynamic Code Injection
46. @h4oxer • Tools: Java Debug Bridge (JDB), Gnu Project Debugger (GDB) • Inspect code execution, paths, variables • In Android alter AndroidManifest.xml > debuggable=true Debuggers
47. @h4oxer • Advanced security tools offer code encryption • Code available in memory • Dumping memory == getting unencrypted code • Tools: Linux Memory Extractor (LiME) Memory Dumping
48. @h4oxer • Application can scan its environment • Should it run on a rooted device? • Should it run on an emulator - which is rooted by default? • Detect dynamic code injection • Detect application tampering Securing Your Environment
49. @h4oxer • Get Google’s opinion on the device status • Response is JSON Web Signature (JWS) • Developer needs to review response and verify signature • SafetyNetApi.attest() SafetyNet API
50. @h4oxer • SafetyNet looks at various device attributes (by @ikoz) • Installed packages • SU Files • Settings (adb enabled, lock screen enabled, …) • SE Linux state • Device admin blacklist • … SafetyNet API
51. @h4oxer • Advantages • Google knows a lot • Updated remotely • Takes a lot into consideration • Disadvantage • You only get a binary answer: compatible/incompatible • Google Play Services dependency • Network requests take time • Developer needs to verify JWS SafetyNet API
52. Conclusion
53. @h4oxer • Implement strong coding practices and strong cryptography • Protect code statically through various layers that protect code and each other • Harden the communications • Scan, detect and protect against insecure execution environments Recap
54. Conclusion
55. Visit us at Booth 3 Win a DexGuard License
56. @h4oxer • https://nelenkov.blogspot.be/2012/04/using-password- based-encryption-on.html • https://koz.io/inside-safetynet/ • https://github.com/scottyab • Android Hacker’s Handbook • Android Security Internals • www.guardsquare.com References