Systematic Fuzzing and Testing of TLS Libraries Juraj Somorovsky

Transcription

1 Systematic Fuzzing and Testing of TLS Libraries Juraj Somorovsky 1 1

2 Transport Layer Security The most important crypto protocol HTTP, SMTP, IMAP 2 2

3 Secure Sockets Layer (SSL), SSLv2 SSLv3 Trasnsport Layer Security TLS History Wagner, Schneier: Analysis of SSLv3 Bleichenbacher s attack Padding oracle attack 2005 TLS 1.1 TLS BEAST, CRIME, BREACH, Lucky 13 TLS

4 Questions How can we test these attacks? Can we find such attacks automatically? 5 5

5 Approach [SP2-17] 1. Collect TLS libraries Profit 6 6

6 Approach [SP2-17] 1. Collect TLS libraries Profit 7 7

7 Contributions Flexible TLS framework Fuzzing, testing, writing attacks High impact vulnerability in OpenSSL Additional vulnerabilities in Botan, MatrixSSL 8 8

8 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 9 9

9 TLS RSA Handshake ClientHello ServerHello Certificate ClientKeyExchange ChangeCipherSpec (Client-) Finished ServerHelloDone ChangeCipherSpec (Server-) Finished Application Application 10 10

10 TLS is complex Different versions Crypto primitives: RSA, EC, AES, 3DES, RC4, Chacha, Poly1305, New Hope Extensions Protocol flows 11 11

11 TLS is complex ClientHello Certificate ClientKeyExchange CertificateVerify ChangeCipherSpec (Client-) Finished ServerHello Certificate ServerKeyExchange ServerHelloDone ChangeCipherSpec (Server-) Finished Application Heartbeat Application Heartbeat 12 12

12 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 13

13 Secure Sockets Layer (SSL), SSLv2 SSLv3 TLS History 1995 Wagner, Schneier: Analysis of SSLv3 Bleichenbacher s attack Trasnsport Layer Security 2000 Padding oracle attack TLS 1.1 TLS BEAST, CRIME, BREACH, Lucky 13 TLS

14 Early CCS ClientHello ServerHello Certificate ServerHelloDone ClientKeyExchange ChangeCipherSpec (Client-) Finished ChangeCipherSpec (Server-) Finished Server computes the master key based on a zero value 15 15

15 Early CCS Man-in-the-Middle attacks Further state machine attacks in 2015: Beurdouche et al.: FREAK de Ruiter and Poll 16 16

16 Heartbleed [TLS Handshake] Server Heartbeat DeepSec Heartbeat DeepSec 17 17

17 Heartbleed [TLS Handshake] Server Heartbeat DeepSec Heartbeat DeepSec. [rsa key]. 18

18 Padding oracle attacks Adaptive chosen-ciphertext attacks Ciphertext C = Enc(M) C 1 valid/invalid C 2 valid/invalid M = Dec(C) (repeated several times) AES-CBC: Vaudenay s attack RSA-PKCS#1: Bleichenbacher s attack 20 20

19 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 21

20 Recent Attacks on TLS Not only crypto attacks Attacks on TLS state machines FREAK Early CCS Buffer overflows / overreads Heartbleed CVE (High) -> CVE (Critical) Tool for flexible protocol executions needed 22 22

21 Framework Prerequisites Flexible protocol flow definition Message modifications Invalid behavior detection Protocol flow reproduction ClientHello ClientKeyExchange ClientKeyExchange ChangeCipherSpec (Client-) Finished ServerHello Certificate ServerHelloDone ChangeCipherSpec (Server-) Finished Application Application 23

22 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 24

23 High-Level Overview 25 25

24 Modifiable variables Define basic data types (integer, byte, arrays) with modifications Example: ModifiableInteger i = new ModifiableInteger(); i.setvalue( 30 ); i.setmodification(new AddModification( 20 )); System.out.println(i.getValue()); // 50 Further modifications: xor, shuffle, delete, 26 26

25 Protocol messages ClientHello ClientHelloMessage ciphersuites: ModifiableByteArray ciphersuitelength: ModifiableInteger getciphersuites() getciphersuitelength() Stored in a message list Serializable in XML 27 27

26 Defining a protocol flow <protocolmessages> <ClientHello> <supportedciphersuites> <CipherSuite>TLS_RSA_WITH_AES_128_CBC_SHA</CipherSuite> </supportedciphersuites> </ClientHello> <ServerHello/> <Certificate/> <ServerHelloDone/> <RSAClientKeyExchange/> <RSAClientKeyExchange/> <ChangeCipherSpec/> <Finished/> <ChangeCipherSpec/> <Finished/> <Application/> </protocolmessages> 29 29

27 Defining a protocol flow <protocolmessages> <ClientHello> <supportedciphersuites> <CipherSuite>TLS_RSA_WITH_AES_128_CBC_SHA</CipherSuite> </supportedciphersuites> </ClientHello> <ServerHello/> <Certificate/> <ServerHelloDone/> <RSAClientKeyExchange/> <ChangeCipherSpec/> <Finished/> <ChangeCipherSpec/> <Finished/> <Heartbeat/> </protocolmessages> <Heartbeat> <payloadlength> <integeraddmodification> </integeraddmodification> </payloadlength> </Heartbeat> 30 30

28 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 32

29 Vulnerability detection How do we detect invalid server behavior? 1. Different TLS alerts Useful by padding oracle attacks 2. Address Sanitizer (ASan) Detects memory errors at runtime Available in recent compilers, e.g. GCC Vulnerability found -> protocol stored in XML 33 33

30 Two-stage concept Currently only server evaluation 1. Crypto Padding oracles, Bleichenbacher attack, invalid curve attacks, POODLE 2. Fuzzing for boundary violations 3 phases 34 34

31 Fuzzing for boundary violations 1. Variable filtering Not all variables suitable 2. Fuzzing with filtered variables ClientHelloMessage ciphersuites ciphersuitelength clientrandom extensions extensionlength. Random modifications (add, delete, xor) Boundary values (-128, -1, 0, 32768, ) 3. Fuzzing with modified protocol flows 35 35

32 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 36

33 Results Padding oracle attack OpenSSL (CVE ) Botan (CVE ) MatrixSSL Bleichenbacher attack MatrixSSL Missing length checks GnuTLS OpenSSL Out-of-bound reads / writes OpenSSL pre1 (stack overflow) Botan (Out-of-bound read) 37 37

34 Padding oracle attack Ciphertext C Decryption failed Valid / Invalid Server Applicable to AES-CBC Challenge: not to reveal padding validity 1. Same error message 2. Constant time padding and HMAC validation 38 38

35 AES-CBC in TLS MAC-Pad-Encrypt Example: Two blocks Message: Hello MAC size: 20 bytes (SHA-1) Padding size: = 7 pad mac H e l l o

36 AES-CBC in TLS Challenge: not to reveal padding validity Always: Padding validation MAC validation Same error message and timing pad mac H e l l o

37 Constant Time Validation pad mac Decrypted data H e l l o Mask data H e l l o

38 Constant Time Validation Decrypted data H e l l o Mask data H e l l o

39 OpenSSL Vulnerability Decrypted data 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F Mask data 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 1F 45 45

40 OpenSSL Vulnerability (CVE ) Introduced by patching Lucky 13 Only when using AES-NI Leads to a different server response C RECORD OVERFLOW / BAD RECORD MAC Can this be even worse? 46 46

41 Yes MatrixSSL Timing attack -> buffer overflow 47 47

42 Overview 1. TLS Protocol 2. Attacks 3. Framework Prerequisites 4. TLS-Attacker Design 5. Fuzzing 6. Results 7. Conclusions 48

43 Conclusions and future work Maintaining a crypto library is hard New code / patches can introduce new flaws Systematic fuzzing and evaluation needed TLS-Attacker For researchers, pentesters For developers Development / fuzzing improvements needed TLS client-side tests Better fuzzing strategies 49 49