Where is the FEEB? The Effectiveness of Instruction Set Randomization Ana Sovarel, David Evans and Nathanael Paul Presented by Sandra Rueda CSE 598A - Spring 2007 - Sandra Rueda Page 1
Code Injection Attacks • High level description: – Look for an input – Insert code – Alter (Transfer) control flow • Main target: – Buffer overflow vulnerabilities • Threat to: – C Programs – CGI Scripts • The attacker needs to know the instruction set to succeed CSE 598A - Spring 2007 - Sandra Rueda Page 2
ISR • If the attacker needs to know the instruction set … • Let’s obscure that set: – Instruction Set Randomization (ISR) Original opcode: 0x3c 0011 1100 Encoding Key: 0x33 0011 0011 XOR New opcode: 0x0F 0000 1111 Original opcode: 1100 Transposing Key: 2103 1001 0011 New opcode: 0110 CSE 598A - Spring 2007 - Sandra Rueda Page 3
ISR Overview • Threat model: – ISR is a defense against external users – It is designed to protect remote servers – (It is not designed as a defense against internal users) • Architecture Loading: Executing: PCB Decoding Register PID, PC, … Key Key Memory e Key (Instruct) Executable: e Key (Instruct) + Key CSE 598A - Spring 2007 - Sandra Rueda Page 4
Proof of Concept • Emulation 1 [Kc et al. CCS’03] – Two ways of randomizing: xor, transposition – 32 bit keys – Mechanisms to randomize binary and perl code • Emulation 2 [Barrantes et al. CCS’03] – Randomizing with xor – Keys as long as the program: they are generated from a seed and a pseudorandom sequence – Mechanism to randomize binary code • Performance: ISR does not considerably slows down I/O intensive processes CSE 598A - Spring 2007 - Sandra Rueda Page 5
How secure is it? • Key length (XOR): – Key as long as the instructions • 32 bits instructions lead to 32 bits keys 2 32 options – Key as long as the program • s = length of the seed 2 s options • Key length (Transposition): – nb = #bits per instruction l = nb * log 2 (nb) – < log 2 (l!) permutations (not all permutations are valid) • It is supposed that injected code will raise an exception after de- randomization: – Invalid opcode – Invalid address access – In some cases the de-randomized instructions may work (tested with real attacks it is a low percentage) [Barrantes et al.] CSE 598A - Spring 2007 - Sandra Rueda Page 6
ISR Drawbacks • Requires special support – Hardware based support | binary to binary translators • Applications must be statically linked – Alternatives: encrypting everything at the beginning or on-demand – Alternatives slow down response time • It can not handle self-modifying code • Vulnerable to denial of service attacks CSE 598A - Spring 2007 - Sandra Rueda Page 7
Effectiveness of ISR • Is it possible to guess the randomization key(s) given time and resource constraints ? CSE 598A - Spring 2007 - Sandra Rueda Page 8
Proposed Attack • Strategy: incrementally break the key • Mechanism: – Trial and error – Select a random key – Inject an identifiable sequence of instructions • Identifiable means … being able to detect if a partial key is right or wrong Socket connection satisfies this requirement • And being able to determine the correct key from the guess XOR satisfies this requirement – Options: • Return attack • Jump attack CSE 598A - Spring 2007 - Sandra Rueda Page 9
Return Attack Stack Layout: 1 byte local buffer local buffer encrypted ret instruction … … saved base pointer saved base pointer saved ret address overwritten ret address saved ret address … … Before After • The stack is compromised the attack can only be used if observable activity occurs before the crash • “We suspect situations where the return attack can be used are rare, but an attacker who is fortunate enough to find such vulnerability can use it …” CSE 598A - Spring 2007 - Sandra Rueda Page 10
Jump Attack The FEEB! Stack Layout: 1 byte local buffer local buffer short jump (eb) offset -2 (fe) … … saved ret address overwritten ret address … … Before After • It produces infinite loops • If the guess is correct the socket remains open • If it is not correct the server crashes and the socket is closed CSE 598A - Spring 2007 - Sandra Rueda Page 11
Incremental Key Breaking • If the key used is as long as the program … – Two keys (from a successful jump attack) are not enough – Repeat the attack using the two already known keys • When the same 32-bit key is used – max_attempts_return = 1024 (4 * 2 8 ) – max_attemps_jump = 66048 (2 16 + 2 * 2 8 ) – Extra attempts may be needed because of false positives • “This can not be done with the described approach” CSE 598A - Spring 2007 - Sandra Rueda Page 12
Eliminating False Positives • Return Attack: • False positives may happen because – The injected guess is decrypted to a instruction similar to near return – The injected guess is decrypted to a harmless instruction and a subsequent instruction behaves like a near return • Options: – Try all 255 possibilities • If none behaves like a return then the mask is the actual instruction (0xc3) • If more than one produce the expected behavior : … use the guess harmless instruction use previous knowledge near return behavior … overwritten ret address CSE 598A - Spring 2007 - Sandra Rueda Page 13
Eliminating False Positives • Jump Attack: • False positives may happen because: – The injected guess is decrypted to a instruction that causes an infinite loop – The injected guess is decrypted to a harmless instruction and a subsequent instruction causes the infinite loop – The injected guess cause a crash but there is a delay that is interpreted as an infinite loop (it is longer than the threshold) • Options: – Only near conditional jumps may generate the same behavior, all of them share the same first four bits 2 13 = 2 5 (opcode) * 2 8 (offset) – It happens when the first byte decrypts to a harmless instruction, the second to a jump instruction and the third to -2 or -3 change the sign to the third byte – Increase the threshold CSE 598A - Spring 2007 - Sandra Rueda Page 14
Extended Attack • ISR with short repeated key the proposed attack is enough • ISR with long keys a larger number of keys is required – Problematic in the case of Jump attacks because they leave active infinite loops causing degradation in service – However after learning some keys it is possible to improve the attack by using additional instructions CSE 598A - Spring 2007 - Sandra Rueda Page 15
Deployment • Inject a micro-VM in the region of memory where masks are known • The micro-VM has a buffer to load the worm code • The micro-VM loads worm code in chunks • The worm code is encrypted with the known keys • To propagate the worm the VM uses the same techniques already described (return and jump attacks) • It requires the old keys thus they are included in the worm code CSE 598A - Spring 2007 - Sandra Rueda Page 16
Evaluation • Client: – Opens a socket to the server – Builds an attack string and sends it – Waits for the acknowledgment – Detects the result • Target: – Echo server with a buffer overflow vulnerability – They modified an ISR implementation so the attack would succeed (?) – Address space layout randomization disabled to test the attack • Results: – After breaking the first bytes, fewer attempts per byte are required to guess the new keys CSE 598A - Spring 2007 - Sandra Rueda Page 17
Results Time to acquire key bytes Jump o Return x 1000 o o x o o o Time x 100 (sec) x x x 10 4 16 64 256 1024 Key Bytes Acquired “On average we can break a 100-byte key in just over 6 minutes with the jump attack .” CSE 598A - Spring 2007 - Sandra Rueda Page 18
Attack Limitations • The attack works for applications that use the same randomization key several times • It will crash a system multiple times – An administrator might notice the attack • It does not work for transposition – It needs to be able to determine the key based on plaintext-ciphertext comparison • It is heavily dependent on x86 architecture – RISC: too long instructions to realistically guess using a brute-force attack • It does not work for higher-level languages as SQL and Perl CSE 598A - Spring 2007 - Sandra Rueda Page 19
Take Away • ISR with short repetitive keys are Security vs. susceptible to the attack. ISR with long Performance fresh keys are not • Long fresh keys mean re-randomizing Is that bad? after crashes. It might cause denial-of- service problems • Like other tools ISR technique requires tuning and constant monitoring! CSE 598A - Spring 2007 - Sandra Rueda Page 20
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