CS 43: Computer Networks Security. Kevin Webb Swarthmore College December 7, 2017
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1 CS 43: Computer Networks Security Kevin Webb Swarthmore College December 7, 2017
2 Topics Spying on network traffic Classic problem: buffer overflow attack Monetizing botnets
3 Once upon a time The Internet was a group of mutually trusting users attached to a transparent network. Result: Not much built-in security headers IP address spoofing IP prefix hijacking
4 Once upon a time Trust is gone, is the network still transparent? Switches help, cables can still be tapped What about wireless?
5 Wireless example/demo
6 Encryption Multiple options: End to end (SSL, TLS): Browsers use this. Link layer (WEP, WPA): Access point uses this. Facebook: enabled E2E encryption? July 2013
7 Cryptography Dates back 1000 s of years Simple substitution cipher (Caesar cipher) Shift each letter by three (a -> d, b -> e, ) Hello world becomes khoor zruog Many other, significantly better ciphers since De facto standard today: AES
8 (Symmetric) Cryptography Problem: Encrypting with a cipher requires shared key information. (prior to 1970 s) Sophisticated cipher doesn t help if we have to communicate the secret key!
9 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read.
10 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read.
11 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read.
12 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read.
13 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read.
14 Box Analogy You want to ship a package to someone. You trust it won t be stolen, but might be read. Shared Secret!?
15 (Asymmetric) Public Key Crypto Analogy: locking and unlocking are asymmetric Anybody can lock Very difficult to unlock without the key Let s apply this to data. We need a function that s easy to apply in one direction and difficult in the other.
16 Factoring Multiplication is easy. Factoring (primes) is hard is the product of two primes. What are they?
17 RSA Algorithm Rivest, Shamir, Adleman (RSA) Everyone computes two items: Public key (kind of like a pad lock, ok if seen) Private key (keep this to yourself) Receiver distributes public key, sender uses it to craft message that only receiver can read.
18 RSA Algorithm Choose two prime numbers of similar length. P = 41 and Q = 29 Compute N = P * Q N = 41 * 29 = 1189 Compute phi(n) = (P 1) * (Q 1) phi(n) = (41 1) * (29 1) = 40 * 28 = 1120 Choose a value e, 1 < e < 1120 e must not divide 1120, we ll pick 13
19 RSA Algorithm P = 41 and Q = 29 N = 1189 phi(n) = 1120 e = 13 Compute modular multiplicative inverse of e Need: (d * e) % phi(n) = 1 d = 517
20 RSA Algorithm P = 41 and Q = 29 N = 1189, phi(n) = 1120, e = 13, d = 517 Public key is (n, e), private key is (n, d) To encrypt message m = 1000: Take % 1189 = 611 To decrypt message 611: % 1189 = 1000
21 In Practice Result: we can exchange secure messages with parties we ve never talked to before! (e.g., your bank) Exchange a secure message containing shared secret via RSA (asymmetric crypto) Subsequently use shared secret for conventional symmetric crypto (e.g., AES)
22 Classic Attack: Buffer Overflow Encryption ruining your (evil) day? Let s try taking control instead!
23 Recall: The Stack Callee s frame. Caller s frame. Callee s local variables. Caller s Frame Pointer Return Address First Argument to Callee Final Argument to Callee Caller s local variables. Older stack frames. Shared by caller and callee.
24 A well intentioned program Suppose we have a protocol that does recv() until it finds \r\n\r\n. char buf[1000] buf[0] Stack Memory buf[999] Ret urn add ress
25 A well intentioned program Suppose we have a protocol that does recv() until it finds \r\n\r\n. char buf[1000] G E T / Stack Memory i n d e x. h t m l \0 \0 \0 \0 \0 Ret urn add ress
26 A well intentioned program What happens if we re sent more than 1000 bytes before we see \r\n\r\n? Keep writing char buf[1000] G E T / Stack Memory i n d e x. h t m l D D D D D D D D D
27 A well intentioned program Uh, if we can overwrite the return address We can control execution on return. char buf[1000] G E T / Stack Memory i n d e x. h t m l D D D D D D D D D
28 A well intentioned program Let s send malicious data that contains a ptr. char buf[1000] G E T / Stack Memory i n d e x. h t m l D D D D D PTR PTR PTR PTR
29 A well intentioned program Let s send malicious data that contains a ptr. char buf[1000] G E T / Stack Memory i n d e x. h t m l D D D D D PTR PTR PTR PTR
30 A well intentioned program Oh, and also some commands up here char buf[1000] E V I L Stack Memory C O D E H E R E D D D D D PTR PTR PTR PTR
31 A well intentioned program Function returns, executes evil code. char buf[1000] E V I L Stack Memory C O D E H E R E D D D D D PTR PTR PTR PTR
32 A well intentioned program Improve chances: NO OP sled char buf[1000] NOP NOP NOP NOP NOP Stack Memory NOP NOP NOP NOP NOP E V I L C O D E PTR PTR PTR PTR
![A well intentioned program See: Smashing the Stack for Fun and Profit char buf[1000]](/docs-images/75/72546448/images/33-0.jpg "NOP NOP NOP NOP NOP Stack Memory NOP NOP NOP NOP NOP E V I L C O D E PTR PTR PTR PTR")
33 A well intentioned program See: Smashing the Stack for Fun and Profit char buf[1000] NOP NOP NOP NOP NOP Stack Memory NOP NOP NOP NOP NOP E V I L C O D E PTR PTR PTR PTR
34 Input from Network Programs that receive user input are susceptible to buffer overflow (& more) attacks. Potentially much more problematic to receive input from the Internet! If attackers can take over program s control flow, they can execute anything.
35 Relevant XKCD #327 Alt text: Her daughter is named Help I'm trapped in a driver's license factory. Bottom line: be careful about what you re accepting from the network! Make sure the memory you re using is bounded and that the data is valid!
36 1988: The Morris Worm Cornell student Robert Morris Exploited buffer overflow in fingerd It had a 512-byte buffer, he exploited it to execute /bin/sh, giving him shell access Told compromised host to download his worm code, it self-replicated by exploiting others Claimed wanted to gauge size of Internet
37 1988: The Morris Worm Worm did a check to see if it needed to replicate itself If machine already compromised (process running) don t infect again. Worried about admins putting up fake process Replicate anyway, at random, 1/7 times. This effectively shut down LOTS of machines.
38 1988: The Morris Worm Robert Morris: First person convicted under Computer Fraud and Abuse Act Sentenced to three years probation, 400 hours community service, $10,000 Where is he now?
39 Exploits Today Worms Trojans (trick user) Browser exploits (drive-by downloads) Often used in BotNets
40 BotNets Having access to 1000 s of machines is lucrative! Send Spam. Flood target with traffic (DDoS). Steal data (CC # s, state secrets, etc.). Mine bitcoins.
41 Questions?
42 Final Exam Friday, December 15, at 9:00 LOCATION: SCI 199
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