EE579T Network Security 9: An Introduction to Network-Based Attacks

1. EE579TNetwork Security9: An Introduction to Network-Based Attacks Prof. Richard A. Stanley WPI

2. Thought for the Day “Everything should be made as simple as possible. But not simpler.” Albert Einstein WPI

3. Overview of Tonight’s Class • Review last week’s lesson • Course project outlines • Network attacks WPI

4. Projects to Present on April 17 • Team 1: 802.11? • Xiangping Xu, Yang, Gao, Yingchun Xu • Team 2: Mobile IP Security • Phadnis, Misra, Shetty, Subramaniam • Team 3: Secure Border Gateway Protocol • Aytek, Baktir, Yadlon • Team 4: Exploiting Firewall Rule Sets • Kurtz, Barrett WPI

5. Projects to Present on April 17 • Team 5: deleted • Team 6: Fibre Channel Security • Elkind, Maki, Deshpande, Nat, Rongfred • Team 7: Bluetooth Security • Mason, Bouchard • Team 8: Flawed WEP & Fixes • Doraisami, Shirali, Shukla, Thurston WPI

6. Projects to Present on April 24 • Team 9: SNMP Vulnerability • Staake, Peterson, Schweinsberg, Toczek • Team 10: TCP Security Flaws • Kavita, Anuj, Nikhil • Team 11: Intrusion Detection • Madhavi, Shankar, Rohan, Swathi • Team 12: Network Worms • Yuefeng, Wei, Xin WPI

7. Projects to Present on April 24 • Team 13:Web Services Security • Geldmacher, Johnston, • Team 14: DDoS • Hill, Voduc, Huynh • Team 15: S/W Firewalls • Page, Poon, Ibrahim, Meawad, Leclerc • Team 16: Honeypots • Hartling, Lawson, Posluszny, Chung WPI

8. Network Based Attacks Oldies and Goodies--It Isn’t Magic WPI

9. Word of Warning • Some of the attacks about to be described are as old as network attacks themselves • This doesn’t make studying them a waste of time • There is nothing new under the sun -- old attacks keep popping up in new clothes “Those who do not study history are condemned to repeat it.” George Santayana WPI

10. Getting Fingered WPI

11. Do You Know Who? WPI

12. TCP Review WPI

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14. TCP Actions • Assumes IP addresses are valid and correct • If sequence number received  sequence number expected, packet is refused (discarded), system waits for correctly numbered packet WPI

15. Sequence Number Prediction • Determine server’s IP address • Sniffing packets • Trying host numbers in order • Connect w/browser, observe address in status • Try addresses in the server’s address space • Monitor packet sequence numbers • Predict and spoof the next sequence number • Hacker now appears to be a legitimate user WPI

16. Purpose, Detection & Defense • Once on net as an internal user, hacker can use net as a base for other attacks, or to access information on the net just spoofed • Detection: look for sequential “Access denied” entries in the audit log • Prevention: if available, enable real-time notification of large number of sequential access denial entries WPI

17. SYN Flood • Send a normal SYN packet to a server, as if to open a TCP connection • When the server returns a SYN/ACK packet, ignore it • Send another SYN packet to the server • Repeat as necessary • ...until server cannot handle any more WPI

18. FINish, But Don’t Start • Attacker sends FIN packet to server, but has not previously established a TCP connection • Server replies with RST packet • Attacker now knows that port on that server is alive and functioning WPI

19. Passive Sniffing • Hacker obtains access to network segment; observes and analyzes traffic • Unauthorized access to legitimate computer (packet monitors standard NT/2000 fixture) • Unauthorized added NIC on segment • Purpose: gather intelligence, read traffic • Defense: • Secure authentication schemes (Kerberos) • Data encryption WPI

20. Desynchronization Attacks • Hacker forces both ends of TCP session into a desynchronized state • Hacker then uses a third-party host (a computer connected to the physical segment under attack) to intercept original packets and create acceptable replacement packets that mimic the real ones that would have been exchanged • NB: desynchronized  disconnected WPI

21. Post-Desynchronization Hijacking - 1 • Assume: • hacker can listen to any packet exchanged on a TCP session • hacker can forge any kind of IP packet desired and replace the original with it • session has been desynchronized WPI

22. Post-Desynchronization Hijacking - 2 • Client sends packet header with • SEG_SEQ = CLT_SEQ • SEG_ACK = CLT_ACK • Because session has been desynchronized, client packet sequence number (CLT_SEQ) will never equal server’s expected sequence number (SVR_ACK) • Server therefore discards packet WPI

23. Post-Desynchronization Hijacking - 3 • Hacker copies server-discarded packet • Hacker waits to give server time to discard the packet • Sends server same packet the client did, but changes SEG_ACK, SEG_SEQ, & checksum to: • SEG_SEQ = SVR_ACK • SEG_ACK = SVR_SEQ WPI

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25. Post-Desynchronization Hijacking - 4 • The sequence numbers are now correct, so the server accepts the packet the hacker sent • Hacker must produce sequence data so that • SEG_SEQ = (SEG_SEQ + CLT_TO_SVR_OFFSET) • SEG_ACK = (SEG_ACK - SVR_TO_CLT_OFFSET) • Where • CLT_TO_SVR_OFFSET = SVR_ACK - CLT_SEQ • SVR_TO_CLT_OFFSET = CLT_ACK - SVR_SEQ WPI

26. Post-Desynchronization Hijacking - 5 • Hacker now interposed between true client and server • All packets now routed through hacker machine, so any desired commands can be added to / removed from the payload • Server responds to both client & hacker requests; hacker filters his requests and sends client requests to true client WPI

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30. ACK Storm • Primary flaw of desynchronization attack • Receipt of unacceptable packet generates ACK packet to source with expected sequence number • First ACK packet from server contains server’s own sequence number • Client refuses packet, because it did not initially send the modified-request packet • Client now sends its own ACK packet, and ... WPI

31. The End of the Storm • In theory, the ACK storm is an infinite loop • BUT… • If ACK packet lost, no further ACK is sent, because the packet contains no data payload • TCP communicates over a lossy network (i.e. packets will get lost) • With non-zero packet loss, storm quickly ends • Self-regulating WPI

32. Early Desynchronization Attack -1 • Breaks client-server connection during the setup stage • Breaks on server side • After break, hacker creates new connection with a different sequence number • Hacker listens for SYN/ACK exchange • Hacker then sends server a RST, then SYN/ACK with same parameters as client packet, but with different sequence number WPI

33. Early Desynchronization Attack -2 • On receipt of hacker’s RST packet, server closes first connection, and opens new connection on same port, but with a new sequence number when it receives hacker SYN. Sends SYN/ACK to original client. • Hacker intercepts server SYN/ACK and sends server its own ACK packet • Server switches to synchronized connection ESTABLISHED state WPI

34. Early Desynchronization Attack -3 • Client had already switched to ESTABLISHED state on receipt of first SYN/ACK from server • Attack success depends on hacker choosing correct value of CLT_TO_SVR_OFFSET • Wrong value makes both client and hacker packets unacceptable • Produces unwanted effects, including disconnect WPI

35. Early Desynchronization Attack -4 • The hacker now has an established connection with the server, and looks just like the real client • Real client cannot establish a connection on this port until the hacker disconnects, because the server believes that the client is already connected WPI

36. Null Data Desynchronization • TCP connection can be desynchronized by sending large amount of null data to both server and client • Data not visible to client • Sheer volume of data interferes with ability to maintain the TCP session, and ultimately desynchronizes connection WPI

37. Telnet Session Attack - 1 • Hacker passively monitors session • When appropriate, hacker sends large volume of null data to server • Hacker sends ATK_SVR_OFFSET bytes containing sequence IAC NOP • Server interprets these as null due to NOP • Telnet daemon removes each byte pair from data stream • Reception of null data interrupts Telnet session WPI

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39. Telnet Session Attack - 2 • Server has now received commands • SVR_ACK = CLTSEQ + ATK_SVR_OFFSET • Telnet session now desynchronized • Same procedure carried out with client to desynchronize • Early desynchronization attack carried out • Hacker now establishes Telnet session with server and client, becomes “man in middle” WPI

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41. Some Caveats • Telnet session has to be able to carry null data • Timing is everything -- if null data sent at wrong time, session may simply break • If your Telnet session appears unpredictable, you might be experiencing an attack WPI

42. More ACK Info • All networks lose packets, so retransmission occurs • When an active attack such as described before occurs, even more retransmission occurs than in the normal course of events • Extra packets due to the ACK storms • One data packet can generate 10-300 empty ACK packets WPI

43. Detecting Attacks • Detect desynchronized states • Use packet reader (i.e., a sniffer) to view sequence numbers at both ends of a connection • Sequence numbers show if desynchronized • Packet percentage counting • Collect statistics on normal network operations • Use statistics to detect packet storms resulting from attacks WPI

44. Spoofing “You can fool all of the people some of the time. You can fool some of the people all of the time. But you can’t fool all of the people all of the time.” Abraham Lincoln Fooling most of the people most of the time is usually good enough! WPI

45. IP Spoofing-1 • Hacker changes masquerade host IP address to the trusted client’s address • Hacker builds source route to server with direct path packets should take to/from server and back to hacker’s host, with trusted client as last hop in route to server • Hacker uses source route to send client request to server • What’s wrong with this picture? WPI

46. IP Spoofing -2 • Simpler approach: wait until client system shuts down and impersonate the system • Example: Unix NFS uses IP only addresses to authenticate clients • Hacker sets up PC with name and IP address of legitimate client, then initiates connection to Unix host • Typical “insider” attack, as needs knowledge of which computers are not active WPI

47. Spoofing E-mail • Open your email client • Change the “Name” field to something else • Change the “Email address” to something else • Delete the Incoming Mail Server address • Delete the value of Mail Server User Name • If you were really bad, you would find an outgoing mail server that allowed anonymous login for outgoing mail, and put its name here • The approach above is good enough to fool most people most of the time WPI

48. Automated Spoofing • C2MYAZZ • Who knows to what this filename refers? • Hijacks session without disrupting connectivity • This clever utility exploits what was intended as a feature for convenience and backwards compatibility • So, since this is well-known, the tool must be hard to get or overtaken by events, yes? WPI

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50. Preventing Spoofing • Firewall packet filtering • Audit incoming traffic. You should never find packets with source and destination addresses in the local domain coming in from outside. BUT…this takes lots of effort • Don’t allow packets that appear to have originated locally to come in from outside • Hard, especially when hacker is inside WPI