TCP/IP: TCP. Network Security Lecture 7. Eike Ritter Network Security - Lecture 7

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1 TCP/IP: TCP Network Security Lecture 7 1

2 TCP spoofing Alice trusts Bob (e.g., logins on Alice are allowed with no password if TCP connection comes from host Bob) Mallory wants to impersonate Bob when opening a TCP connection to Alice Steps M kills B (e.g., flooding) M sends SYN segment to A with source IP address set to B s IP address A sends a SYN/ACK to B, with its initial sequence number I A M completes the 3-way handshake, with ACK set to I A + 1 M eavesdrops A s response containing I A [we know how to do this] M guesses the correct I A value ( blind spoofing ) [we will focus on this] 2

3 TCP spoofing Described in R. T. Morris, A Weakness in the 4.2BSD UNIX TCP/IP Software Used by Kevin Mitnickattack in his attack against the San Diego Supercomputer Center Addressed by S. Bellovin, RFC 1984, Defending Against Sequence Number Attacks Set initial sequence number to the timer prescribed originally +the value of a cryptographic hash function of each connection: ISN = M + F(localhost, localport, remotehost, remoteport) It is vital that F not be computable from the outside, so it is keyed with with some secret data True random number Per-host secret and boot time of the machine Thus, each connection is given a separate sequence number space That s the theory, at least 3

4 Initial sequence number revisited How about the actual implementations? Are the different OSes correctly implementing initial sequence number generators? How do you test it? MichalZalewskilooks at this problem in Strange Attractors and TCP/IP Sequence Number Analysis and the follow-up study One Year Later Derive properties of ISN generators by observing the generated ISNs(instead of, say, looking at their implementation) For example: , , , , , , , , ,... Phase space analysis to visualize hidden dependencies Represent a sequence of ISNsin a 3-d space: x[t] = seq[t] -seq[t-1] y[t] = seq[t-1] -seq[t-2] z[t] = seq[t-2] - seq[t-3] Look for unexpected regularities in this space ( attractors ) Use attractor to build Spoofing Sets, i.e., likely values for the next ISN 4

5 Linux 2.2 5

6 Windows

7 Windows 98 7

8 Cisco IOS -before 8

9 Cisco IOS -after 9

10 TCP hijacking We saw: TCP spoofing + address-based authentication = attacker wins What if authentication is performed by other means, e.g., password-based? Attacker needs a more powerful attack Attacker waits for TCP connection to be established and then creates a desynchronized state on both ends of the connection Two points cannot communicate Then creates acceptable packets for both ends At this point, attacker completely controls the connection Performed by using spoofed TCP segments to Insert data in the streams Reset an existing connection (denial of service) First described in L. Joncheray, Simple Active Attack Against TCP 10

11 TCP hijacking Scenario: attacker wants to inject data into existing TCP connection between two hosts Think of telnet connection between A and B M wants to hijack connection to inject commands that will be executed on B Attacker waits until the connection to hijack is quiet All the sent data has been acknowledged Attacker injects data into the stream ( desynchronization ) echo + + > ~/.rhosts Source IP is spoofed to A s IP address SEG.SEQ = last ACK from the server SEG.ACK = last SEQ from the server Datagram is acceptable and will be processed by the server 11

12 TCP hijacking What happens next? Server sends ACK to A Remember: attacker is spoofing A s address SRV.ACK = M Client receives segment, notices it is out of order, and sends ACK with the right sequence number CLN.ACK = N (N < M) Server receives segment, notices it is out of order, and sends ACK with the right sequence number SRV.ACK = M... Ack storm 12

13 ACK storm > : P : (21) ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack > :. seq ack

14 Fixing the ACK storm How to prevent/stop? Disable the client so that it does not reply to server s ACKs Resynchronize Send null data to client so that it resyncswith the server In telnet, NOP commands Just wait ACKsthat carry no data are not retransmitted if lost ACK storm will congest the network So eventually, ACKswill be lost ARP spoofing Spoofs client so that server ACKsonly reach the attacker 14

15 SYN flooding Denial of service attack Attacker sends a SYN packet Victim replies with SYN/ACK segment Internally, the victim allocates resources to keep track of the half-open connection Transmission Control Block (TCB) In practice, some memory Attacker does not send ACK The victim has limited number of available TCBsto keep track of the half-open connection When limit is reached, it will not accept any other connection Can the attacker easily hide (spoof) its source address? 15

16 SYN flooding defenses How would you protect from this attack? Filtering To block spoofed packets Increase the length of the queue storing half-open connections Recycle existing half-open connections when the limit is reached and new connections arrive Reduce the SYN-RECEIVED timeout Time waited between receiving a SYN and recycling the resources allocated to this connection Do not allocate any state at all for a received SYN SYN cookie 16

17 SYN cookie Encode the connection state in the initial sequence number and/or other fields (e.g., the Timestamp option) Set ISN to t m s t: timestamp m: MSS s: result of crypto function on server address and port, client address and port, t When ACK is received Recover the ISN (ACK 1) Check t against current timestamp and check that it is close enough Recomputes and check that it is valid Decode m 17

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