Network sniffing tools like Wireshark can capture network traffic containing usernames, passwords, emails and other data. ARP poisoning allows an attacker to intercept traffic by falsifying ARP entries and acting as a man-in-the-middle using tools like Cain and Abel, enabling the capture and modification of network traffic.

1. JMU GenCyber Boot Camp
Summer, 2015

2. Network Sniffing
• Sometimes it is possible observe/record traffic
traveling on a network
• Network traffic may contain valuable information:
– Usernames and passwords
•Encrypted
•Unencrypted
– E-mail, web requests (and replies), data files
– Etc.
• A sniffer is a piece of software that captures
network traffic

3. Analogy - Wiretapping
• The FBI conducts wiretaps
– Go to a judge and get a court order authorizing the
wiretap
•Who?
•What?
•When?
•Why?
– With the help of the phone company, can listen
to/record a suspect’s phone conversations to obtain
evidence

4. Analogy – Wiretapping (cont)
• Sniffer allows an administrator (or attacker) to
record/listen in on conversations between
computers
– May need authorization to monitor network traffic
–Electronic Communications Privacy Act
–https://www.cdt.org/issue/wiretap-ecpa
– May not need authorization to monitor network traffic
–“Trap and Trace”/”Pen register”
–Consent
– May not care - attackers

5. Sniffing - Environment
• Some networks use shared media so passive
sniffing is very easy
– Network interface cards can be placed in
“promiscuous” mode so that they do not ignore
traffic to other hosts
• Wireless network traffic can also be
captured (but may be encrypted)
• Sniffing is more difficult (but not
impossible) in switched environments

6. Protocol Analysis
• Captured network packets contain binary
data which is difficult to interpret
• Most sniffers include a protocol analysis
component which organizes and displays
the (human-readable) contents of the traffic
– Example: Wireshark

7. Example – An Nmap Port Scan
• Target host: 192.168.78.141
– Start Wireshark
• Source host: 192.168.78.142
– Perform a TCP-connect scan
•nmap –sT <target host>
• View results

8. Example – A Web Connection
• Target host: 192.168.78.141
– Start Wireshark
• Source host: 192.168.78.142
– Open a text-based web browser
•Get default web page on the target host
• View results

9. Example – An FTP Connection
• Target host: 192.168.78.141
– Start Wireshark
• Source host: 192.168.78.142
– Use the ftp client
•ftp <target host>
• View results

10. Example – An SFTP Connection
• Target host 192.168.78.142
• Source host 192.168.78.141
– Use the sftp client
•sftp guest@<target host>
• View results

11. Man-in-the-Middle
• In a switched environment a host only
receives:
– Traffic destine for itself
– Broadcast traffic
• Cannot see traffic between other hosts
• Man-in-the-middle = insert yourself as an
(undetected) intermediary between
communicating hosts

12. Man-in-the-middle (cont)
• Normal:
• Man-in-the-middle:
Alice Bob
I
Alice Bob
I

13. Man-in-the-middle (cont)
• How to achieve man-in-the-middle in a
switched environment?
• Exploit address resolution protocols

14. Address Resolution
• All network communications must be carried out
over physical networks
– Each machine has a unique physical address
• Programs (and humans) use IP addresses to
specify the machine to which a message is sent
• The address resolution problem – need to map IP
address to physical address

15. The Address Resolution Problem
Hosts A and B are on the same physical network
B wants to communicate with A but only knows A’s
IP address
E
D
C
B
A

16. The Address Resolution Protocol (ARP)
Host A wants to resolve the IP address IB
Host A broadcasts a special (ARP) packet that asks
the host with IP address IB to respond with its
physical address
All hosts receive the request
Host B recognizes its IP address
Host B sends a reply containing its physical address

17. ARP
• Phase 1:
• Phase 2:
A X B Y
A X B Y

18. ARP Caches
• Each host maintains a cache of recently-
used mappings
– Information in the cache expires after a set time
has elapsed
• When sending an ARP request a host
includes its IP-to-physical address binding
• All machines on a physical network
“snoop” ARP packets for mappings

19. Demo – ARP Cache
• Host.141 has not communicated with .143
– .141’s ARP cache probably doesn’t contain an
entry for .143
• Host .141 makes a web request to .143
– ARP for .143’s physical address
•Added to .141’s cache
– Web request sent and reply received

20. ARP Cache Poisoning
• Broadcast ARP replies associating your
physical address with a given IP address
– Other hosts receive this message and put the
mapping into their ARP cache
– When a machine wants to communicate with
the given IP address it sends the frame to your
physical address
– You read the frame and then forward it on to
the real destination host

21. Cain and Abel
• A man-in-the-middle LAN attack tool
– Sniffer
– Protocol analyzer
• URL: http://www.oxid.it/cain.html
• Can be used to poison hosts ARP caches

22. Demo – ARP Cache Poisoning
• Hosts .142 and .143 may or may not have
communicated
– ARP caches may or may not contain entries for
each other
• Start Cain (on .141) and poison both .142
and .143’s ARP caches:
– .142’s HW address associated with .141’s IP
– .143’s HW address associated with .141’s IP

23. ARP Cache Poisoning - Result
• .142 and .143 will communicate with each other
– May not realize that their communications are flowing through a
third-party
• All communications will flow through .141
– .141 can read/store traffic
– .141 forwards between the two hosts

24. Example – An FTP Connection
• Switched Environment
– Source host: .143
– Destination host: .142
– Attacker: .141
• Using:
– Cain and Abel

25. ARP Poisoning
• Can:
• Read traffic
• Modify traffic

26. Example – DNS Spoofing
• Switched Environment
– Source host: .143
– Destination host: Google
– Attacker: .141
• Using:
– Cain and Abel

27. Example – SSH Downgrade
• Switched Environment
– Source host: my laptop
– Destination host: .147
– Attacker: .141
• Using:
– Cain and Abel

28. ARP Poisoning
• What attackers look for:
– Sensitive, unencrypted communications
•Web requests/replies, e-mail, FTP
– Weakly-encrypted communications
•Old versions of SSH, RDC

29. ARP Poisoning - Countermeasures
• Static ARP tables/smart switch
• ARPwatch
• IDS

30. Summary
• Network traffic may contain valuable information:
– Usernames and passwords
•Encrypted
•Unencrypted
– E-mail, web requests (and replies), data files
– Etc.
• ARP poisoning can allow an attacker to capture and
modify network traffic as a man-in-the-middle:
– Cain and Abel