Computer Security
CS 426
Lecture 33
Network Security (1)
CS426
Fall 2010/Lecture 33
1
Network Protocols Stack
Application
Application protocol
TCP protocol
Transport
p
Application
Transport
p
Network
IP protocol
IP
IP protocol
Network
Link
Data
Link
Network
Access
Data
Link
Link
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Fall 2010/Lecture 33
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Types
yp of Addresses in Internet
• Media Access Control (MAC) addresses in the network
access layer
– Associated w/ network interface card (NIC)
– 48 bits or 64 bits
• IP addresses for the network layer
y
– 32 bits for IPv4, and 128 bits for IPv6
– E.g., 128.3.23.3
• IP addresses + ports for the transport layer
– E.g., 128.3.23.3:80
• Domain names for the application/human layer
– E.g., www.purdue.edu
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Routing and Translation of
Addresses
• T
Translation
l ti b
between
t
IP addresses
dd
and
d MAC
addresses
– Address Resolution Protocol (ARP)
(
) for
f IPv4
– Neighbor Discovery Protocol (NDP) for IPv6
• Routing with IP addresses
– TCP, UDP, IP for routing packets, connections
– Border Gateway Protocol for routing table updates
• Translation between IP addresses and domain
names
– Domain
o a Name
a e Syste
System ((DNS)
S)
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Fall 2010/Lecture 33
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Threats in Networking
g
• Confidentiality
C fid ti lit
– Packet sniffing
• Integrity
– Session hijacking
• Availability
– Denial of service attacks
• Common
– Address translation poisoning attacks
– Routing attacks
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Fall 2010/Lecture 33
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Concrete Security
y Problems
• ARP iis nott authenticated
th ti t d
– APR spoofing (or ARP poisoning)
• Network packets pass by untrusted hosts
– Packet sniffing
• TCP state can be easy to guess
– TCP spoofing
p
g attack
• Open access
– Vulnerable to DoS attacks
• DNS is not authenticated
– DNS poisoning
i
i attacks
tt k
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Fall 2010/Lecture 33
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Address Resolution Protocol (ARP)
(
)
• Primarily used to translate IP addresses to Ethernet MAC
addresses
– The device drive for Ethernet NIC needs to do this to send a
packet
• Also used for IP over other LAN technologies, e.g., FDDI,
or IEEE 802.11
• Each host maintains a table of IP to MAC addresses
• Message types:
– ARP request
– ARP reply
– ARP announcement
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Fall 2010/Lecture 33
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http://www.netrino.com/Embedded-Systems/How-To/ARP-RARP
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Fall 2010/Lecture 33
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ARP Spoofing
p
g (ARP
(
Poisoning)
g)
• Send fake or 'spoofed'
'spoofed', ARP messages to an Ethernet
LAN.
– To have other machines associate IP addresses with the
attacker’s MAC
• Defenses
– static ARP table
– DHCP snooping (use access control to ensure that hosts only
use the IP addresses assigned to them
them, and that only authorized
DHCP servers are accessible).
– detection: Arpwatch (sending email when updates occur),
• Legitimate use
– redirect a user to a registration page before allow usage of the
network
t
k
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Fall 2010/Lecture 33
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IP
Internet Protocol
Version
• Connectionless
C
ti l
– Unreliable
– Best effort
Flags
Fragment Offset
Time to Live
Protocol
Header Checksum
• Transfer datagram
– Header
– Data
Header Length
Type of Service
T t l Length
Total
L th
Identification
Source Address of Originating Host
Destination Address of Target Host
Options
P ddi
Padding
IP Data
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Fall 2010/Lecture 33
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IP Routing
g
Meg
Office gateway
Packet
121.42.33.12
121 42 33 12
Source 121.42.33.12
Destination 132.14.11.51
5
Sequence
Tom
132 14 11 1
132.14.11.1
ISP
132 14 11 51
132.14.11.51
121.42.33.1
• Internet routing uses numeric IP address
• Typical route uses several hops
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Fall 2010/Lecture 33
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Packet Sniffing
g
• Promiscuous Network Interface Card reads all
packets
– Read all unencrypted
yp
data ((e.g.,
g , “ngrep”)
g p)
– ftp, telnet send passwords in clear!
Eve
Alice
Network
P
Prevention:
ti
E
Encryption
ti
CS426
Bob
(IPSEC TLS)
(IPSEC,
Fall 2010/Lecture 33
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User Datagram
g
Protocol
• IP provides
id routing
ti
– IP address gets datagram to a specific machine
• UDP separates traffic by port (16-bit number)
– Destination port number gets UDP datagram to
particular application process, e.g., 128.3.23.3:53
– Source port number provides return address
• Minimal guarantees
– No acknowledgment
– No flow control
– No message
g continuation
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Fall 2010/Lecture 33
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Transmission Control Protocol
• Connection-oriented, preserves order
– Sender
• Break data into packets
• Attach sequence numbers
– Receiver
• Acknowledge receipt; lost packets are resent
• Reassemble packets in correct order
Book
Mail each page
Reassemble book
1
19
5
1
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Fall 2010/Lecture 33
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14
TCP Sequence
q
Numbers
• Sequence number (32 bits) – has a dual role:
– If the SYN flag is set, then this is the initial sequence number.
The sequence
q
number of the actual first data byte
y is this
sequence number plus 1.
– If the SYN flag is clear, then this is the accumulated sequence
number of the first data byte of this packet for the current session
session.
• Acknowledgment number (32 bits) –
– If the ACK flag is set then this the next sequence number that the
receiver is expecting.
– This acknowledges receipt of all prior bytes (if any).
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Fall 2010/Lecture 33
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TCP Handshake
C
S
SYN (seq=x)
Listening
Store data
SYN ACK (ack
(ack=x+1
x+1 seq
seq=y)
y)
ACK (ack=y+1,seq=x+1)
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Fall 2010/Lecture 33
Wait
Connected
16
TCP sequence
q
p
prediction attack
• Predict the sequence number used to identify the packets
in a TCP connection, and then counterfeit packets.
• Adversary: do not have full control over the network
network, but
can inject packets with fake source IP addresses
– E.g.,
g , control a computer
p
on the local network
• TCP sequence numbers are used for authenticating
packets
• Initial seq# needs high degree of unpredictability
– If attacker knows initial seq # and amount of traffic sent, can
estimate likely current values
– Some implementations are vulnerable
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Fall 2010/Lecture 33
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Blind TCP Session Hijacking
j
g
• A, B trusted connection
Server A
– Send packets with predictable seq
numbers
• E impersonates B to A
E
B
– Opens
Ope s connection
co ect o to A to get initial
ta
seq number
– DoS B’s queue
– Sends
S d packets
k
to A that
h resemble
bl
B’s transmission
– E cannot receive,, but mayy execute
commands on A
Attack can be blocked if E is outside firewall.
firewall
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Fall 2010/Lecture 33
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Risks from Session Hijacking
j
g
• Inject data into an unencrypted server
server-to-server
to server traffic
traffic,
such as an e-mail exchange, DNS zone transfers, etc.
• Inject
j
data into an unencrypted
yp
client-to-server traffic,,
such as ftp file downloads, http responses.
• IP addresses often used for preliminary checks on
fi
firewalls
ll or att th
the service
i llevel.
l
• Hide origin of malicious attacks.
• Carry out MITM attacks on weak cryptographic protocols
protocols.
– often result in warnings to users that get ignored
• Denial of service attacks, such as resetting the
connection.
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Fall 2010/Lecture 33
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DoS vulnerability caused by
session hijacking
• S
Suppose attacker
tt k can guess seq. number
b for
f an
existing connection:
– Attacker can send Reset packet to
close connection. Results in DoS.
– Naively,
N i l success prob.
b iis 1/232 (32-bit
(32 bit seq. #’
#’s).
)
– Most systems allow for a large window of
acceptable seq
seq. #’s
#s
• Much higher success probability.
• Attack is most effective against long lived
connections, e.g. BGP.
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Fall 2010/Lecture 33
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Categories of Denial-of-service
Attacks
Stopping services
Exhausting resources
• Process killing
• Spawning processes
to fill the process table
Locally • Process crashing
• System reconfiguration • Filling up the whole
file system
fil
• Saturate comm
b d idth
bandwidth
• Malformed packets to • Packet floods
(Smurf SYN flood
(Smurf,
flood,
Remotely
R
t l crash buggy services
DDoS, etc)
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Fall 2010/Lecture 33
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SYN Flooding
g
C
S
SYNC1
SYNC2
SYNC3
Listening
Store data
SYNC4
SYNC5
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Fall 2010/Lecture 33
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SYN Flooding
g
• Attacker sends many connection requests
– Spoofed source addresses
• Victim allocates resources for each request
– Connection requests exist until timeout
– Old implementations have a small and fixed bound on
half-open connections
• Resources exhausted  requests rejected
• No more effective than other channel capacity
capacitybased attack today
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Fall 2010/Lecture 33
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Smurf DoS Attack
1 ICMP Echo Req
Src: Dos Target
Dest: brdct addr
DoS
Source
3 ICMP Echo Reply
Dest: Dos Target
gateway
DoS
Target
• S
Send
d ping
i requestt tto broadcast
b d
t addr
dd (ICMP E
Echo
h R
Req))
• Lots of responses:
– E
Every h
hostt on target
t
t network
t
k generates
t a ping
i reply
l (ICMP
Echo Reply) to victim
– Ping
g reply
p y stream can overload victim
Prevention: reject external packets to broadcast address
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Fall 2010/Lecture 33
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Internet Control Message
g Protocol
• Provides feedback about network operation
– Error reporting
– Reachability testing
– Congestion Control
• Example message types
–
–
–
–
–
–
CS426
Destination unreachable
Time-to-live
Time
to live exceeded
Parameter problem
Redirect to better gateway
Echo/echo reply - reachability test
Timestamp request/reply - measure transit delay
Fall 2010/Lecture 33
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Distributed DoS (DDoS)
(
)
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Fall 2010/Lecture 33
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Hiding
g DDoS Attacks
• Reflection
R fl ti
– Find big sites with lots of resources, send packets with
spoofed source
so rce address
address, response to victim
ictim
• PING => PING response
• SYN => SYN-ACK
SYN ACK
• Pulsing zombie floods
– each
h zombie
bi active
ti b
briefly,
i fl th
then goes d
dormant;
t
– zombies taking turns attacking
– making tracing difficult
ff
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Fall 2010/Lecture 33
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Cryptographic
yp g p network protection
p
• Solutions above the transport layer
– Examples: SSL and SSH
– Protect against session hijacking and injected data
– Do not protect against denial-of-service attacks caused by
spoofed packets
• Solutions at network layer
– Use cryptographically random ISNs [RFC 1948]
– More
M
generally:
ll IP
IPsec
– Can protect against
• session hijacking and injection of data
• denial-of-service attacks using session resets
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Fall 2010/Lecture 33
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Readings
g for This Lecture
• Optional Reading
• Steve Bellovin: A Look Back at
“S
“Security
it P
Problems
bl
iin th
the
TCP/IP Protocol Suite”
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Fall 2010/Lecture 33
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Coming
g Attractions …
• DNS S
Security
it
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Fall 2010/Lecture 33
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