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Network Attacks
Bharatha Yajaman
ISQS 6342
Outline
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Sniffing
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IP Address Spoofing
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Passive Sniffing
Active Sniffing
Changing the IP address
Undermining UNIX r – commands
With source routing
Session Hijacking
NetCat
Sniffing
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Sniffers are most common tools used by
attackers
Data link layer of the protocol stack is targeted
Sniffer is a program that gathers traffic from local
network & is useful for attackers for attackers to
get all information as well as network
administrators trying to trouble shoot problems
An attacker can read data passing by a given
machine in real time or can even store the data
in a file
Type of Data Sniffer can capture?
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User IDs and Passwords
DNS queries and responses
email messages
FTP passwords
Files shared using Network File System or
Windows Shares
An Island Hopping Attack
Attacker now
has UserID
and
Password on
new victim
LAN
User telnets to
another system
Sniffer gathers
passwords sent in
clear text
Various Sniffing tools
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tcpdump, freeware for UNIX platform,
www.tcpdump.org
windump, freeware of tcpdump for Windows
9x, NT and 2000
Snort, freeware sniffer and network-based
intrusion detection system, www.snort.org
Ethereal, freeware for UNIX and WinNT with
nice User Interface, www.ethereal.com/
Sniffit, widely used in attacker community,
reptile.rug.ac.be/ ~coder/sniffit/sniffit.html
Sniffing through a Hub – Passive Sniffing
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Transmitting data across a Hub based LAN
just like shouting in a crowded room
A hub implements a broadcast medium
shared by all systems on the LAN
Any data sent across the LAN is actually sent
to each and every machine connected to the
LAN
Sniffing through a Hub – Passive Sniffing
HUB
BROADCAST ETHERNET
A LAN implemented with a hub
Sniffing through a Hub – Passive Sniffing
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If an attacker runs a Sniffer on one system on
LAN, he can gather data sent to and from any
other system on the LAN
Majority of the Sniffer tools are well suited to
sniff data in a hub environment
These tools are called passive sniffers as
they passively wait for the data to be sent to
them, silently gathering the data from the
LAN
Active Sniffing: Sniffing through a Switch
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Switched Ethernet does not broadcast all
information to all systems on the LAN
A switch which is more intelligent than the
hub looks at the MAC address associated
with each frame passing through it and sends
the data to the required connection on the
switch
Active Sniffing: Sniffing through a Switch
SWITCH
SWITCHED ETHERNET
A LAN implemented with a switch
Active Sniffing: Sniffing through a Switch
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The switch limits the data that a passive
Sniffer can gather
If there is a passive Sniffer activated on a
switched LAN the sniffer will only be able to
see data going to and from one machine –
the system with sniffer installed
Active Sniffing: Sniffing through a Switch
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So how do attacker over come this difficulty
of sniffing a switched LAN?
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Attackers have created a variety of tools that
support sniffing a switched environment
These tools actively inject traffic into LAN to sniff a
switched LAN
Dsniff is an active sniffing tool,
www.monkey.org/~dugsong/dsniff
Foiling Switches with Floods ( MAC
flooding )
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MAC address are physical hardware
addresses unique to every Ethernet Card
LAN switches hold the MAC addresses in the
switch memory
What happens if the traffic is flooded with
random MAC address on LAN ?
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As number of different MAC addresses in use
increases the switch dutifully stores the MAC
addresses used by each link on the switch
Foiling Switches with Floods ( MAC
flooding )
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What happens if the traffic is flooded with
random MAC address on LAN ?
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Eventually, the switches memory is exhausted
with bogus MAC addresses
Some switch implementations start forwarding
data to all links connected to all the links if their
memory links are exhausted
The attacker can take advantage of this flaw
and use any passive switching tool to sniff
the switched LAN
Foiling Switches with Spoofed ARP
Messages
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Some switches are immune to MAC flooding
These switches stops storing new MAC
addresses once their memory reaches a
given limit
To sniff in a switched environment where
MAC flooding doesn't work
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Attacker can use DSniff’s tool called arpspoof
arpspoof allows an attacker to manipulate ARP
traffic on LAN
Foiling Switches with Spoofed ARP
Messages
Victim’s
traffic isn’t
sent to
attacker
SWITCH
DEFAULT
ROUTER FOR
LAN
OUTSID
E
WORLD
Foiling Switches with Spoofed ARP
Messages
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To use arpspoof the attacker
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First consults a map of the network likely
generated during the scanning phase of the attack
Looking at the network topology the attacker
observes the IP address of the default router for
the LAN
Sets up the attack by configuring the IP layer of
the attacker’s machine to forward any packet it
receives from the LAN to the IP address of the
default router (IP forwarding)
Foiling Switches with Spoofed ARP
Messages
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To use arpspoof the attacker
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Sends the fake ARP replies to the victims
machine
This ARP changes the victims ARP table by
remapping the default router’s IP (layer 3) to
attacker own MAC address (layer2)
The victims sends the data, forwarding it to what it
thinks is the default router (but using the attackers
MAC address)
Foiling Switches with Spoofed ARP
Messages
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To use arpspoof the attacker
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The attacker sniffs the information using any kind
of sniffing tool
The attacker’s machine will promptly forward the
victim’s traffic to default router on the LAN
Upon reaching the default router the traffic is
transmitted to the outside world
The attacker is now sniffing in a switched
environment
Foiling Switches with Spoofed ARP
Messages
2. Send fake ARP
response to re-map
default router IP to
attacker’s MAC
SWITCH
3. Victim sends traffic
destined for outside world
based on poisoned ARP
table entry
1. Configure IP
Forwarding
4. Sniff the traffic
from the link
5. Packets are forwarded
from attacker’s machine
to the actual default
router for delivery to the
outside world
DEFAULT
ROUTER
FOR LAN
OUTSID
E
WORLD
Sniffing and Spoofing DNS
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The attacker fires up the dnsspoof program
which sniffs the LAN for DNS queries for
specific hosts ( www.bank.com ) - using
Dsniff
At sometime later victim tries to resolve the
name www.bank.com using DNS by trying to
surf the website
Now the attacker sniffs the DNS query and
immediately sends a fake DNS response
Sniffing and Spoofing DNS
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This response will have a lie claiming that
www.bank.com should resolve to IP address
of the attackers machine (say 10.1.1.56)
The victim machine will cache this incorrect
DNS entry
The victims machine will ignore the real
response from the real DNS server as it has
the mapping for www.bank.com
Finally the victims browser makes a
connection with the attackers machine
Sniffing and Spoofing DNS
1. Attacker activates
dnsspoof program
4. Attacker quickly sends
fake DNS response with any
IP attacker wants the victim
to use
3. Sniff the DNS
request from the line
Desired
destination
www.bank.com
SWITCH
2. Victim tries
to resolve a
name using
DNS
5. Victim now
surfs the
attackers site
instead of
desired
location
DEFAULT
ROUTER
FOR LAN
OUTSID
E
WORLD
Attacker’s
machine at
10.10.1.56
Sniffing HTTPS and SSH
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Even though the S’s in HTTPS, SSL and SSH
stand for secure, it is built on a trust model of
underlying public keys
When an HTTPS connection is established
the server sends a certificate which the
browser verifies
If the certificate was signed by a trusted
Certificate Authority, an SSL connection will
be established
Sniffing HTTPS and SSH
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SSL connection uses a session key to
encrypt all data sent by server and client
SSH does not support digital certificates but
is based on same public key encryption idea
SSL and SSH protocols are sound from a
security perspective but the problem is
exploited in the trust certificates and public
keys
Sniffing HTTPS and SSH
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For SSL if a web server sends a browser a
certificate and if the browser does not
recognize the browser will ask the user
whether to accept this untrusted certificate or
not
For SSH the user will be warned that server’s
public key has changed but will still be
permitted to establish connection
Sniffing HTTPS and SSH
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How Dsniff exploits the problem ?
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Dsniff uses webmitm and sshmitm tools for
attacking HTTPS or SSH
mitm stands for “monkey in the middle” a
reference to a classic person-in-the-middle attack
Attackers position themselves between two
systems and actively participate in the connection
to gather data
Sniffing HTTPS and SSH
Person–in–the–middle attack
NETWORK
NETWORK
ATTACKER
Sniffing HTTPS and SSH
Person–in–the–middle attack
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To conduct a person in the middle attack
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The attacker runs the dnsspoof program
configured to send false DNS information so that
a DNS query for a given website will resolve to the
attacker’s IP address
The attacker will also activate webmitm program
that will transparently proxy all HTTP and HTTPS
traffic it receives
The DNS spoof program detects DNS request for
www.bank.com and redirects the client to
attacker’s machine
Sniffing HTTPS and SSH
Person–in–the–middle attack
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To conduct a person in the middle attack
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Victim’s browser starts to establish a secure
connection
All messages for establishing SSL connection are
sent to webmitm on attacker’s machine
webmitm acts as a SSL proxy, establishing 2 SSL
connections one from victim to the attacker's
machine and the other from attacker’s machine to
the actual web server
Sniffing HTTPS and SSH
Person–in–the–middle attack
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To conduct a person in the middle attack
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webmitm when establishing the SSL session
between victim machine to the attacker will send
the attacker’s own certificate
Victim’s browser will notice that the certificate is
not signed by a trusted Certificate Authority and
show a message to the user asking the user
whether to accept this un-trusted certificate or not
Sniffing HTTPS and SSH (Modify)
10.1.2.3
1. Attacker activates
dnsspoof and webmitm
program
2. Dnsspoof sends fake
DNS response with IP
address of the machine
running webmitm
(10.1.2.3)
4. Webmitm proxies the
https connection establishing
an https connection to server
and sending attacker’s own
certificate
SWITCH
3. Victim
establishes SSL
connection not
knowing attacker
is proxying the
connection
5. Victim now
accesses the
desired server,
but all the traffic
is viewable by
attacker using
webmitm as
proxy
DEFAULT
ROUTER
FOR LAN
OUTSID
E
WORLD
Desired
destination
10.22.12.41
IP address spoofing
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Simply changing the IP address
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Simplest way of spoofing another system’s IP
address can be achieved by simply changing the
IP of system to the other’s system address
This can be done by using ifcommand in UNIX,
or the Windows NT network control panel
This method is remarcable in achieving limited
goals if the attacker wants
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To send packets that look like they look like they come
from some where else
Obscure the source of packet flood or other Denial-ofservice attack
IP address spoofing (3 way handshake
diagram)
SYN (A , ISNA )
ACK (A, ISN A ), SYN ( B, ISNB )
A
B
RESET
TCP three way handshake inhibits simple spoofing
IP address spoofing - Undermining UNIX
r-commands
EVE
ALICE
BOB
IP address spoofing - Undermining UNIX
r-commands
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Eve interacts with Bob by sending TCP SYN
packets to one or more of his open ports again
without spoofing
This connection initiation allows Eve to
approximate rate at which the initial TCP
sequence numbers in Bob’s SYN-ACK response
are changing with time
Eve launches a denial-of-service against Alice
(SYN flood or smurf attack)
IP address spoofing - Undermining UNIX
r-commands
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Alice is dead for a period of time. This prevents
Alice from sending RESET packet and dropping
our spoofed TCP connection
Eve initiates a connection to Bob using Alice’s IP
address
The first part of the 3 way handshake is complete
Bob responds to 2nd part of 3 way handshake to
Alice but Alice is dead and cannot respond with a
RESET
IP address spoofing - Undermining UNIX
r-commands
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Using the information gathered, Eve sends a ACK
to Bob including a guess at sequence number,
ISBB, spoofing Alice’s IP.
If the sequence number is incorrect a connection
will not be established but if the sequence number
is correct, Eve will open a TCP connection with
Bob, pretending to be Alice
IP address spoofing - Undermining UNIX
r-commands
EVE
DIE
Open several connections
to Bob to observe the
initial sequence number in
response
SYN (A , ISNA )
ACK (B , ISNB )
ACK (A, ISN A ), SYN ( B, ISNB )
ALICE
BOB
IP address spoofing - Spoofing with
Source Routing
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A far easier method for IP address spoofing is
based on source routing
This technique will let the attacker get
responses in interactive sessions and even
avoid to conduct Denial-of-service attack
Source routing is an option in IP that allows
the source machine sending packets to
specify path it will take on the network
IP address spoofing - Spoofing with
Source Routing
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EVE
ALICE
PACKET
Route
1. Bob
2. Eve
3. Alice
Packet
Contents
PACKET
Route
1. Alice
2. Eve
3. Bob
Packet
Contents
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BOB
Eve generates packets
with fake source route
Packets claim to come
from Alice
Source route includes
Eves address making
Eve look like a router
Packets include
destination Bob
IP address spoofing - Spoofing with
Source Routing
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EVE
ALICE
PACKET
Route
1. Bob
2. Eve
3. Alice
Packet
Contents
PACKET
Route
1. Alice
2. Eve
3. Bob
Packet
Contents
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BOB
Eve injects these
spoofed packets on the
network
Bob will take action on
packets and responds
When Bob responds
packets will be
transmitted to Eve who
is a part of the source
route
Eve will intercept the
packet but will not
forward packet to Alice
IP Spoofing Defense
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The initial set of sequence numbers
generated by TCP stacks must be difficult to
predict
For UNIX machines in particular, using of
weak r-commands must be avoided
Implementation of anti-spoof packet filters at
border routers and firewalls connecting
organization to the Internet and business
partners
IP Spoofing Defense
NETWORK A
FILTERIN
G
DEVICE
NETWORK B
Dropped
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Packet with
IP source
address on
Network A
The filtering devise drops all packets coming
in on one interface that have a source
address of network on another interface
These packets indicate a possible spoofing
attack
IP Spoofing Defense
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Source routed packets through the network
gateways must not be allowed
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This can be done by configuring routers using no
ip sourceroute (this woks for Cisco routers) to
drop all source routed packets at gateways
Careful with trust relationships throughout the
environment
Avoid extending UNIX and Win NT trust
relationship to systems across an
unprotected network
Session Hijacking
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Marriage of sniffing and spoofing is called
session hijacking
An attacker can hijack a session even if
strong authentication is used assuming the
conversation following the initial
authentication is not cryptographically
protected
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As the session is sent in clear text after initial
authentication and a attacker can hijack the it at
any point after authentication
Network-based Session Hijacking
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Alice Telnet
NETWO
RK
BOB
ALICE
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EVE
A has established a
telnet connection
across n/w to B
Attacker sits on a
segment in the
network where traffic
is passing from A to
B
Attacker can see the
session using
sniffing techniques
Network-based Session Hijacking
Alice Telnet
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NETWO
RK
BOB
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EVE
Attacker can also
monitor TCP
sequence numbers
of these packets
while observing the
session
At some point the
attacker will decide
to hijack the
connection
Network-based Session Hijacking
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Alice Telnet
NETWO
RK
BOB
Hi
I’m
Alice
EVE

Now he will start
injecting spoofed traffic
source with A’s IP into
the network using
proper TCP sequence
numbers on all packets
If successful Bob will
listen the commands
sent by attacker
thinking it came from A
Session Hijacking – ACK storm
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When a system receives a packet with TCP
connection that is out of order it resends its
last ACK packet
Retransmission of ACK is supposed to help
the systems resynchronize their sequence
numbers
Session Hijacking – ACK storm
ACK ACK ACK ACK ACK

NETWO
RK
BOB
What happens if the
attacker is injecting
traffic in the TCP
connection ?

Packets
with
increasing
sequence
number
EVE

The sequence number
between attacker and B
will go on increasing
As traffic gets routed back
to A it will see sequence
numbers increasing even
though A has not sent any
packets
Session Hijacking – ACK storm

ACK ACK ACK ACK ACK
NETWO
RK
BOB

Packets
with
increasing
sequence
number
EVE
TCP stacks of A and B
get confused as
attacker sends the
traffic in increasing
sequence number
To resynchronize
connection A will
resend ACK messages
again and again
consuming a good
amount of bandwidth in
what is know an ACK
storm
Session Hijacking – ACK storm
ACK ACK ACK ACK ACK

NETWO
RK
BOB
Packets
with
increasing
sequence
number
EVE
During an ACK storm
the performance
quickly starts to suffer
the attacker will be able
to get one or two
commands executed
on B before the ACK
storm causes the
connection to be
dropped
Session Hijacking Defenses
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Utilizing all defense techniques used for
spoof attacks
Using encryption tools like SSH or VPN for
securing sessions
Encrypted sessions will prevent session
hijacking as attackers will not have keys to
encrypt or decrypt
Netcat : General purpose network tool
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Netcat is one of the most useful tools
available for interacting with systems across
network
Netcat is often referred as “Swiss Army knife
of network tools”
The idea behind Netcat is it allows a user to
move data across a network while functioning
much like the UNIX cat command
Instead of dumping data on the local system
Netcat moves arbitrary data over any TCP or
UDP port
Netcat : General purpose network tool
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Netcat executable operates in one of two
modes
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Client mode
Listen mode
In client mode Netcat can be used to initiate
connection to any TCP or UDP port on
another machine
In listen mode Netcat opens any TCP or UDP
port on the local system
Netcat supports source routing
Netcat : General purpose network tool
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Using these basic building blocks people
have devices many different scenarios based
on Netcat
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File Transfer
Port Scanning
Making connections to open ports
Vulnerability scanning
To create a passive backdoor command shell
To actively push a backdoor command shell
Relaying traffic
Netcat : Defence
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Preventing Netcat file transfers
Securing against port scanning
Blocking arbitrary connections to a port
Protecting against vulnerability scanning
Stopping backdoors
Preventing relay attacks
References
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Counter Hack
Hacking Exposed
http://www.cert.org/advisories/CA-1995-01.html
http://ciac.llnl.gov/ciac/bulletins/f-08.shtml
http://www.iss.net/security_center/advice/Undergrou
nd/Hacking/Methods/Technical/default.htm
http://www.althes.fr/ressources/avis/smartspoofen.pdf
Thank You