Transcript EE579T-Class 7

EE579T
Network Security
7: An Introduction to Network-Based Attacks
Prof. Richard A. Stanley
Spring 2001
© 2000, 2001, Richard A. Stanley
WPI
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Thought for the Day
“Everything should be made as simple
as possible. But not simpler.”
Albert Einstein
Spring 2001
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WPI
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Overview of Tonight’s Class
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Review last week’s lesson
Look at network security in the news
Course project outlines
Network attacks
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WPI
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Last Week...
• Protocols exist to provide end-to-end
security over the Internet and other hop-byhop networks
• The existence of such protocols is not a
guarantee of security
• Steganography is one way for information
to leak out of a system
• Steganography can be very hard to find, but
it is very easy to implement at low cost
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WPI
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Network Security Last Week- 1
• Gnutella worm finds new way to squirm
into PCs
• Government e-security measures
inadequate, according to some experts
• Germany closer to finalizing regulations
that would allow monitoring every e-mail
• Security hole in Java may expose servers
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Network Security Last Week- 2
• New flaw discovered in Lotus Domino
• Consultant tells CEOs and COOs that most
security problems come from within a
company, for financial or political gain
• Last year businesses throughout the world
lost $1.6 trillion due to computer down time
resulting from security breaches and virus
attacks (Mary Pat McCarthy, Vice Chairwoman , KPMG)
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Network Security Last Week- 3
• Harvard's Dr. Michael Rabin claims he has
developed an unbreakable encryption
technique using a disposable key and that he
has mathematical proof of its security
• Verizon administrative error causes e-mail
outage
• OfficeMax customer data, including credit
card numbers, forwarded to other shoppers
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WPI
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Network Security Last Week- 4
• Columbia House breach exposes customer
information
• Home page redirections on the rise
• Hacker downloads names, SSN’s of 3,000
University of Indiana students
• e-commerce and Internet risks rank as
number one European risk concern and
number two concern of US risk managers
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Course Projects - 1
1. Port scanning technology
– Sullivan, Toomey
2. Extensible authentication protocol
– Mizar, Hirsch, Tummala
3. Honey Pot
– Kaps, Gaubatz
4. Wired/Wireless security comparison
– Azevedo, Nguyen, H. Tummala
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Course Projects - 2
5. SOHO network security
– Davis, Syversen, Kintigh
6. Sniffing switched networks
– Michaud, Lindsay, VanRandwyk
7. Broadband access security
– Sumeet, Nirmit, Harsh
8. Trojan Horse security
– Aparna, Subramanian
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Course Projects - 3
9. Java security
– Malloy
10. Router security
– Mansour,
11. DDoS Security
– Gorse, Pushee
12. Network Security Processors
– McLaren, Brown
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Projects -4
13. Network cryptography
– Lee
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Schedule Options
• Exam on 5 April + 1-2 projects, balance on
12, 19 April
• Exam on 12 April, with projects week
before and after, and 1-2 after exam on 12th
• Others?
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Network Based Attacks
Oldies and Goodies--It Isn’t Magic
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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
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TCP Review
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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
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WPI
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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
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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
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IP Spoofing
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Passive Sniffing
• Hacker obtains access to network segment;
observes and analyzes traffic
– Unauthorized access to legitimate computer
– Unauthorized added NIC on segment
• Purpose: gather intelligence, read traffic
• Defense:
– Secure authentication schemes (Kerberos)
– Data encryption
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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
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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
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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
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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
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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
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WPI
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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
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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 ...
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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
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WPI
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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
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WPI
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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
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WPI
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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
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WPI
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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
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WPI
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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
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WPI
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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
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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”
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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
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WPI
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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
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WPI
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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
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WPI
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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!
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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
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Spoofing -2
• Simpler approach: wait until client system
shuts down and impersonate the system
– Example: Unix NFS uses IP addresses only 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
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WPI
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Spoofing E-mail
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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
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WPI
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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?
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WPI
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WPI
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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
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WPI
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Buffer Overflows
• Sending oversize ICMP packets
• Sending IIS 3.0 a 4048 byte URL request
• Sending email with 256-character file name
attachments to Netscape/MS email clients
• SMB logon to NT with incorrect data size
• Sending Pine user an email with “from”
address > 256 characters
• Connect to WinGate POP3 port with user
name of 256 characters
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WPI
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What Do You Intend?
• Take over a session
– Why?
– What information do you want to get/put?
• Associate with a network more or less
permanently
• Deny service to selected servers / networks
/ clients?
• Anything else?
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WPI
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Summary
• TCP/IP was not intended as a secure
protocol; as a result, it has vulnerabilities
that can be exploited
• There are many types of attacks that can be
mounted over network connections in order
to gain unauthorized access to resources
• Never forget, the best access is hands-on
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WPI
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Homework - 1
1. How would you prevent postdesynchronization hijacking attacks?
2. Research attack scenarios and tools that
you find in literature or on the Internet.
Describe two attack scenarios and the tools
required (if any) that would enable you to
break into the WPI network from outside.
Don’t actually break in, or try to!!
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WPI
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Homework - 2
3. Describe how a SMURF attack works
(don’t just parrot the textbook description).
Describe how to stop it.
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WPI
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Assignment for Next Week
• Read course text, Chapter 14
• Next week’s topic: More Network-Based Attacks
Spring 2001
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WPI
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