Transcript EE579S Computer Security

EE579T
Network Security
5: An Introduction to Network-Based Attacks
Prof. Richard A. Stanley
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #1
Thought for the Day
“Everything should be made as simple
as possible. But not simpler.”
Albert Einstein
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #2
Overview of Today’s Class
• Review last two week’s lessons
• Network attacks
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #3
Last two times...
• Attacking a network is no different from robbing a
bank; you have to plan if you expect to be
successful
• There are three basic steps to planning, which is
called vulnerability assessment:
– Acquire the target (case the joint)
– Scan for vulnerabilities (find the entry points)
– Identify poorly protected data (enumeration)
• This applies if you are inside or outside the
protected perimeter!
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #4
SSL Handshake Overview
Server
Client
1. Supported ciphers, random number
2. Cipher choice, certificate, random number
Verify certificate,
Generate
PreMasterSecret
3. PreMasterSecret (encrypted in server’s public key)
4. Generate keys
4. Generate keys
5. HMAC over handshake messages sent
6. HMAC over handshake messages received
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #5
Network Based Attacks
Oldies and Goodies--It Isn’t Magic
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #6
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #7
Target Acquisition
?
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #8
TCP Review
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #9
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #10
TCP Assumptions
• Assumes IP addresses are valid and correct
• If sequence number received  sequence
number expected, packet is refused
(discarded); system waits for correctly
numbered packet
– If correct packet doesn’t arrive before the timer
winds down, then a retransmission occurs
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #11
TCP Handshake Overview
Server
Client
1. SYN
2. SYN, ACK
3. ACK
What if Step 3 never happens?
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #12
Getting Fingered
Aimee Girard (agirard)
Home: /usr3/agirard
Shell: /sh/tcsh
Building: Unknown
Work phone: Unknown
Directory: /usr3/agirard
Shell: /sh/tcsh
No unread mail.
Aimee Girard has never logged on.
No plan.
Home phone: Unknown
Andrew George Marut (agmarut)
Home: /usr2/agmarut
Shell: /sh/tcsh
Building: Unknown
Work phone: Unknown
Home phone: Unknown
Directory: /usr2/agmarut
Shell: /sh/tcsh
Mail forwarded to:
[email protected]
Andrew George Marut (agmarut) is not presently logged in.
Last seen on ece.wpi.edu at Tue Mar 27 03:06:03 2001
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #13
Do You Know Who?
ece(ttyp9):~> who
crcalvo
ttyp0
rcl
ttyp1
renato
ttyp2
anshul
ttyp4
pavan
ttyp5
lavanya
ttyp6
clements
ttyp7
aelliott
ttyp8
rstanley
ttyp9
bram
ttypa
gaubatz
ttypb
Summer 2003
© 2000-2003, Richard A. Stanley
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
14
20
20
20
19
20
20
20
20
20
20
17:52
07:53
08:38
09:18
04:08
08:53
09:45
10:46
12:18
10:42
10:42
EE579T_GD/5 #14
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #15
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #16
Enumeration
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #17
Finding out What’s There
• Scans
– Stealthy
– Not so stealthy
• Fingerprinting
• Tools
– Nmap
– Retina (eeye.com)
– ...etc.
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #18
Attacks
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #19
SYN Flood
• Send a normal SYN packet to a server, as if
to open a TCP connection
• When the server returns a SYN/ACK
packet, ignore it
• Send another SYN packet to the server
• Repeat as necessary
• ...until server cannot handle any more
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #20
SYN Flood
Server
Client
1. SYN
2. SYN, ACK
Repeat quickly, over and over.
After awhile, the server is able to do
nothing except wait for the pending
connections. Thus, DoS.
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #21
FINish, But Don’t Start
• Attacker sends FIN packet to server, but has
not previously established a TCP connection
• Server replies with RST packet
• Attacker now knows that port on that server
is alive and functioning
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #22
Passive Sniffing
• Hacker obtains access to network segment;
observes and analyzes traffic
– Unauthorized access to legitimate computer
(packet monitors standard NT/2000 fixture)
– Unauthorized added NIC on segment
• Purpose: gather intelligence, read traffic
• Defense:
– Secure authentication schemes (Kerberos)
– Data encryption
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #23
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #24
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #25
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #26
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #27
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #28
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #29
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #30
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #31
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #32
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #33
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 ...
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #34
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #35
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #36
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #37
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #38
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #39
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #40
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #41
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #42
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”
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #43
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #44
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #45
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #46
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #47
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!
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #48
IP 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
• What’s wrong with this picture?
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #49
IP Spoofing -2
• Simpler approach: wait until client system
shuts down and impersonate the system
– Example: Unix NFS uses IP only addresses 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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #50
Spoofing E-mail
•
•
•
•
•
•
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #51
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?
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #52
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #53
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #54
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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #55
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?
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #56
The Dreaded Cookie
# Netscape HTTP Cookie File
# http://www.netscape.com/newsref/std/cookie_spec.html
# This is a generated file! Do not edit.
home.netscape.com FALSE /
FALSE 942189161
cc98a714-14298-900987956-4
.doubleclick.net TRUE /
FALSE 1920499140
.netscape.com
TRUE /
FALSE 1293840000
24.128.181.249:0921530518:183152
www.netscape.com FALSE /
FALSE 942189161
cfc84b26-10757-921530518-1
.imgis.com TRUE /
FALSE 1078108157 JEB2
A80C29F3DBB5C25F1880B5F93004CF94
Summer 2003
© 2000-2003, Richard A. Stanley
NGUserID
id3aa44cd0
UIDC
NGUserID
EE579T_GD/5 #57
If You Don’t Like Cookies?
• Use a utility or your browser tools to
remove them (IE and Netscape 6 and later)
– Find them using the FIND function; they’re all
over the place (especially in Windows)
– But they keep coming back!
• In Windows, accept those you want, set the
C:/Windows/Cookies folder as Read Only
• In Unix, make cookies.txt zero-length R/O
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #58
How to Keep Up?
• Common Vulnerabilities and Exposures
– http://www.cve.mitre.org/
• CVE is
– A dictionary, NOT a database
– A community effort
– Freely available
• In short, this is not a “how to hack” list
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #59
What About Hacker Sites?
• Can provide an idea of the current state of
affairs, and also toolkits
• BE CAREFUL!
– What you download may come with little
“surprises”
• If you download, quarantine and test
– These sites don’t just exist to serve hackers;
some also exist to hack
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #60
Summary
• TCP/IP was not intended as a secure
protocol; as a result, it has vulnerabilities
that can be exploited
• There are many ways to get access to info
• 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
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #61
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!!
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #62
Homework - 2
3. Describe how a SMURF attack works
(don’t just parrot the description you find).
Describe how to stop it.
Summer 2003
© 2000-2003, Richard A. Stanley
EE579T_GD/5 #63