Transcript Lecture 6: Session Layer Security

Session Layer Security
Lecture 6
Supakorn Kungpisdan
[email protected]
NETE4630
1
Roadmap
•
•
•
•
•
•
Introduction
SYN Attack
Session Hijacking
DNS Poisoning
SSH Downgrade Attack
Authentication Techniques and Attacks
2
NETE4630
Introduction
• Session layer provides a set of features that
contributes to the reliability and usefulness of
modern network communications
–
–
–
–
Session Checkpoint
Session Adjournment
Session Termination
Half- and Full-Duplex Operations
3
NETE4630
Session Checkpoint
• TCP acknowledgement (ACK) packets are regularly
passed between hosts to identify the last packet that
was received
• TCP delays the transmission of an ACK packet until
either a timeout is reached or a number of packets equal
to the TCP window size have been sent
• This delays increases the efficiency of the protocol and
establishes checkpoints
• At any point, TCP can resume transmission from the
previous checkpoint if a delivery failure occurs
4
NETE4630
Session Adjournment
• TCP sessions may be adjourned through setting
the TCP window to 0 byte.
– This informs the sending host that no buffer is
available to hold transmitted data and halts
communications without losing the connection
5
NETE4630
Session Termination
• TCP provides a means for both graceful and immediate
session terminations
• Graceful termination occur by setting a finish (FIN) flag
that is subsequently acknowledged by the recipient
• Immediate termination occur by using packets with the
reset (RST) flag set
Half- and Full-Duplex Operations
• While TCP operates at full duplex, the session layer
allows for both full- and half-duplex operations
6
NETE4630
Attacking the Session Layer
• Rely primarily on abuses of the TCP and IP headers
• Several behavior designed into the TCP specification
allow a wide variety of attacks
• In particular, TCP flags and Sequence and
Acknowledgement numbers enable several methods of
attack
• Newer attacks may focus on higher layer protocol like
Session Description Protocol (SDP) and Session
Initiation Protocol (SIP)
7
NETE4630
SYN (Flood) Attack
• Use legitimate TCP functions permits attackers with a
small number of hosts to conduct DoS, which can
completely saturate the bandwidth of a corporation
• In TCP three-way handshake, a new source port is
selected on the client host for each new connection that
is opened to a particular port on a server
• The server has to allocate a number of resources to
handle each connection
• A large number of hosts can use this to great effect
when attacking a web site
8
NETE4630
SYN Attack (cont.)
• From an attacker’s perspective, this approach is
less than ideal:
1. Creating multiple connections is extremely
inefficient
• Every established connection consumes a lot of resources
on the server and the attacking client
2. This kind of attack is not anonymous
3. Many servers limit the number of connections that
they will accept from a single host
9
NETE4630
Performing SYN (Flood) Attack
• Our goal is to consume resources on the victim
server but not on the DoS client
• We want to avoid using any system calls to open
network connections
10
NETE4630
SYN Attack with hping3
• Hping2 tool provides a simple means for producing
crafted packets
• Executing a single SYN packet to port 6666 on the victim
Packet
server
SYN flag set
count
Dest port
$ hping3 –c 1 –p 6666 –S 10.10.1.9
• In this case, we use the attacking machine’s IP as
source IP
11
NETE4630
SYN Attack with hping2 (cont.)
12
NETE4630
SYN Attack with hping2 (cont.)
• However, the DoS client was stymied by attempts to
circumvent its resource consumption
• Any TCP stack that meets an unsolicited SYN/ACK
packet will respond with an RST
• The solution is to spoof a source IP address
$ hping –c 1 –a 10.12.250.250 –p 6666 –S 10.1.1.9
Spoofed IP address
13
NETE4630
SYN Attack with hping2 (cont.)
Target keeps sending SYN/ACK to the spoofed source
until reaching timeout
14
NETE4630
SYN Attack with hping2 (cont.)
• The victim server attempts to reply to the non-existent
host with SYN/ACK
• TCP tries to ensure reliable delivery and will continue to
complete the handshake until timeout
• The DoS client can now produce packets as fast as it
can spoof them, while at the same time the victim server
attempts to complete handshakes in vain
15
NETE4630
Note on SYN Attack
• Careful selection of the spoofed IP is necessary to
conduct a successful DoS attack
• The most successful method to ensure delivery of a
spoofed packet is to select an unused IP on the same
subnet as the attacking host
16
NETE4630
Reflective Attack
• A variation of SYN attack
• Launched by sending a large number of SYN packets to
a web server but alters the source address so that it is to
match the address of the victim
• The web server responds to the large number of SYN
packets by issuing a flood of traffic back to the spoofed
victim’s address
17
NETE4630
Session Hijacking
• Session hijacking works by taking advantage of the fact
that most communications are protected (by providing
credentials) at session setup, but not thereafter.
• These attacks generally fall into three categories:
– Man-in-the-middle (MITM)
– Blind Hijacking
– Session Theft
Ref: http://technet.microsoft.com/en-us/magazine/cc160809(TechNet.10).aspx
18
NETE4630
MITM Attacks
• Attacker intercepts all communications between two
hosts.
• With communications between a client and server now
flowing through the attacker, he or she is free to modify
their content.
• Protocols that rely on the exchange of public keys to
protect communications are often the target of these
types of attacks
19
NETE4630
Blind Hijacking
• An attacker injects data such as malicious commands into
intercepted communications between two hosts commands
like "net.exe localgroup administrators /add
EvilAttacker".
• This is called Blind Hijacking because the attacker can only
inject data into the communications stream, but cannot see
the response to that data (such as "The command completed
successfully.")
• Essentially, the blind hijack attacker is shooting data in the
dark, but this method is still very effective
20
NETE4630
Session Theft Attacks
• Attacker neither intercepts nor injects data into existing
communications between two hosts.
• Instead, the attacker creates new sessions or uses old
ones.
• This type of session hijacking is most common at the
application level, especially Web applications.
21
NETE4630
Hijacking A TCP Session
Session
establishment
Data transfer
22
NETE4630
Hijacking A TCP Session (cont.)
• If the attacker wanted to inject data into the TCP session
as the client, he or she would need to:
– Spoof the client's IP address
– Determine the correct sequence number that is expected by the
server from the client
– Inject data into the session before the client sends its next
packet
• To achieve the third, the attacker could just send the
data to inject and hope it is received before the real
client does
• Or, the attacker could perform a DoS attack on the
client, or use ARP spoofing
23
NETE4630
Blind Injection
When the client receives the ACK packet, it will be
confused, either because it did not send any data or
because the next expected sequence is incorrect.
24
NETE4630
Hijacking A TCP Session (cont.)
• Maybe the attacker can send something "nice" like "mv
`which emacs` /vmunix && shutdown –r now" and not
just a single character)
• This confusion can cause a TCP ACK storm, which can
disrupt a network
• Attackers can automate the session hijacking process
with tools such as Juggernaut, Hunt, and Ettercap
25
NETE4630
Hijacking A UDP Session
• Attackers do not have to worry about the overhead of
managing sequence numbers and other TCP mechanisms.
• Since UDP is connectionless, injecting data into a session
without being detected is extremely easy
DNS queries, online
games like the Quake
series and Half-Life, and
peer-to-peer sessions
are common protocols
that work over UDP; all
are popular targets for
this kind of session
hijacking
26
NETE4630
Determining Susceptibility
• One way to check if your network is vulnerable to
session hijacking is to hijack actual network sessions
using common attacker tools e.g. Juggernaut or Hunt
(now Ettercap)
• Alternatively, try to find out if using transport protocols
that do not use cryptographic protection
• Protocols such as Telnet and FTP are extremely
susceptible to hijacking when not protected inside
encrypted tunnels
• Countermeasure is to use SSL, SSH, and IPSec
27
NETE4630
Tricks and Techniques
•
•
•
•
TCP ACK Storm
ARP Table Modification
TCP Resynchronizing
Remotely Modifying Routing Table
28
NETE4630
TCP ACK Packet Storm
As the attacker
injects more and
more data, the size
of the ACK storm
increases and can
quickly degrade
network
performance.
If neither the attacker nor the client explicitly closes the
session, the storm will likely stop itself eventually when ACK
packets are lost in the storm.
29
NETE4630
ARP Table Modification
Finding owner of MAC address
Spoofed reply
30
NETE4630
ARP Table Modification (cont.)
Stopping TCP ACK Storm
31
NETE4630
TCP Resynchronizing
• To hide his/her tracks, an attacker who is finished
session hijacking might want to resynchronize the
communicating hosts.
• The problem is that, after the attack, the two hosts
whose session was hijacked will be at different points in
the session.
• In other words, each host will be expecting different
sequence numbers.
• For example, server might think that it is 40 bytes into
the session when the client might have sent only 29
bytes.
32
NETE4630
TCP Resynchronizing (cont.)
• Since sequence numbers move in only a positive
direction, it's not possible to manipulate the server so
that its expected sequence number moves downward
to match the client's sequence number.
• Tools like Hunt try to solve this problem by sending a
message to the client
msg from root: power failure – try to type 13 chars
33
NETE4630
Remotely Modifying Routing Table
• Attacker who wants to hijack a session wants to route all
communications between a client and server through him or
her making it easy to monitor, modify, and inject data into the
session, as in MITM attacks.
• Attacker modifies the routing table of the host is to forge
ICMP Redirect (type 5) packets and advertise them as the
route to take when sending data.
• To protect Windows® hosts from forged ICMP redirect, set
the EnableICMPRedirect value to 0 under the registry key
HKLM\System\CurrentControlSet\Services\AFD\Parameters
34
NETE4630
DNS Poisoning
• A more common example of session hijacking is DNS
poisoning
• DNS poisoning allows you to convince a DNS server that
a hostname resolves to an arbitrary IP
35
NETE4630
DNS Resolution
34
56
1
2
NETE4630
Client does not query
the canonical
nameserver because of
the efficiency provided
by caching at the local
nameserver
36
DNS Poisoning (cont.)
34
Attacker’s
nameserver
6
5
7
Spoofed
web server
1
2
NETE4630
37
DNS Poisoning (cont.)
• Implementing DNS poisoning is difficult
• Each DNS query contains a 2-byte identification field
that allows responses to be matched to queries
• An attacker has a 1 in 65,536 (2^16) chance of guessing
the correct identification value
• Normally an attacker needs to sniff the identification
number of the query in order to successfully spoof a
response
38
NETE4630
DNS Poisoning with Ettercap
1
3
2
39
NETE4630
DNS Poisoning with Ettercap (cont.)
4
5
40
NETE4630
DNS Poisoning with Ettercap (cont.)
6
8
7
41
NETE4630
DNS Poisoning with Ettercap (cont.)
Ettercap.dns
9
10
42
NETE4630
SSL Spoofing with Ettercap
43
NETE4630
SSH Downgrade Attack
• SSH is the most famous example of a downgrade attack
where the attacker forces the client and the server to use
the insecure SSH1 protocol.
• The client sends a request to establish a SSH link to the
server and asks it for the version it supports
• The server answers either with:
– ssh-2.xx The server supports only SSH2
– ssh-1.99 The server supports SSH1 and SSH2
– ssh-1.51 The server supports only SSH1
• This attack occurs at the server that supports both SSH1
and SSH2
Ref: http://openmaniak.com/ettercap_filter.php
44
NETE4630
SSH Downgrade Attack (cont.)
45
NETE4630
SSH Downgrade Attack (cont.)
46
NETE4630
SSH Downgrade Attack with ettercap
1. Configure SSH server to support SSH1 and SSH2
#apt-get install openssh-server
#vim /etc/ssh/sshd_config
– Protocol 1, 2
2. Create a SSH1 key pair
#ssh-keygen –t rsa1 –f /etc/ssh/ssh_host_key –N “”
3. Add the key path into sshd_config file:
HostKey /etc/ssh/ssh_host_key
4. Try to telnet to server to check if it has SSH1
Trying server_ip_address...
Connected to server_ip_address.
Escape character is '^]'.
SSH-1.99-OpenSSH_4.6p1 Debian-5ubuntu0.1
47
NETE4630
Client’s PuTTY Screen
Version 2 is preferred
but not restricted
48
NETE4630
Ettercap Filter
49
NETE4630
SSH Downgrade Attack Filter
/usr/share/ettercap/ettercap.filter.ssh
50
NETE4630
Compiling the Filter
51
NETE4630
Loading the Compiled Filter
52
NETE4630
SSH Downgrade Attack Result
53
NETE4630
Avoiding SSH Downgrade Attack
• Never use SSH1 on both server and client
– At /etc/ssh/sshd_config file
Protocol 2
– #telnet server_ip_address 22
Trying server_ip_address…
Connected to server_ip_Address.
Escape character is ‘^’.
SSH-2.0-OpenSSH_4.6p1 Debian-5ubuntu0.1
54
NETE4630
Avoiding SSH Downgrade Attack (cont.)
SSH Client
55
NETE4630
Authentication
• Two main categories of authentication:
– Synchronous and asynchronous authentication
protocols
• Synchronous authentication protocols provide
credentials at the start of the authentication
process
• Asynchronous authentication involves a
challenge-response model
56
NETE4630
Password Authentication Protocol
• Password Authentication Protocol (PAP) is one of the
least secure authentication protocol
• Password and username are sent in cleartext to the
authentication server after a connection has been
established.
• Some systems revert to PAP if they cannot agree on any
other authentication protocol.
• Both entities will try to negotiate and agree upon the
most secure method of authentication
– Start with EAP, CHAP, then PAP
57
NETE4630
Challenge Handshake Authentication Protocol
Enter password
Compare hash value
h(password, challenge)
58
NETE4630
NT Lan Manager v1 (NTLMv1)
Enter password
Compare hash value
R1, R2
59
NETE4630
NTLMv1 (cont.)
• User password and challenge are used to calculate
LANMAN hash and MD4 hash
–
–
–
–
–
–
–
C = 8-byte random challenge
Hash1 = MD4(password)
{K1, K2, K3} = {Hash1, 5-byte-0s}
R1 = DES(K1, C), DES(K2, C), DES(K3, C)
Hash2 = LM-hash(password)
{K4, K5, K6} = {Hash2, 5-byte-0s}
R2 = DES(K4, C), DES(K5, C), DES(K6, C)
• Client sends {R1, R2} as a response to the server
60
NETE4630
LM challenge/response
uppercase(password[1..7])
as KEY
magic word
DES
LM_hash[1..8]
uppercase(password[8..14])
as KEY
magic word
0000000000
DES
LM_hash[9..16]
LM_hash[17..21]
magic word is “KGS!@#$%”
[email protected], Cracking NTMLv2 Authentication
Cracking NTLMv2 Authentication
LM challenge/response (cont.)
LM_hash[1..7]
as KEY
challenge code
DES
LM_response[1..8]
LM_hash[8..14]
as KEY
challenge code
DES
LM_hash[15..21]
LM_response[9..16]
0000000000
as KEY
challenge code
DES
LM_response[17..24]
[email protected], Cracking NTMLv2 Authentication
Cracking NTLMv2 Authentication
NTLM 2 Authentication
unicode(password)
MD4
unicode(
uppercase(account name)
+domain_or_hostname)
as KEY
HMAC_MD5
as KEY
server_challenge
+client_challenge
HMAC_MD5
NTLMv2
Response
[email protected], Cracking NTMLv2 Authentication
Cracking NTLMv2 Authentication
LM, NTLMv1, NTLMv2
LM
NTLMv1
NTLMv2
Password case sensitive
No
Yes
Yes
Hash key length
56bit + 56bit
-
-
Password hash algorithm DES (ECB mode)
MD4
MD4
Hash value length
64bit + 64bit
128bit
128bit
C/R key length
56bit + 56bit + 16bit
56bit + 56bit + 16bit
128bit
C/R algorithm
DES (ECB mode)
DES (ECB mode)
HMAC_MD5
C/R value length
64bit + 64bit + 64bit
64bit + 64bit + 64bit
128bit
[email protected], Cracking NTMLv2 Authentication
64
NETE4630
Authenticating with Kerberos
• Default authentication mechanism used by Windows
2000, XP, and 2003 hosts when part of an active
directory
• Strong protocol, relying on a central server (normally
Active Directory Controller) to grant access privileges to
systems
• The main weakness of Kerberos is that all authentication
tokens have a lifespan
• Any network using Kerberos must synchronize clocks on
all systems using a protocol e.g. Network Time Protocol
(NTP)
65
NETE4630
Attacks Against Password Hashes
• Brute Force Attack
– Iterate through every possible input and hashes it, comparing
the output with the hash value
– Guaranteed to crack the hash if run long enough
• Dictionary Attack
– Iterate through possible passwords and common substitutions of
these words
– Not guaranteed to produce results
– E.g. John The Ripper (or John The Ripper Pro for Commercial
version) available at http://www.openwall.com/john/
66
NETE4630
Attacks Against Password Hashes (cont.)
• Rainbow Table Attack
– Compute every hash ahead of time, allowing the attacker to
check his/her database of hashes just for one he/she is trying to
crack
– Several tools can be used for password cracking including
windows password in SAM, LM, NTLM password hashes
• Rainbow Crack, Ophcrack, John the Ripper, Cain and Abel
– Rainbow Crack can be used to crack LM, MD5, Office hashes
67
NETE4630
Rainbow Table
Hash
function
Reduction
function
Hash
function
Reduction
function
Hash
function
Reduction
function
Rainbow table stores only the first and the last value of each chain
68
NETE4630
Rainbow Table (cont.)
69
NETE4630
Cracking LM Password with Rainbow Crack
1. Dump password hashes using samdump, pwdump,
fgdump
2. Install Rainbow Crack
3. Before cracking the password, generate the rainbow
table first:
–
LM Configuration#0 – #6
4. Sort the rainbow table using rtsort command
5. Crack the password using rcrack command
Ref: http://www.ethicalhacker.net/content/view/94/24/
70
NETE4630
Password Hashes from pwdump
testuser1:"":0F20048EFC645D0A179B4D5D6690BDF3:1120ACB74670C7DD46F
1D3F5038A5CE8:::
remote:"":E52CAC67419A9A224A3B108F3FA6CB6D:8846F7EAEE8FB117AD06BD
D830B7586C:::
joeuser:"":E52CAC67419A9A224A3B108F3FA6CB6D:8846F7EAEE8FB117AD06B
DD830B7586C:::
averageguy:"":299CCF964D9A359BAAD3B435B51404EE:A5C07214487C87B584
E8877DE72DCA0B:::
harderpass:"":B75838F7A57EE67993E28745B8BF4BA6:EC50F8A8149C93EF45
AECB8AF96658E6:::
demouser:"":261A6631FE44BA4993E28745B8BF4BA6:371D5760453C1B000BCC
016F8E23A83C:::
randy:"":98B5AFEB67293D6AAAD3B435B51404EE:A9F34664151F6360757B316
44F37E025:::
Asmith:"":E165F0192EF85EBBAAD3B435B51404EE:E4EBE0E7EF708DC9FD2401
35D3D43D89:::
71
NETE4630
Generating Rainbow Table
• To generate other configuration, use rtgen command
#
#
#
#
#
rtgen
rtgen
rtgen
rtgen
rtgen
lm
lm
lm
lm
lm
alpha-numeric
alpha-numeric
alpha-numeric
alpha-numeric
alpha-numeric
1
1
1
1
1
7
7
7
7
7
0
1
2
3
4
2400
2400
2400
2400
2400
40000000
40000000
40000000
40000000
40000000
all
all
all
all
all
72
NETE4630
Generating Rainbow Table (cont.)
• “1” and “7” are our plaintext ranges. So we want passwords
from “A” to “ZZZZZZZ.”
– If we had put plaintext length range "4-6", "AAAA" and "ZZZZZZ" would
be among the key space
• 0, 1, 2, 3, 4 are table numbers
• 2400 is chain length. Chain length increases the success rate
per table but does not increase table size.
– It computes more hashes per chain but also takes longer to create and
search the tab
• “8000000” is chain count of each rainbow table.
– Chain count is simply how many chains you want per table. Increasing
this value produces larger files with higher success rates, but the overall
computation time isn’t affected.
73
NETE4630
Rainbow Table Configuration
74
NETE4630
Generating Rainbow Table with Winrtgen
• Winrtgen (now in Cain and Abel) is a graphical Rainbow
Tables Generator that supports LM, FastLM, NTLM,
LMCHALL, HalfLMCHALL, NTLMCHALL, MSCACHE,
MD2, MD4, MD5, SHA1, RIPEMD160, MySQL323,
MySQLSHA1, CiscoPIX, ORACLE, SHA-2 (256), SHA-2
(384) and SHA-2 (512) hashes.
• Winrtgen can generate only rainbow table for LM
configuration#0
75
NETE4630
Generating Rainbow Table with Winrtgen
(cont.)
Generating 1 configuration#0 table takes 2 days on P3 1GHz machine
76
NETE4630
Generating Rainbow Table with Winrtgen
(cont.)
Generating 5 configuration#0 tables take 12 days on P3 1GHz machine
77
NETE4630
Generating Rainbow Table (cont.)
•
•
•
•
•
128,000,000
128,000,000
128,000,000
128,000,000
128,000,000
bytes
bytes
bytes
bytes
bytes
lm_alpha#1-7_0_2100x8000000_all.rt
lm_alpha#1-7_1_2100x8000000_all.rt
lm_alpha#1-7_2_2100x8000000_all.rt
lm_alpha#1-7_3_2100x8000000_all.rt
lm_alpha#1-7_4_2100x8000000_all.rt
• Sort the rainbow table:
#
#
#
#
#
rtsort
rtsort
rtsort
rtsort
rtsort
lm_alpha#1-7_0_2100x8000000_all.rt
lm_alpha#1-7_1_2100x8000000_all.rt
lm_alpha#1-7_2_2100x8000000_all.rt
lm_alpha#1-7_3_2100x8000000_all.rt
lm_alpha#1-7_4_2100x8000000_all.rt
78
NETE4630
Cracking the Password
C:\rainbowcrack-1.2-win\rainbowcrack-1.2-win>rcrack
RainbowCrack 1.2 - Making a Faster Cryptanalytic Time-Memory Trade-Off
by Zhu Shuanglei < [email protected] This e-mail address is being
protected from spam bots, you need JavaScript enabled to view it >
http://www.antsight.com/zsl/rainbowcrack/
usage: rcrack rainbow_table_pathname -h hash
rcrack rainbow_table_pathname -l hash_list_file
rcrack rainbow_table_pathname -f pwdump_file
rainbow_table_pathname: pathname of the rainbow table(s), wildchar(*, ?)
supported
-h hash:
use raw hash as input
-l hash_list_file:
use hash list file as input, each hash in a line
-f pwdump_file:
use pwdump file as input, this will handle LAN
Manager hash only
example: rcrack *.rt -h 5d41402abc4b2a76b9719d911017c592
rcrack *.rt -l hash.txt
rcrack *.rt -f hash.txt
79
NETE4630
Cracking the Password (cont.)
• rcrack c:\rainbowcrack\*.rt -f pwdumpfile.txt
• rcrack c:\rainbowcrack\*.rt -l justhashlist.txt
• rcrack c:\rainbowcrack\*.rt –h
213D466DB5B288F0F82E44EC0938F4F4
• Where pwdumpfile.txt is the results of using a hash dumping
utility like pwdump2, pwdump3, samdump, etc to dump the LAN
Manager's passwords.
• If your password consists of only letters only, rcrack should be able
to crack it with a success rate of 99.9%.
80
NETE4630
Cracking the Password (cont.)
81
NETE4630
Cracking the Password (cont.)
• 26 of our 41 hashes found in about 12 minutes.
• Also notice that the hash for the password “password” is
the same because there is no salting with the LAN
Manager hashing algorithm.
82
NETE4630
Protecting yourself against RainbowCrack
attacks and other password attacks
•
•
•
•
•
•
•
•
Limiting physical access
Continue to force the use of special characters
Use ALT-XXX characters in your passwords
Keep up with updates
Use Pass phrases
Use Multi-factor authentication
Password Policy
Use NTLM or NTLMv2
83
NETE4630
Limiting Physical Access
• One common attack if you have physical access to a machine is to
use a bootable Linux distro to simply boot into Linux and grab the
SAM file off the windows partition.
• IronGeek wrote a good tutorial on this method and even has a video
you can watch. You can get it here:
http://www.irongeek.com/i.php?page=security/localsamcrack2.
• Another interesting tool released by Eeye is SysRQ2:
http://research.eeye.com/html/tools/RT20060801-8.html.
“SysRq is a bootable CD image that allows a user to open a fully
privileged (SYSTEM) command prompt on Windows 2000, Windows
XP, and Windows Server 2003 systems by pressing
Ctrl+Shift+SysRq at any time after startup.”
84
NETE4630
Continue to force the use of special
characters
• Rainbow tables can rip thru a LM password with any
type of special character it still takes a large amount of
time (1-2 years) to generate them
• In LC4 we go from 9-11 hours to brute force alphanumeric password to 91 days to brute force passwords
with the possibility of all special characters (not including
ALT-XXX passwords).
85
NETE4630
Use ALT-XXX characters in your passwords
• ALT characters are produced by holding down the ALT
key and pressing a three or four digit number sequence
on your keypad.
• Most password crackers cannot crack passwords with
ALT characters.
• Most ALT characters also have the added benefit that
passwords that have ALT characters in them cannot be
stored as LM hashes.
• It causes password hashes to disappear
86
NETE4630
ALT-XXX Characters
87
NETE4630
Use Passphrases
• Easiest and simplest way to protect you network from password
cracking.
• Use of pass phrases that are greater than 14 characters AND use
special characters you can protect yourself from all but the
determined attackers.
• If your network is Windows 2000 and above you have a maximum
length of 127 characters on your password/pass phrase; so sky’s
the limit.
• A pass phrase like “This is my Stupid Pass Phrase!” is long enough
to be stored as NTLM or NTLMv2, has Uppercase, Lowercase,
Spaces, and Special Characters, and is easy to remember.
• This is a much more secure password than even
“@w3cjd$Beu=mDr”.
88
NETE4630
Question?
Next week
Presentation Layer Security
NETE4630
89
Password Cracking with Rainbow Table
• Rainbow tables are sophisticated tables that utilize
reduction algorithms to reduce the time needed to crack
a password
• The only information stored in the table is an initial value
and a final value
• When the initial value is passed through the algorithm, it
produces a hash, when put through a reduction
algorithm. The result of the reduction is then used as
plaintext for another hash function
90
NETE4630
Password Cracking with Rainbow Table
(cont.)
1. Given a hash value, apply reduction function to it and
query the table for the result
2. If not matched in the current chain, proceed with
another chain, then another table, until a match is
found
3. When you find a match, recover the initial value for that
chain and reconstruct the chain until you are the point
where the match occurred
4. When you reach this point, the initial value is the
corresponding password
91
NETE4630