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- Intrusion Detection
The Big Picture
Stephen Northcutt
Intrusion Detection - The Big Picture - SANS GIAC © 2000 1
S. Northcutt – v1.0 – Jul 2000
Edited by J. Kolde – v1.1 – Aug 2000
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- Pagers and Cell Phones
The high rate of slide delivery means that distractions
will cause your fellow students to miss material. If
you are a “high interrupt” person, please consider
moving to the back of the room or disabling your
pagers and phones. Questions are fine anytime.
Intrusion Detection - The Big Picture - SANS GIAC © 2000 2
In this course we’ll be covering the following types of security tools and countermeasures:
• firewalls
• host-based intrusion detection
• network-based intrusion detection
• vulnerability scanners
• honeypots
We’ll also touch on incident response and discuss less technical issues of information security, such
as risk assessment and how to justify these tools to management.
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- Frequently Referred to URLs
• SANS
– www.sans.org
• NSWC CD2S web page
– www.nswc.navy.mil/ISSEC
– click on forms to get the knowledge-based
risk assessment forms for WinNT, Unix,
Win95, Mac 8.X, etc.
Intrusion Detection - The Big Picture - SANS GIAC © 2000 3
The SANS website is home to GIAC, the Global Incident Analysis Center, and to the SANS training
materials, with courses like this one available online.
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- More URLs
• SHADOW & CIDER
– www.nswc.navy.mil/ISSEC/CID
• Coast
– ftp://coast.cs.purdue.edu
• SecurityFocus
– www.securityfocus.com
• Snort
– www.snort.org
(Win32 version at
www.datanerds.net/~mike/snort.html)
Intrusion Detection - The Big Picture - SANS GIAC © 2000 4
SHADOW and CIDER are free intrusion detection system projects.
The Coast archive is Gene Spafford’s security tool archive.
SecurityFocus is home of the Bugtraq mailing list, and has a good vulnerability database and tool
archive.
Snort is currently the most popular free network intrusion detection system “as seen on GIAC”.
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- URLs Continued
• DTK Deception Toolkit
– www.all.net
• CIDF
– www.gidos.org
– www.isi.edu/gost/brian/cidf/
• Tripwire
– ftp://coast.cs.purdue.edu/pub/tools/unix/Tripwire
– www.Tripwiresecurity.com/
• SPI
– ciac.llnl.gov/cstc/
Intrusion Detection - The Big Picture - SANS GIAC © 2000 5
Fred Cohen’s DTK (Deception Toolkit) is an excellent tool kit for building honeypots.
CIDF is the Common Intrusion Detection Framework, a standards initiative by the IETF’s Intrusion
Detection working group, designed to improve IDS interoperability.
Tripwire is the de facto standard in file and registry integrity checking.
SPI does integrity checks for US government systems.
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- Even More URLs
• Vulnerability Scanners
– Saint: wwdsilx.wwdsi.com/saint/
– Nessus: www.nessus.org
– Nmap: www.insecure.org/nmap/
– Cerberus: www.cerberus-
infosec.co.uk/cis.shtml
• Phonesweep
– www.sandstorm.net
Intrusion Detection - The Big Picture - SANS GIAC © 2000 6
SAINT and NESSUS are general vulnerability scanners. Nmap does stealthy port scanning, OS
identification and too many other functions to list. CIS is a vulnerability scanner for improving the
security of Windows NT machines. They were all free last time we looked. (Editor’s note: nmap
was ported to Windows NT in July 2000 by eEye Digital Security. The Windows version can be
downloaded from http://www.eeye.com. – JEK)
Phonesweep is a ‘wardialer’ or modem-finding tool.
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- URLs URLs URLs
• NukeNabber (from Puppet’s Place)
– www.dynamsol.com/puppet/
• Legion (detect unprotected shares)
– Rhino9 has disbanded; you will need to do
a net search.
NOTE: Appendix A has a glossary
Intrusion Detection - The Big Picture - SANS GIAC © 2000 7
NukeNabber can be considered a personal host intrusion detector for stand-alone PC’s, which will
notify you of attempted connections to user-defined ports.
Legion can be quite hard to find. Most other vulnerability scanners also now look for unprotected
shares.
In the back of your materials are additional references. (Editor’s note: for students taking this
course online, the Glossary is included as a separate download file. – JEK)
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- Goal of This Course
To understand how the primary
components of intrusion detection
capability (such as vulnerability
assessments, firewalls, network- and
host- based IDS systems) work
together to provide information
assurance.
Intrusion Detection - The Big Picture - SANS GIAC © 2000 8
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- GIAC Tracks
• Information Security KickStart
• Security Essentials Certification
• Firewalls and Perimeter Protection
• Intrusion Detection In-Depth
• Advanced Incident Handling and Hacker Exploits
• Windows NT and Windows 2000 Security
• Unix Security
• Systems and Network Auditing
Intrusion Detection - The Big Picture - SANS GIAC © 2000 9
Clearly, there will be some repetition between the classes. These classes have been designed to be
very high content. There is more material than people can normally absorb in a single sitting; when
we repeat, this is done to help the student learn as much of the total material as possible.
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- Introduction
• Introductory Example - Mitnick Attack
• Is There a Business Case for Intrusion
Detection?
• What We Will Cover in This Course
Intrusion Detection - The Big Picture - SANS GIAC © 2000 10
Let’s get started then. In our introductory section, we are first going to show you a real attack, so we
can see the type of things an attacker does in the real world, and we’ll discuss how the security
components of this course could have detected or prevented it.
We’ll then take a step back and put our business hats on when we examine the question of a business
case for intrusion detection. Because the fact is, this stuff costs money and even with free tools, it
takes up valuable time. So we’ll see how to decide on it’s worth to your organization.
Finally, we’ll look at how we are going to divide up the rest of the course.
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- What better introduction to
Intrusion Detection
than the Mitnick Attack?
Intrusion Detection - The Big Picture - SANS GIAC © 2000 11
We start by examining the intrusion by possibly the world’s most infamous computer criminal, Kevin
Mitnick, on the system of Tsutomu Shimomura. This system compromise and the subsequent
successful pursuit of Mitnick have been described in several books and elsewhere, but the technical
details described come from Shimomura’s original posting on the comp.security.misc newsgroup, 25
Jan 1995.
The obvious first question is why we are bothering with an attack which is over 5 years old, when
several new attacks are discovered every week.
First, because it uses well-known techniques like SYN flooding and IP Spoof to accomplish trust
hijacking. The second, more disturbing point is that little has changed since late 1994. These attacks
still work on many systems and so are still common attacks today.
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- Two Systems, Trust
Relationship
A trusts B
A B
A is talking to B
Intrusion Detection - The Big Picture - SANS GIAC © 2000 12
A trust relationship existed between two machines, both administered by the good guy. (One was an
office machine, the other a home machine.) Administrators often set up these sort of relationships,
usually as a convenience.
In this particular example, the systems are Unix and the trust relationship is the use of “r” utilities.
But similar trust relationships exist in other systems (for example, Windows “shares”). The attacker
is going to pretend to be one side of the trust relationship using a technique called IP Spoof to appear
to be computer B and then take advantage of the trust relationship.
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- Enter the Badguy(tm)
A trusts B
A B
A is talking to B
Attacker
probes to determine
a trust relationship,
A trusts B. Attacker
Intrusion Detection - The Big Picture - SANS GIAC © 2000 13
The attack started when the attacker detected a trust relationship was in place between two systems
of interest. The trust relationship in particular was that A allows B to make rshell connections,
providing a remote shell service.
The badguy™ uses finger, showmount, rpcinfo, and so forth to ferret out the trust
relationship. It should be noted there is often a recon phase for complex attacks.
If these recon probes can be detected, they can provide a valuable early-warning function.
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- Set Up the Attack
A trusts B
A
A B
B
A is talking to B
Attacker
predicts the sequence SYN Attack to B
number A will expect renders B unable
“IP Spoof” to reply to A
Attacker
Intrusion Detection - The Big Picture - SANS GIAC © 2000 14
After the recon phase, the initial attack occurs.
He first gags B with a flood of SYN packets, a technique that involves bombarding B with TCP
connection requests until B is too busy to respond to anyone. (A SYN packet is the first part of
TCP’s three-part handshake for connection establishment, which goes SYN, SYN/ACK, ACK).
Next, he sends a connection request (SYN) to A,spoofing the source address so the packet is
apparently from B. Since A allows connections from B, it will reply with a SYN/ACK packet that
gives an initial sequence number for the connection. This reply goes to B, which would usually deny
sending it and close the connection with a RST packet, but because it’s been gagged, it can’t reply.
Since the attacker hasn’t seen the reply, he must predict the sequence number if he is to continue the
connection. Sequence number prediction code has been widely available on the Internet for a
number of years.
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- Make ‘A’ Defenseless
Content of rshell packet “+ + in /.rhosts” open A to attack
A B
SYN Attack to B
Attacker sends keeps B unable
expected to warn A
ACK with fake SRC
IP ADDRESS to Attacker
establish rshell
Intrusion Detection - The Big Picture - SANS GIAC © 2000 15
Having guessed the next sequence number, and assuming A has sent the SYN/ACK back to B, the
attacker completes the connection establishment by sending a final ACK, still with B’s source
address.
Now the attacker has a connection to A, that A believes is from it’s trusted friend B. That trust is
exploited to gain further access.
To maintain the hijacked connection and continue successfully masquerading as B, the attacker must
keep B gagged, since every reply from A goes back to B, not the attacker, and B would refute the
connection if it could.
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- Finish the Job
Content of rshell packet “+ + in /.rhosts” open A to attack
A
Attacker uses Attacker
# rlogin -l root
to takeover ‘A’
Intrusion Detection - The Big Picture - SANS GIAC © 2000 16
Now, the attacker goes in for the kill to crack open A’s security. He sends an rshell command to
add the string “++” to the file “/.rhosts”.
This string is a wild-card which says “trust as root all users on all systems”.
Once ./rhosts has been modified the attacker can stop the masquerade, and stop gagging B, and he
simply logs in directly as root.
Game Over.
Fortunately, Shimomura noticed the attack. Would you notice a similar one on your system?
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- What Common
Tools Could Have Prevented
The Attack?
Intrusion Detection - The Big Picture - SANS GIAC © 2000 17
“An ounce of prevention is worth a pound of cure.” This statement was probably coined by an
ancient incident handler.
Of the 3 parts of the security cycle “Prevention-Detection-Response”, prevention can be the most
cost-effective.
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- Network Vulnerability Scanner
AA B
Scanner Warning:
A trusts B
A has potential rshell vulnerability
Intrusion Detection - The Big Picture - SANS GIAC © 2000 18
Vulnerability scanners can probe a network or host to identify problems that, if fixed, can prevent an
attack from succeeding.
The fact that A trusts B isn’t a vulnerability in itself, but may be a violation of your organization’s
security policy.
It is the combination of that trust decision and the rshell vulnerability that allowed the attack to
succeed.
Since networks and host configurations change so often, vulnerability scans must be frequent.
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- Firewalls
Violation, the “R”
Protocols are not
allowed
Cat “+ +” > ./rhosts
Many attack attempts fail to penetrate well-configured
firewalls, especially if they have a
“deny everything not specifically allowed” policy.
Intrusion Detection - The Big Picture - SANS GIAC © 2000 19
Firewalls or filtering routers can be configured not to let “risky” services pass into the protected
network. This is normally by blocking access to the ports used by those services.
Most firewall administrators would call letting inbound connections to the “r” services through a
firewall very risky indeed. There are more secure replacements available for these services, like
SSH for remote shells.
Many firewalls would also stop the spoofed packets from the attacker, correctly noticing that packets
from machine B shouldn’t be originating from outside the firewall (assuming both A and B were
inside the firewall). Similarly, responsible egress filtering on the part of the attacker’s organization
or ISP would also have blocked the spoofed packets.
The firewall wouldn’t have protected from the attack if B had been outside the firewall, and hence
connections pretending to be from B would have been allowed through.
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- What Intrusion Detection
Techniques Could Have Detected
The Attack?
Intrusion Detection - The Big Picture - SANS GIAC © 2000 20
Detecting the attack is one thing. Most intrusion detection systems would also have detected the
recon probes before the attacker went in for the kill. Early warning is much better than real-time or
after the fact notification of system compromise.
(The problem is, a recon probe is often hard to distinguish from a legitimate query. The bad guys’
recon techniques are becoming stealthier, which is both good and bad. Harder to spot, but if you can
spot it, it’s easier to recognize as hostile, since legitimate users don’t sneak about.)
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