Chapter 10
Network
Security
Networking
in the
Internet Age
by Alan Dennis
1
Copyright © 2002 John Wiley & Sons, Inc.
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2
Chapter 10. Learning Objectives
• Be familiar with the major threats to network
security
• Be familiar with how to conduct a risk
assessment
• Understand how to prevent, detect, and correct
disruptions, destruction, and disaster
• Understand how to prevent, detect, and correct
unauthorized access
3
Chapter 10. Outline
• Introduction
– Why networks need security
– Types of Security Threats
– Network Controls
• Risk Assessment
– Develop a Control Spreadsheet
– Identify and Document the Controls
– Evaluate the Network’s Security
• Controlling Disruption, Destruction and Disaster
– Preventing Disruption, Destruction and Disaster
– Detecting Disruption, Destruction and Disaster
– Correcting Disruption, Destruction and Disaster
• Controlling Unauthorized Access
– Preventing Unauthorized Access
– Detecting Unauthorized Access
– Correcting Unauthorized Access
4
Introduction
5
Introduction
• Security is a major networking concern. 90% of
the respondents to the 2000 Computer Security
Institute/FBI Computer Crime and Security
Survey reported security breaches in the last 12
months.
• Information Week estimates the annual cost of
security losses worldwide at $1.6 trillion.
• It means more than preventing a hacker from
breaking into your computer, it also includes being
able to recover from temporary service problems,
or from natural disasters (Figure 10-1).
6
Figure 10-1 Threats to Network Security
7
Types of Security Threats
• Disruptions are the loss or reduction in network
service.
• Some disruptions may also be caused by or result
in the destruction of data.
• Natural (or manmade) disasters may occur that
destroy host computers or large sections of the
network.
• Unauthorized access is often viewed as hackers
gaining access to organizational data files and
resources. However, most unauthorized access
incidents involve employees.
8
Security Problems Are Growing
• The Computer Emergency Response Team
(CERT) at Carnegie Mellon University was
established with USDoD support in 1988 after a
computer virus shut down 10% of the computers
on the Internet (Figure 10-2).
• In 1989, CERT responded to 137 incidents.
• In 2000, CERT responded to 21,756 incidents.
• By this count, security incidents are growing at a
rate of 100% per year.
• Breaking into a computer in the U.S. is now a
federal crime.
9
Figure 10-2 Number of Incidents Reported to CERT
Source: CERT Statistics, www.cert.org/stats/cert_stats.html
10
Network Controls
• Developing a secure network means developing
mechanisms that reduce or eliminate the threats to
network security, called controls.
• There are three types of controls:
– Preventative controls - mitigate or stop a person from
acting or an event from occurring (e.g. passwords).
– Detective controls - reveal or discover unwanted events
(e.g., auditing software).
– Corrective controls - rectify an unwanted event or a
trespass (e.g., reinitiating a network circuit).
11
Network Controls
• It is not enough to just establish a series of
controls; personnel need to be designated as
responsible for network control and security.
• This includes developing controls, ensuring that
they are operating effectively, and updating or
replacing controls.
• Controls must also be periodically reviewed to:
– ensure that the control is still present
(verification)
– determine if the control is working as specified
(testing)
12
Risk Assessment
13
Risk Assessment
• Risk assessment is the process of making a
network more secure, by comparing each security
threat with the control designed to reduce it.
• One way to do this is by developing a control
spreadsheet (Figure 10-3).
• Network assets are listed down the side.
• Threats are listed across the top of the spreadsheet.
• The cells of the spreadsheet list the controls that
are currently in use to address each threat.
14
Threats
Assets (with Priority)
Disruption, Destruction, Disaster
Fire Flood Power Circuit Virus
Loss Failure
Unauthorized Access
External Internal Eavesdrop
Intruder Intruder
(92) Mail Server
(90) Web Server
(90) DNS Server
(50) Computers on 6th floor
(50) 6th floor LAN circuits
(80) Building A Backbone
(70) Router in Building A
(30) Network Software
(100) Client Database
(100) Financial Database
(70) Network Technical staff
Figure 10-3 Sample control spreadsheet with some assets and threats
15
Network Assets (Figure 10-4)
• Network assets are the network components
including hardware, software and data files.
• The value of an asset is not simply its replacement
cost, it also includes personnel time to replace the
asset along with lost revenue due to the absence of
the asset.
• For example, lost sales because a web server is
down.
• Mission critical applications are also important
assets. These are programs on an information
system critical to business operations.
16
Hardware
 Servers, such as mail servers, web servers, DNS servers, DHCP servers, and LAN file
servers
 Client computers
 Devices such as hubs, switches, and routers
Circuits
 Locally operated circuits such LANs and backbones
 Contracted circuits such as MAN and WAN circuits
 Internet access circuits
Network Software


Server operating systems and system settings
Applications software such as mail server and web server software
Client Software


Operating systems and system settings
Application software such as word processors
Organizational
Data

Databases with organizational records
Mission critical
applications
 For example, for an Internet bank, the Web site is mission critical
Figure 10-4 Types of Assets
17
Security Threats
• A network security threat is any potentially
adverse occurrence that can harm or interrupt the
systems using the network, or cause a monetary
loss to an organization.
• Once the threats are identified they are then
ranked according to their occurrence.
• Figure 10-5 summarizes the most common threats
to security.
• For example, the average cost to clean up a virus
that slips through a security system and infects an
average number of computers is $150,000/virus.
18
Figure 10-5 Common Security Threats
19
Identifying and Documenting Controls
• Once the specific network threats and controls
have been identified, you can begin working
on the network controls.
• Each network component should be
considered along with the specific threats to it.
• Controls to address those threats are then
listed in terms of how each control will
prevent, detect and/or correct that threat.
20
Threats
(92) Mail Server
Disruption, Destruction, Disaster
Fire Flood Power Circuit Virus
Loss Failure
1,2 1,3
4
5, 6
7, 8
Unauthorized Access
External Internal Eavesdrop
Intruder Intruder
9, 10, 11 9, 10
(90) Web Server
1,2 1,3
4
5, 6
7, 8
9, 10, 11
9, 10
(90) DNS Server
1,2 1,3
4
5, 6
7, 8
9, 10, 11
9, 10
(50) Computers on 6th floor
1,2 1,3
7, 8
10, 11
10
(50) 6th floor LAN circuits
1,2 1,3
(80) Building A Backbone
1,2 1,3
(70) Router in Building A
1,2 1,3
9
9
Assets (w/ priority)
6
(30) Network Software
7, 8
9, 10, 11
9, 10
(100) Client Database
7, 8
9, 10, 11
9, 10
(100) Financial Database
7, 8
9, 10, 11
9, 10
(70) Network Technical staff
1
1
Figure 10-6 Sample control spreadsheet
listing assets, threats, and controls
21
Figure 10-6 (cont.) Sample control
spreadsheet list of controls
Controls
1. Disaster Recovery Plan
2. Halon fire system in server room. Sprinklers in rest of building
3. Not on or below ground level
4. Uninterruptible Power Supply (UPS) on all major network servers
5. Contract guarantees from inter-exchange carriers
6. Extra backbone fiber cable laid in different conduits
7. Virus checking software present on the network
8. Extensive user training on viruses and reminders in monthly newsletter
9. Strong password software
10. Extensive user training on password security and reminders in
monthly newsletter
11. Application Layer firewall
22
Evaluate the Network’s Security
• The last step in designing a control spreadsheet is
evaluating the adequacy of the controls and the
degree of risk associated with each threat.
• Based on this, priorities can be decided on for
dealing with threats to network security.
• The assessment can be done by the network
manager, but it is better done by a team of experts
chosen for their in-depth knowledge about the
network and environment being reviewed.
23
Controlling Disruption,
Destruction and Disaster
24
Preventing Disruption, Destruction
and Disaster
• Preventing disruptions, destructions and
disasters mean addressing a variety of
threats including:
–
–
–
–
–
Creating network redundancy
“Preventing” natural disasters
Preventing theft
Preventing computer virus attacks
Preventing denial-of-service attacks
25
Network Redundancy
• The key to in preventing or reducing disruption,
destruction and disaster - is redundancy.
• Examples of components that provide redundancy
include:
–
–
–
–
Uninterruptible power supplies (UPS)
Fault-tolerant servers
Disk mirroring
Disk duplexing
• Redundancy can be built into other network
components as well.
26
Preventing Natural Disasters
• Disasters are different from disruptions since the
entire site can be destroyed.
• The best solution is to have a completely
redundant network that duplicates every network
component, but in a different location.
• Generally speaking, preventing disasters is
difficult. The most fundamental principle is to
decentralize the network resources.
• Other steps depend on the type of disaster to be
prevented.
27
Preventing Theft
• Equipment theft can also be a problem if
precautions against it are not taken.
• Industry sources indicate that about $1
billion is lost each year to theft of
computers and related equipment.
• For this reason, security plans should
include an evaluation of ways to prevent
equipment theft.
28
Preventing Computer Viruses
• Special attention must be paid to preventing
viruses that attach themselves to other programs
and spread when the programs are executed.
• Macroviruses attach themselves to documents and
become active when the files are opened are also
common. Anti-virus software packages are
available to check disks and files to ensure that
they are virus-free.
• Incoming e-mail messages are the most common
source of viruses. Attachments to incoming e-mail
should be routinely checked for viruses.
• The use of filtering programs that ‘clean’
incoming e-mail is also becoming common.
29
Worms
• A worm is a special type of virus that
spreads itself without human intervention.
• Most viruses attach themselves to other
programs but a worm copies itself from
computer to computer.
• Worms spread when the install themselves
on a computer and then send copies to other
computer, such as by e-mail or by using a
security hole in the target computer’s
operating system.
30
Detecting Disruption, Destruction & Disaster
• One function of network monitoring software is to
alert network managers to problems so that these
can be corrected.
• Detecting minor disruptions can be more difficult.
• The network should also routinely log fault
information to enable network managers to
recognize minor service problems.
• In addition, there should be a clear procedure by
which network users can report problems.
31
Disaster Recovery Plans (DRP)
• The goal of the disaster recovery plan (DRP) is to
plan responses to possible disasters, providing for
partial or complete recovery of all data,
application software, network components, and
physical facilities.
• Critical to the DRP are backup and recovery
controls that enable an organization to recover its
data and restart its application software should
some part of the network fail.
• The DRP should also address what to do in a
variety of situations, such as, if the main database
is destroyed or if the data center is destroyed.
32
Elements of a Disaster Recovery Plan
(see Figure 10-7)
•
•
•
•
•
•
•
•
•
Names of responsible individuals
Staff assignments and responsibilities
List of priorities of “fix-firsts”
Location of alternative facilities.
Recovery procedures for data communications
facilities, servers and application systems.
Actions to be taken under various contingencies.
Manual processes.
Updating and Testing procedures.
Safe storage of data, software and the disaster
recovery plan itself.
33
Two-Level Disaster Recovery Plans
• Most large organizations have a two-level disaster
recovery plan.
• Level 1: When they build networks they build
enough capacity and have enough spare equipment
to recover from a minor disaster, such as loss of a
major server or portion of the network.
• Level 2: most large organizations rely on
professional disaster recovery firms to provide
second level support for major disasters.
34
Disaster Recovery Firms
• Many large organizations outsource their disaster
recovery efforts to disaster recovery firms.
• Disaster recovery firms offer a range of services
from secure storage for backups, to a complete
networked data center that clients can use should
their network be destroyed by some disaster.
• Full services are not cheap, but may be
worthwhile when millions of dollars of lost
revenue may be at stake.
35
Controlling Unauthorized Access
36
Preventing Intruder Access
• Four types of intruders attempt to gain
unauthorized access to computer networks.
1. Casual hackers who only have limited knowledge
of computer security.
2. Security experts whose motivation is the thrill of
the hunt.
3. Professional hackers who break into corporate or
government computers for specific purposes.
4. Organization employees who have legitimate
access to the network but who gain access to
information they are not authorized to use.
37
Preventing Unauthorized Access
• A proactive approach that includes routinely
testing your security systems is key to preventing
unauthorized access.
• Access related security issues include:
–
–
–
–
–
–
–
Security policies
User profiles
Physical security
Dial-in security
Firewalls
Network address translation
Encryption
38
Developing a Security Policy
• The security policy should clearly define
the important network components to be
safeguarded along with controls needed to
do that (Figure 10-8).
• The most common way for a hacker to
break into a system is through “social
engineering” (breaking security simply by
asking how).
39
Elements of a Security Policy
(see Figure 10-8)
•
•
•
•
•
•
•
•
Names of responsible individuals.
Incident reporting system and response team.
Risk assessment with priorities.
Controls on access points to prevent or deter
unauthorized external access.
Controls within the network to ensure internal users
cannot exceed their authorized access.
An acceptable use policy.
User training plan on security.
Testing and updating plans.
40
User Profiles and Forms of Access
• The limits of what users have access to on a network are
determined by user profiles assigned to each user account
by the net manager.
• The profile specifies access details such as what data and
network resources a user can access and the type of access
(e.g., read, write, create, delete).
• Most access is still password based, that is, users gain
access based on something they know.
• Many systems require users to enter a password in
conjunction with something they have, such as a smart
card. ATM cards work in this way.
• In high-security applications, users may be required to
present something they are, such as a finger, hand or the
retina of their eye for scanning by a biometric system.
41
User Profiles: Managing User Access
• User profiles can limit the allowable log-in
days, time of day, physical locations, and
the allowable number of incorrect log-in
attempts.
• Creating accounts and profiles is simple, as
they are created when new personnel arrive.
• One security problem is often created
because network managers forget to remove
user accounts when someone leaves an
organization.
42
Managing Users
• It is important to screen and classify both users
and data (need to know).
• The effect of any security software packages that
restrict or control access to files, records, or data
items should also be reviewed.
• Adequate user training on network security should
be provided through self-teaching manuals,
newsletters, policy statements, and short courses.
• A well publicized security campaign can also help
deter potential intruders.
43
Physical Security
• Physical security means implementing access
controls so only authorized personnel have access
to areas where network equipment is located.
• Each network component should have its own
level of physical security.
• Two important areas of concern are network
cabling and network devices.
• Network cables should be secured behind walls.
• Network devices such as hubs and switches should
be secured in locked wiring closets.
44
Dial-In Security
• Any organization that permits staff members to
access its networks via dial-in modems opens
itself to a broader range of intruders.
• One strategy is to routinely change modem
numbers.
• Another strategy is to use call-back modems &
automatic number identification (ANI) so only
users dialing in from authorized locations are
granted access.
• One-time passwords provide a strategy for
traveling employees who can’t use call-back
modems and automatic number identification.
45
Firewalls
• Firewalls are used to prevent intruders on the
Internet from making unauthorized access and
denial of service attacks to your network.
• A firewall is a router, gateway, or special purpose
computer that examines packets flowing into and
out of the organization’s network (usually via the
Internet or corporate Intranet), restricting access to
that network.
• The two main types of firewalls are packet level
firewalls and application-level firewalls.
46
Fig. 10-9 Using a firewall to protect networks.
47
Packet Filters
• A packet-level firewall (or packet filter) examines
the source and destination address of packets that
pass through it, only allowing packets that have
acceptable addresses to pass.
• Since each packet is examined separately, the
firewall can’t understand what the sender’s goal is.
• Packet filters may be vulnerable to IP spoofing,
accomplished by changing the source address on
incoming packets from their real address to an
address inside the organization’s network.
• While packet filters have strengthened their
security since the first cases of IP spoofing, IP
spoofing remains a problem.
48
Application-Level Firewalls
• An application-level firewall or application gateway acts
as an intermediate host computer, separating a private
network from the rest of the Internet, but it works on
specific applications, such as Web site access.
• The application gateway acts as an intermediary between
the outside client making the request and the destination
server responding to that request, hiding individual
computers on the network behind the firewall.
• Because of the increased complexity of what they do,
application level firewalls require more processing power
than packet filters which can impact network performance.
49
Network Address Translation
• Network address translation (NAT) is used to
shield a private network from outside interference.
• An NAT proxy server uses an address table,
translating network addresses inside the
organization into aliases for use on the Internet.
So, internal IP addresses remain hidden.
• Many organizations combine NAT proxy servers,
packet filters and application gateways,
maintaining their online resources in a “DMZ
network” between the two (Figure 10-10).
50
Fig. 10-10 Typical network design using firewalls.
51
Security Holes
• Security holes are made by flaws in network
software that permit unintended access to the
network. Operating systems often contain security
holes, the details of which can be highly technical.
• Once discovered, knowledge about the security
hole may be quickly circulated on the Internet.
• A race can then begin between hackers attempting
to break into networks through the security hole
and security teams working to produce a patch to
eliminate the security hole.
52
Encryption
• Encryption systems include 4 main components:
– Plaintext: the unencrypted message
– An encryption algorithm: that works like the
locking mechanism to a safe
– A key that works like the safe’s combination
– Ciphertext is produced from the plaintext
message by the encryption function.
– Decryption is the same process in reverse (like
a modulation/demodulation), but it doesn’t
always use the same key or algorithm. Plaintext
results from decryption.
53
Encryption Techniques
• There are three important encryption
techniques now in use:
– Symmetric or private key encryption
– Asymmetric or public key encryption
– Digital signatures, which are based on a
variation of public key encryption.
54
Symmetric Encryption
• Symmetric or private key encryption, uses
the same algorithm and key to both encrypt
and decrypt a message.
• Historically, this is the most common
encryption technique.
• Since the key must be distributed, however,
it is vulnerable to interception. This is an
important weakness of symmetric key
encryption.
55
Symmetric Encryption
• Strong encryption doesn’t only depend on
keeping the algorithm secret, it also depends
on the length of the key.
• A common way to break encryption is by
“brute force”, meaning trying all possible
combinations until the correct key is found.
• Since longer keys have more possible
combinations, they are more difficult to
crack.
56
Data Encryption Standard (DES)
• DES is a symmetric encryption algorithm
developed by IBM and maintained by the National
Institute of Standards and Technology.
• A 56-bit version of DES is commonly used, but
can be broken by brute force.
• Other symmetric encryption techniques include:
– RC4 uses a 40 bit key, but can use up to 256 bits.
– Triple DES (3DES) uses DES three times, effectively
giving it a 168 bit key.
– Advanced Encryption Standard (AES), designed to
replace DES uses 128, 192 and 256 bit keys.
57
Encryption: a “dual use” technology
• The U.S. government limits the export of
encryption techniques since they can also be
used for military purposes.
• The limit is 56 bit keys, based on the DES
technique were developed in the 1970s.
• US policy is the focus of an ongoing policy
debate between security agencies and the
software industry.
58
Asymmetric or Public Key Encryption
• A second popular technique is asymmetric or
public key encryption (PKE).
• PKE is called asymmetric since it uses two
different “one way” keys:
– a public key used to encrypt messages, and
– a private key used to decrypt them.
• PKE greatly reduces the key management problem
since the private key is never distributed.
• The most popular form of PKE is called RSA
named after the initials of its inventors.
59
Public Key Encryption (Figure 10-11)
• Public key encryption works as follows:
– B (the message recipient) makes his/her public
key widely available (say through the Internet).
– A (the sender) then uses B’s public key to
encrypt the message to be sent to B.
– B then uses the B’s own private key to decrypt
the message.
• No security hole is created by distributing
the public key, since B’s private key has
never been distributed.
60
Figure 10-11
Public Key
Encryption
61
Digital Signatures (see Figure 10-12)
• PKE also permits authentication (digital signatures),
which essentially uses PKE in reverse. The digital
signature, is a small part of the message, and includes
the name of the sender and other key contents.
• The digital signature in the outgoing message is
encrypted using the sender’s private key
• The digital signature is then decrypted using the
sender’s public key thus providing evidence that the
message originated from the sender.
• Digital signatures and public key encryption combine
to provide secure and authenticated message
transmission (see Figure 10-12).
62
Figure 10-12
Digital
Signatures
63
Certificate Authorities (CA)
• One problem with digital signatures involves
verifying that the person sending the message is
really who he or she says they are.
• A certificate authority (CA) is a trusted
organization that can vouch for the authenticity of
the person of organization using authentication.
• The CA sends out a digital certificate verifying the
identity of a digital signature’s source.
• For higher level security certification, the CA
requires that a unique “fingerprint” (key) be issued
by the CA for every message sent by the user.
64
Other Encryption Techniques: PGP
• Pretty Good Privacy (PGP) is a PKE freeware
package developed by Phil Zimmerman often used
to encrypt e-mail.
• PGP users make their public keys available by
posting them on Web pages.
• Anyone wishing to send an encrypted message to
that person, simply cuts and pastes the public key
from the Web page into the PGP software. The
PGP software then encrypts and sends the message
using that key.
• PGP servers are also available that allow you to
search for someone’s public key.
65
Other Encryption Techniques: SSL
• Secure Sockets Layer (SSL) is a technique used
on the Web that operates between the application
and transport layers.
• SSL combines symmetric encryption with digital
signatures. SSL has four steps:
– Negotiation: browser and server first agree on the
encryption technique they will use (e.g., RC4, DES).
– Authentication: the server authenticates itself by
sending its digital signature to the browser.
– Symmetric Key Exchange: browser and server
exchange sym. keys used to encrypt outgoing messages.
– Sym. Key Encryption w/ Dig. Signatures: encrypted
messages are then sent that include digital signatures.
66
Other Encryption Techniques: IPSec
• The IP Security Protocol (IPSec) technique works
between the transport and network layers.
• First, sender and receiver exchange two numbers
using Internet Key Exchange (IKE). These are
combined to create encryption keys, which are
then exchanged.
• Next, sender and receiver negotiate the encryption
technique to be used, such as DES or 3DES.
• Sender and receiver then begin transmitting data.
• IPSec transmits using either transport mode, in
which only the IP payload is encrypted, or tunnel
mode, in which the entire IP packet is encrypted.
67
Detecting Unauthorized Access
• Since unauthorized access can not always be
prevented, managers need to try to detect when it
has occurred. This is done using one of three types
of Intrusion Detection Systems (IDSs):
– Network-based IDSs install IDS sensors on
network circuits and monitor packets
– Host-based IDSs monitor all activity on the
server as well as incoming server traffic
– Application-based IDSs are a special form of
host-based IDSs that monitor just one
application, such as a Web server.
68
Detecting Intrusions
• Intrusion detection systems use two main techniques
to determine if an intrusion is in progress:
• Misuse detection compares monitored activities with
signatures of known attacks. If an attack is
recognized the IDS issues an alert.
• Anomaly detection operates in stable computing
environments and looks for major deviations from
the “normal” parameters of network operation.
When one is detected, (e.g., a large number of failed
logins), an alert is issued.
• IDSs are often used in conjunction with firewalls
and other security tools (See Figure 10-13).
69
Figure 10-13 Intrusion Detection System
70
Correcting Unauthorized Access
• Once an unauthorized access is detected, the first
step is to identify where the security breach
occurred and fix it so that it will not reoccur.
• In order to deter such break-ins, there has been a
stiffening of computer security laws and in the
legal interpretation of other laws that pertain to
computer networks.
• Many organizations have also taken their own
steps to detect or deter intruders such by using
entrapment techniques that lure hackers to a server
with fake information and may even have special
software to track the hacker’s origin.
71
End of Chapter 10
72