Introduction to Network Security

© N. Ganesan, Ph.D.


Chapter Focus

• Introduction to computer security

• Overview of security threats

• Outline of security measures

• Summary

Understanding the Threats


• Intentional attacks on computing resources and networks persist for a number of reasons

• Complexity of computer software and newly emerging hardware and software combinations make computer and the network susceptible to intrusion

– It is difficult to thoroughly test an application for all possible intrusions

Security Threats

1. Trojan horse programs

2. Back door and remote administration programs

3. Denial of service

4. Being an intermediary for another attack

5. Unprotected Windows shares

6. Mobile code (Java, JavaScript, and ActiveX)

7. Cross-site scripting

8. Email spoofing

9. Email-borne viruses

10. Hidden file extensions

11. Chat clients

12. Packet sniffing

Source: CERT

Trojan Horse Programs

• Trojan horses are programs that are installed without the knowledge of the user

• Trojan horse programs can perform a wide variety of covert talks such as modifying and deleting files, transmitting files to the intruder, installing programs, installing viruses and other Trojan horse programs etc.

Backdoor and Remote

Administration Programs

• Covert installation of remote administration programs such as

BackOrifice, Netbus and SubSeven

• Such programs give remote access to the computer from anywhere on the


Intermediary for Other Attacks

• Client computer is used to launch mostly denial of service attacks on other computers

• An agent is usually installed using a

Trojan horse program to launch the denial of service attack on other computers

Unprotected Windows Share

• Malicious code can be stored in protected Windows share for propagation

Mobile code


• Mobile codes in Java, JavaScript, and ActiveX can be executed by a web browser is generally useful, but it can also be used to run malicious code on the client computer.

• Disabling Java, JavaScript, and ActiveX from running in the Web browser must be considered when accessing websites that cannot be trusted

• Email received in HTML format is also susceptible to mobile code attack because it could also carry the mobile code

Cross-site Scripting

• A malicious script can be sent and stored by a web developer on a website to be downloaded by an unsuspecting surfer

• When this website is accessed by a user, the script is transferred to the local web browser

• Ways of acquiring malicious scripts include

“following links in web pages, email messages, or newsgroup, using interactive forms on an untrustworthy site, viewing online discussion groups, forums, or other dynamically generated pages where users can post text containing HTML tags” - CERT

Email Spoofing

• Email “spoofing” tricks the user in believing that the email originated from a certain user such as an administrator although it actually originated from a hacker

• Such emails may solicit personal information such as credit card details and passwords

• Examining the email header may provide some additional information about the origin of the email

Email Borne Viruses

• Malicious code is often distributed through email as attachments

• Attachments must thus be opened with caution

Hidden File Extensions

• An attachment may have a hidden file extension

– Such files may execute the attachment

• Examaple:

– Downloader (MySis.avi.exe or


– VBS/Timofonica (TIMOFONICA.TXT.vbs)

– VBS/CoolNote


– VBS/OnTheFly (AnnaKournikova.jpg.vbs)

• In the above files, the hidden extension is .vbs pertaining to an executable Visual Basic script

Chat Clients

• Internet chat applications such as instant messaging applications and

• Internet Relay Chat (IRC) involve the exchange of information including files that may contain malicious executable codes

• The same caution that applies to email attachments apply here as well

Packet Sniffing

• Packet sniffer programs capture the contents of packets that may include passwords and other sensitive information that could later be used for compromising the client computer

• For example, a sniffer installed on a cable modem in one cable trunk may be able to sniff the password from other users on the same trunk

• Encryption of network traffic provides one of the defenses against sniffing

Providing Security

Reasons to Secure Computing and

Network Resources

• Many businesses rely heavily on computers to operate critical business processes

• Individuals are using computers for tasks that required confidentiality

• Advent of Internet has provided a physical path of entry for every computer connected to the Internet

– An always connected broadband connection is always vulnerable in this case

Providing Security

• Providing security requires action on two fronts, namely the management and the technical fronts respectively

• The management aspect relates to organizational policies and behavior that would address security threats and issues

• The technical aspect relates to the implementation of hardware and software to secure access to computing resources and the network

Management Aspect

• Best practice approach is to ensure secure behavior

• The above can be done by established guidelines for managing, addressing and rectifying security related issues

Technical Aspect

• Introduce security related hardware and software to secure access to computers and computing resources

Technical Approaches

• From an implementation point of view, the following are some of the steps that could be taken to provide security

– Implement security patches and other updates pertaining to an operating system and other venerable software such as the Internet Explorer

– Install self-monitoring an anti-virus, anti-spam and antihacker and pop-up blocker software

– Install a firewalls

– Use encryption wherever feasible

• All the approaches can be used to complement one another

Self-Monitoring Software and

Security Patches

• Security patches are issued by mainly the OS vendor such as Microsoft to patch security holes as they are discovered

• Examples of self-monitoring software include anti-virus, spyware elimination, pop-up blocking, and anti-spam software

• Both the security patches and the selfmonitoring software act at the local client level

Examples of Self-Monitoring


• Antivirus

– Mcafee

• Spyware elimination

• Pop-up blocker

• Anti-Spam


• Firewalls are used for controlling access to the computing resources

• In general, it acts at the network level controlling network access to computing resources

• Firewalls can be implemented in software as well as in hardware


• By encryption, the data can be made illegible to the intruder

• It can be implemented at the network level as well as the client level

• For example, locally stored data can be encrypted and the network traffic could equally well be encrypted

Some Applications of Encryption



• Digital Certificates

More on Security Techniques

• Firewalls and encryption will be discussed further in separate modules under the section entitled “Network









Web References



html ity/2.5.1



• s/hackers.html


• round/Hacking/Methods/Technical/




• .

Tim Rains • Technical Lead •

Networking Team

• Q310099 , "Description of the Portqry.exe Command-

Line Utility"

The End