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Introduction To Network Security - Part 2

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This article builds upon our first article Introduction to Network Security - Part 1. This article is split into 5 pages and covers a variety of topics including:

  • Tools and Attacker Uses
  • General Network Tools
  • Exploits
  • Port Scanners
  • Network Sniffers
  • Vulnerability Scanners
  • Password Crackers
  • What is Penetration Testing
  • More Tools
  • Common Exploits
  • A Brief Walk-through of an Attack
  • and more.

Tools An Attacker Uses

Now that we've concluded a brief introduction to the types of threats faced by both home users and the enterprise, it is time to have a look at some of the tools that attackers use.

Keep in mind that a lot of these tools have legitimate purposes and are very useful to administrators as well. For example I can use a network sniffer to diagnose a low level network problem or I can use it to collect your password. It just depends which shade of hat I choose to wear.

General Network Tools

As surprising as it might sound, some of the most powerful tools especially in the beginning stages of an attack are the regular network tools available with most operating systems. For example and attacker will usually query the 'whois' databases for information on the target. After that he might use 'nslookup' to see if he can transfer the whole contents of their DNS zone (called a zone transfer -- big surprise !!). This will let him identify high profile targets such as webservers, mailservers, dns servers etc. He might also be able to figure what different systems do based on their dns name -- for example would most likely be a database server. Other important tools include traceroute to map the network and ping to check which hosts are alive. You should make sure your firewall blocks ping requests and traceroute packets.


An exploit is a generic term for the code that actually 'exploits' a vulnerability in a system. The exploit can be a script that causes the target machine to crash in a controlled manner (eg: a buffer overflow) or it could be a program that takes advantage of a misconfiguration.

A 0-day exploit is an exploit that is unknown to the security community as a whole. Since most vulnerabilities are patched within 24 hours, 0-day exploits are the ones that the vendor has not yet released a patch for. Attackers keep large collections of exploits for different systems and different services, so when they attack a network, they find a host running a vulnerable version of some service and then use the relevant exploit.

Port Scanners

Most of you will know what portscanners are. Any system that offers TCP or UDP services will have an open port for that service. For example if you're serving up webpages, you'll likely have TCP port 80 open, FTP is TCP port 20/21, Telnet is TCP 23, SNMP is UDP port 161 and so on.

A portscanner scans a host or a range of hosts to determine what ports are open and what service is running on them. This tells the attacker which systems can be attacked.
For example, if I scan a webserver and find that port 80 is running an old webserver -- IIS/4.0, I can target this system with my collection of exploits for IIS 4. Usually the port scanning will be conducted at the start of the attack, to determine which hosts are interesting.

This is when the attacker is still footprinting the network -- feeling his way around to get an idea of what type of services are offered and what Operating Systems are in use etc. One of the best portscanners around is Nmap ( Nmap runs on just about every operating system is very versatile in how it lets you scan a system and has many features including OS fingerprinting, service version scanning and stealth scanning. Another popular scanner is Superscan ( which is only for the windows platform.

Network Sniffers

A network sniffer puts the computers NIC (network interface card or LAN card) into 'promiscuous mode'. In this mode, the NIC picks up all the traffic on its subnet regardless of whether it was meant for it or not. Attackers set up sniffers so that they can capture all the network traffic and pull out logins and passwords. The most popular network sniffer is TCPdump as it can be run from the command line -- which is usually the level of access a remote attacker will get. Other popular sniffers are Iris and Ethereal.

When the target network is a switched environment (a network which uses layer 2 switches), a conventional network scanner will not be of any use. For such cases, the switched network sniffer Ettercap ( and WireShark ( are very popular. Such programs are usually run with other hacking capable applications that allow the attacker to collect passwords, hijack sessions, modify ongoing connections and kill connections. Such programs can even sniff secured communications like SSL (used for secure webpages) and SSH1 (Secure Shell - a remote access service like telnet, but encrypted).

Vulnerability Scanners

A vulnerability scanner is like a portscanner on steroids, once it has identified which services are running, it checks the system against a large database of known vulnerabilities and then prepares a report on what security holes are found. The software can be updated to scan for the latest security holes. These tools are very simple to use unfortunately, so many script kiddies simply point them at a target machine to find out what they can attack. The most popular ones are Retina (, Nessus ( and GFI LanScan ( These are very useful tools for admins as well as they can scan their whole network and get a detailed summary of what holes exist.

Password Crackers

Once an attacker has gained some level of access, he/she usually goes after the password file on the relevant machine. In UNIX like systems this is the /etc/passwd or /etc/shadow file and in Windows it is the SAM database. Once he gets hold of this file, its usually game over, he runs it through a password cracker that will usually guarantee him further access. Running a password cracker against your own password files can be a scary and enlightening experience. L0phtcrack cracked my old password fR7x!5kK after being left on for just one night !

There are essentially two methods of password cracking :

Dictionary Mode - In this mode, the attacker feeds the cracker a word list of common passwords such as 'abc123' or 'password'. The cracker will try each of these passwords and note where it gets a match. This mode is useful when the attacker knows something about the target. Say I know that the passwords for the servers in your business are the names of Greek Gods (yes Chris, that's a shout-out to you ;)) I can find a dictionary list of Greek God names and run it through the password cracker.

Most attackers have a large collection of wordlists. For example when I do penetration testing work, I usually use common password lists, Indian name lists and a couple of customized lists based on what I know about the company (usually data I pick up from their company website). Many people think that adding on a couple of numbers at the start or end of a password (for example 'superman99') makes the password very difficult to crack. This is a myth as most password crackers have the option of adding numbers to the end of words from the wordlist. While it may take the attacker 30 minutes more to crack your password, it does not make it much more secure.

Brute Force Mode - In this mode, the password cracker will try every possible combination for the password. In other words it will try aaaaa, aaaab, aaaac, aaaad etc. this method will crack every possible password -- its just a matter of how long it takes. It can turn up surprising results because of the power of modern computers. A 5-6 character alphanumeric password is crackable within a matter of a few hours or maybe a few days, depending on the speed of the software and machine. Powerful crackers include l0phtcrack for windows passwords and John the Ripper for UNIX style passwords.

For each category, I have listed one or two tools as an example. At the end of this article I will present a more detailed list of tools with descriptions and possible uses.

2. Penetration-Testing

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