A network intrusion is any unauthorized activity on a computer network. Detecting an intrusion depends on the defenders having a clear understanding of how attacks work.

In most cases, such unwanted activity absorbs network resources intended for other uses, and nearly always threatens the security of the network and/or its data. Properly designing and deploying a network intrusion detection system will help block the intruders.

As a first stepof defense, here's a brief rundown of popular attack vectors.

Asymmetric Routing

In this method, the attacker attempts to utilize more than one route to the targeted network device. The idea is to have the overall attack evade detection by having a significant portion of the offending packets bypass certain network segments and their network intrusion sensors. Networks that are not set up for asymmetric routing are impervious to this attack methodology.

Buffer Overflow Attacks 

This approach attempts to overwrite specific sections of computer memory within a network, replacing normal data in those memory locations with a set of commands that will later be executed as part of the attack. In most cases, the goal is to initiate a denial of service (DoS) situation, or to set up a channel through which the attacker can gain remote access to the network. Accomplishing such attacks is more difficult when network designers keep buffer sizes relatively small, and/or install boundary-checking logic that identifies executable code or lengthy URL strings before it can be written to the buffer.

network intrusion detectionCommon Gateway Interface Scripts

The Common Gateway Interface (CGI) is routinely used in networks to support interaction between servers and clients on the Web. But it also provides easy openings—such as "backtracking"—through which attackers can access supposedly secure network system files. When systems fail to include input verification or check for backtrack characters, a covert CGI script can easily add the directory label ".." or the pipe "|" character to any file path name and thereby access files that should not be available via the Web.

Protocol-Specific Attacks

When performing network activities, devices obey specific rules and procedures. These protocols—such as ARP, IP, TCP, UDP, ICMP, and various application protocols—may inadvertently leave openings for network intrusions via protocol impersonation ("spoofing") or malformed protocol messages. For example, Address Resolution Protocol (ARP) does not perform authentication on messages, allowing attackers to execute "man-in-the-middle" attacks. Protocol-specific attacks can easily compromise or even crash targeted devices on a network.

Traffic Flooding

An ingenious method of network intrusion simply targets network intrusion detection systems by creating traffic loads too heavy for the system to adequately screen. In the resulting congested and chaotic network environment, attackers can sometimes execute an undetected attack and even trigger an undetected "fail-open" condition.


These programs present themselves as benign and do not replicate like a virus or a worm. Instead, they instigate DoS attacks, erase stored data, or open channels to permit system control by outside attackers. Trojans can be introduced into a network from unsuspected online archives and file repositories, most particularly including peer-to-peer file exchanges.


A common form of standalone computer virus, worms are any computer code intended to replicate itself without altering authorized program files. Worms often spread through email attachments or the Internet Relay Chat (IRC) protocol. Undetected worms eventually consume so many network resources, such as processor cycles or bandwidth, that authorized activity is simply squeezed out. Some worms actively seek out confidential information—such as files containing the word "finance" or "SSN"—and communicate such data to attackers lying in wait outside the network.

Once these attack vectors are thoroughly understood, network security teams can look for opportunities to deploy technologies and strategies that will mitigate the potential effectiveness of each one.