Malicious code in general refers to computer programs that are written specifically to cause mischief or, worse, cause damage to infected computers. Malicious code can be used to describe all Viruses, Worms, Trojan Horses etc. but is more commonly used to describe active content, Java or ActiveX for instance, which can be used to either corrupt or steal data or even install a Trojan Horse or Virus for later use. Java and ActiveX are normally used to enhance the features of a web site but can be used for these more sinister purposes.

A computer virus is a piece of software with the ability to copy itself and spread itself to other computers. A virus can enter a victims computer either through e-mail, by downloading infected software from the Internet, or by using infected media such as floppy disks or CD's. With the wide use of e-mail, malicious viruses and scripts have the capability to reach almost anyone who is connected to the Internet. Like their biological equivalent, a computer virus often carries with it a destructive element (known as a payload), and can be difficult to contain and destroy. Well-known viruses like Melissa, and ILOVEYOU.VBS have been able to spread so quickly that they overloaded Internet e-mail systems and company networks within a few hours of their introduction.

A virus may be merely annoying, or completely destructive. The most severe viruses can erase the contents of the computer’s hard drive, or render it completely useless. If no back-ups are kept, important data may be lost or damaged beyond repair, which could ultimately result in serious financial loss.

There are different types of Virus and Malicious Code including:

Virus

Boot Sector

Boot sector viruses modify the contents of either the master boot sector or the DOS boot sector, depending on the virus and type of disk, usually replacing the legitimate contents with their own version.
 

Parasitic

Parasitic viruses attach themselves to COM and/or EXE files and divert the execution flow in such a way that the virus code executes first.
 

Multipartite

Multipartite viruses exhibit the characteristics of both boot sector and parasitic viruses. They infect COM and/or EXE files (like parasitic viruses) and boot sectors (like boot sector viruses). Thus their chances of replication are increased.
 

Macro

Macro viruses use macros contained of certain document types to become active and to infect other documents. The viral macro usually makes transparent changes to the global macro environment so that all new or edited documents become infected.

 

Malicious Code

Examples of malicious code include:
 

Worm

Worms are rogue programs similar to viruses, but do not need a carrier such as a floppy disk or macro to replicate as they are able to created exact copies of themselves. Currently, Worms are the most widespread form of malicious code. Most are written in Visual Basic and are "email aware" such as Kakworm and Loveletter. In the "olden days" you had to open a file or run an application manually in order to be infected. Now you can become infected and then infect hundreds or thousands of people by simply reading your email. Recently, Friends Greetings emails  circulated round the Internet. It was initially thought to be a worm because after people installed the application everyone in their personal address book was sent an email. It turned out that this was not an email aware worm but an attempt at viral marketing. Unfortunately people did not read the licensing conditions before agreeing to them. These conditions clearly stated that their personal address book would be used.

Trojan Horse

A Trojan horse is a program that performs functions other than those stated in its specifications. These functions are often malicious. It may inflict damage or allow somebody from a remote site to take control of the computer. Key loggers, programs that record keystrokes (including passwords) are sometimes installed as part of a Trojan program. Details of your work can then be passed to 3rd parties without your knowledge. A Trojan horse sometimes masquerades as a legitimate program, but once installed on the victim machine runs an illicit, damaging program.
 

Spyware / Adware

Spyware and Adware refers to code that sends details of a PC to a 3rd party. Unlike malicious code, Spyware is intentionally coded into certain applications (or is a self contained application installed on a computer without the knowledge of the user). This allows software suppliers and advertising companies ((to target browser banner advertisements that match you habits for instance), hackers and even family members to monitor your computing habits online habits and learn about how you use your software. Some say that government agencies use spyware too. Some perceive that software used by a business to monitor staff usage in compliance of corporate policies is also Spyware. Organisations should make staff aware of corporate policies and any monitoring that takes place, the use of software to openly do this should be acceptable.

See http://www.spywareinfo.com, http://www.xblock.com or http://www.spycop.com for more information on aspects of Spyware.

Thiefware

Thiefware refers to the use of software and Internet technology to steal custom from a legitimate web site. Some software places links on victimised sites for visitors to click on leading them away to other competitor sites. Many site owners would say the software is stealing visitors which makes Thiefware an appropriate name for such services and software.
Another aspect of Thiefware appears related to Trojan Horses. You download and install an application and during the setup process your are requested to install another application. You believe that the second program is a support application for the first but it is in fact Thiefware.

When you surf with Thiefware installed you may notice that there are additional links of adverts on some web sites (if you have visited it before). These links may seem out of place (such as hard core pornography in a children's site) The site owner has not granted permission for their site to be viewed in this way and would not be aware of these links as it is the software on your PC that has placed them there in your browser. The site owner could also lose revenue if the links / adverts point the user to a competitors site. Some affiliate programmes rely on people clicking on links on a page to earn commission, these could be hijacked by Thiefware so that someone else gets the commission.
Some aspects of Thiefware are also similar to Spyware in that your browsing habits may be logged, see the effects section for more issues. . See http://www.thiefware.com for further details.

Hoaxes

Sometimes seen as almost as much trouble as the real thing, hoaxes are widespread throughout the Internet community. Where a real virus is spread by the software itself, usually without the users knowledge, a virus hoax, uses social engineering techniques to make the recipient spread it by in an effort to inform their co-workers, friends or relatives of the latest threat to their data. Although not containing malicious code, hoaxes take time and effort to dispel. In the past, real viruses have been created to emulate certain aspects of hoaxes in order to spread. The idea being they might be able to spread further if they were not taken seriously. Effort needs to taken to evaluate all threats to information, even if it turns out to be a hoax.
 

Effects

Malicious Code can produce unwanted side affects in computers such as displaying on screen messages, playing tunes or making words drop off the screen. More sinister effects include password-protecting files so that they cannot be accessed, modification to data in spreadsheets, sending information to third parties via email and information corruption/deletion.

If it can be shown that an organisation have been negligent in allowing a virus or malicious code to be transferred to a third party, they may be held legally liable for any damage caused.
It may be possible for viruses, malicious code and Spyware or Thiefware to collect system passwords and other data stored on your machine, such as credit card details. It’s your connection to the Internet that is used to pass the information to the to a third party, therefore it needs to use some of your valuable bandwidth (the amount of information you can send and receive in a period of time) thus slowing down you connection, and perhaps even causing you to lose your connection.