COMPUTER VIRUSES

Computer Viruses - Learn more about the threats to your PC in our comprehensive special report.

Any computer connected to the internet faces a daunting range of electronic threats. Perhaps the biggest single threat to any computer is the humble software bug. Seemingly innocuous programming errors can be exploited to force entry into a computer and also provide the weak spots that allow computer worms and viruses to proliferate.

Many software bugs will simply cause a computer to crash. But an expert programmer can sometimes figure out how to make a computer malfunction in a creative way, so that it provides access to secure parts of a system, or shares protected data.

When a software vulnerability is revealed, it is often a race against the clock to apply the correct software patch before an attacker can convert the bug into an "exploit" that can be used to cause major damage.

Viruses and worms

A computer virus is a program that spreads between computers by hiding itself within a - seemingly innocent - document or application. A worm, on the other hand, is a program that replicates and travels without "infecting" anything else on a system.

Many modern specimens of malevolent code, however, use a mixture of tricks to cheat their way onto computer systems, blurring the line between worms and viruses. The terms are now often used interchangeably.

The first worms appeared in the 1970s and spread slowly between computers connected to the same network. They simply displayed an annoying message on the screen of each infected machine. The first computer virus, called Elk Cloner, was written in 1982 and infected computers via floppy disks.

Trojans and zombies

But viruses and worms no longer just provide a way for malevolent hackers to gain notoriety. Today's viral code can contaminate computers at lightning speed, spreading via email, peer-to-peer file-sharing networks and even instant messaging programs. The most successful ones cause serious damage, forcing companies around the globe to close down while infected computers are cleaned up.

A string of recent specimens have been designed to snatch passwords or credit card information and install programs that can be used to remotely control infected machines. These programs are known as trojan horses.

There is evidence that virus writers can earn large amounts of money by leasing access to networks of compromised computers - often referred to as "botnets". These groups of remote-controlled "zombies" have been used to extort money from websites, by threatening to crash them with a denial-of-service (DoS) attack. This involves overloading a server with bogus page requests, so that real messages cannot get through.

Spam, Spam, Spam

Spammers have also begun using botnets to forward unsolicited bulk email advertising, or spam, through scores of zombie PCs. This makes it far more difficult for spam hunters to block the messages at source and catch the culprits.

Once considered a fairly minor problem, spam is rapidly spiralling out of control, and much more than half of all email messages are now thought to consist of unwanted advertising messages.

To combat computer scientists' best efforts to stem the tide of junk email, the spammers have had to become more cunning and sophisticated. More recently, spim (spam by instant messenger) and spit (spam by internet telephony) have joined the fray.

Phishing

Spam's more sinister cousin is the phishing email. This is a con trick that arrives as an email and tries to trick a recipient into handing over money or sensitive personal information like their bank account details or a username and password.

The simplest phishing tricks try to dupe a target into sending money as part of a get-rich-quick scheme. But phishing tricksters are also getting more devious and recent scams pose as customer service emails and send users to bogus banking or commercial websites where they are invited to "re-enter" their account information.

Some genuine sites have even proven vulnerable to software glitches that can be exploited to capture information from regular users. Phishing is especially threatening because it can be used to steal a person's digital identity.

Spyware

Along with spam and phishing, spyware represents the third of an unhappy trinity of internet pests. These insidious and clandestine programs typically find their way onto a computer system alongside another, often free, software application, although some can also exploit software bugs to get onto a machine. The programs are used to serve up unwanted adverts, change system settings and gather information on a user's online behaviour for marketing purposes.

Hackers

The term "computer hacker" was first coined in the 1960s and originally meant someone capable of developing an ingenious solution to a programming problem. But the phrase has since fallen into disrepute, entering the popular vocabulary as a term for a programmer with criminal intent.

The earliest "criminal" hackers were in fact relatively harmless, interested in testing the boundaries of their knowledge and their ability to get around security measures. They mainly performed innocuous pranks, for example employing low-tech tricks to get free calls through the US phone networks.

There are many tools in the modern hacking kit, including network scanners, packet sniffers, rootkits and decompilers. But "social engineering" - for example, putting a particularly enticing message in an email header to encourage people to open it - and even search engines can also be useful weapons for the hacker.

Computer crime

As the number of computers networks has grown, so have the possibilities for more serious misuse. And, as money increasingly becomes a digital commodity, the world has seen the emergence of serious computer criminals.

Criminal gangs have also started to get in on the action, attracted by the huge quantities of money now spent online every day. There is evidence that unscrupulous experts can also earn serious money from crime syndicates by breaking into computer systems, writing viruses and creating phishing scams.

And it is not just ordinary desktop computers that are under threat. Governments, banks and critical infrastructure can also be brought to a standstill by an expert armed only with a laptop computer and a net connection.

Mobile menace

The biggest new target for computer hackers is the mobile device. Virus writers are already experimenting with code designed for smartphones and experts predict more may be on the way, while hackers are also looking at ways to crack handheld devices.

While the internet has transformed global communication beyond recognition, the arms race between those intent on harnessing its power for criminal purposes and those tasked with preventing them has only just begun.