Routes of infection

Spyware does not directly spread in the manner of a computer virus or worm: generally, an infected system does not attempt to transmit the infection to other computers. Instead, spyware gets on a system through deception of the user or through exploitation of software vulnerabilities.

Bundled Software

The most direct route by which spyware can get on a computer involves the user installing it. However, users are unlikely to install software if they know that it will disrupt their working environment and compromise their privacy. So many spyware programs deceive the users, either by piggybacking on a piece of desirable software, or by tricking the users to do something that installs the software without their realizing it. Recently, spyware has come to include "rogue anti-spyware" programs, which masquerade as security software while actually doing damage.

Classically, a Trojan horse, by definition, smuggles in something dangerous in the guise of something desirable. Some spyware programs get spread in just this manner. The distributor of spyware presents the program as a useful utility — for instance as a "Web accelerator" or as a helpful software agent. Users download and install the software without immediately suspecting that it could cause harm. For example, Bonzi Buddy, a spyware program targeted at children, claims that:

He will explore the Internet with you as your very own friend and sidekick! He can talk, walk, joke, browse, search, e-mail, and download like no other friend you've ever had! He even has the ability to compare prices on the products you love and help you save money! Best of all, he's FREE!

Spyware can also come bundled with shareware or other downloadable software, as well as music CDs. The user downloads a program (for instance, a music program or a file-trading utility) and installs it, and the installer additionally installs the spyware. Although the desirable software itself may do no harm, the bundled spyware does. In some cases, spyware authors have paid shareware authors to bundle spyware with their software, as with the Gator spyware now marketed by Claria. In other cases, spyware authors have repackaged desirable free software with installers that add spyware.

Tricking the User

A third way of distributing spyware involves tricking users by manipulating security features designed to prevent unwanted installations. The Internet Explorer Web browser, by design, prevents websites from initiating an unwanted download. Instead, a user action (such as clicking on a link) must normally trigger a download. However, links can prove deceptive: for instance, a pop-up ad may appear like a standard Windows dialog box. The box contains a message such as "Would you like to optimize your Internet access?" with links which look like buttons reading Yes and No. No matter which "button" the user presses, a download starts, placing the spyware on the user's system. Later versions of Internet Explorer offer fewer avenues for this attack.

Fake Anti-Spyware programs

Malicious programmers have released a large number of fake anti-spyware programs, and widely distributed Web banner ads now spuriously warn users that their computers have been infected with spyware, directing them to purchase programs which do not actually remove spyware — or worse, may add more spyware of their own.

The recent proliferation of fake or spoofed antivirus products has occasioned some concern. Such products often bill themselves as antispyware, antivirus, or registry cleaners, and sometimes feature popups prompting users to install them.

Known offenders include:

• SpyAxe
• AntiVirus Gold
• SpywareStrike
• SpyFalcon
• WorldAntiSpy
• WinFixer
• SpyTrooper
• Spy Sheriff
• SpyBan
• SpyWiper
• PAL Spyware Remover
• Spyware Stormer
• PSGuard

Taking Advantage of Security Holes

Some spyware authors infect a system by attacking security holes in the Web browser or in other software. When the user navigates to a Web page controlled by the spyware author, the page contains code which attacks the browser and forces the download and install of spyware. The spyware author would also have some extensive knowledge of commercially-available anti-virus and firewall software. This has become known as a "drive-by download", which leaves the user a hapless bystander to the attack. Common browser exploits target security vulnerabilities in Internet Explorer and in the Microsoft Java runtime.

Microsoft Internet Explorer is Highly Suceptible to Infection

The installation of spyware frequently involves Microsoft's Internet Explorer. As the most popular Web browser, and with an unfortunate history of security issues, it has become the largest target. Its deep integration with the Windows environment and its scriptability make it an obvious point of attack into Microsoft Windows operating systems. Internet Explorer also serves as a point of attachment for spyware in the form of browser helper objects, which modify the browser's behavior to add toolbars or to redirect traffic.

In a few cases, a worm or virus has delivered a payload of spyware. For instance, some attackers used the W32.Spybot.Worm worm to install spyware that popped up pornographic ads on the infected system's screen.  By directing traffic to ads set up to channel funds to the spyware authors, they can profit even by such clearly illegal behavior.