What is spyware? The definition dilemma

Defining spyware is complicated by the variety of behavior that it is capable of. Here, Ed Skoudis outlines what different types of spyware do, from logging keystrokes to serving personalized advertisements.

What is spyware? Generally speaking, most people think of spyware as software that grabs or alters personal information on unsuspecting users' machines. But this overly broad and amorphous notion covers everything from adware that gathers information on browsing habits to keyloggers that steal personal information. That presents problems for the security industry.

Examining the range of activities that fall under the general and nebulous spyware umbrella underscores the dilemma:

  • Aggregating surfing habits across multiple users for advertising. Through detailed analysis of this information, Web-based advertisers can place their ads on the most heavily surfed sites of their target demographics.

  • Grabbing surfing habits of individual users for advertising. By knowing an individual user's interests, advertisers can create very focused ads and deliver them at a time when the user is most inclined to click-through or buy a product. This is often most pronounced in association with pornographic Web sites, and access from within an enterprise could embarrass the organization, or may even be a factor in a sexual harassment lawsuit.

  • Placing pop-up and banner ads generated by locally installed spyware when a user surfs the Web. As a user accesses Web sites, the spyware running on the user's machine injects ads into the surfing session. Users will think that these ads are associated with the sites they are browsing. Beyond mere annoyance, such activities often bog down a computer or even render it unusable.

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  • Customizing search results based on advertiser's needs. Search engine advertising is a multibillion dollar business. Some spyware companies try to exploit this by watching for when the local browser accesses a popular search engine. The spyware can inject additional ads into the search results, or even filter out some of the legit ads from competitors who are advertising on the search engine.

  • Blocking access to antivirus and antispyware update sites. In an effort to remain lodged onto a system, some spyware blocks access to antivirus and antispyware update sites.

  • Stealing personal financial information for fraud and identity theft. This is one of the biggest threats from spyware, increasingly perpetrated by organized crime. Such attacks are often implemented with keystroke loggers or related software that waits for a user to access particular financial services or e-commerce sites. By stealing bank account numbers, e-commerce login information and credit card numbers, criminals can hijack accounts or even steal the identity of a victim user.

    AV tools typically quarantine or delete it when they get a signature match, but one person's spyware is sometimes another person's business model. There's real concern about legal action from "legit" adware companies.

    Therefore, most AV tools have signatures only for the most egregious spyware specimens.

    In particular, traditional antivirus vendors, who have, at least until recently, have been slow to jump on this threat, in part because of legal adware that users may have knowingly or inadvertently approved when they check "I agree" to some dense end-user licensing agreement when they install software or sign on for some service.

    Antispyware programs, on the other hand, usually don't automatically quarantine or uninstall detected specimens. Instead, they leave the decision for deletion up to end users or administrators, somewhat alleviating the lawsuit issue. That's why we often see antispyware labeling a detected specimen as Potentially Unwanted Programs (PUPs), instead of calling it malicious code. Spyware is only potentially unwanted, and it's up to the user or administrator to make the final call.

    About the author
    Ed Skoudis, CISSP, is a contributing editor for SearchSecurity's sister publication,
    Information Security magazine. He is also cofounder of the security consultancy Intelguardians and author of Malware: Fighting Malicious Code and Hack-Counter Hack.


  • This was first published in September 2005

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