A new study details the extent and seriousness of potentially destructive spyware on the Internet, finding that it is still prevalent but declined significantly.

University of Washington computer scientists sampled more than 20 million Internet sites looking for programs that can covertly enter computers.

While most spyware can be a nuisance — generating pop-ups, loading unwanted programs — it can also perform such malicious tasks as gathering personal data or using your modem to dial costly toll numbers.

The study examined popular categories of Web sites including games, news and celebrity sites. Among the findings:

— More 5 percent of executable files contain piggybacked spyware.

— One in 62 Internet domains performs "drive-by download attacks" to force spyware on users who simply visit the site.

— Game and celebrity Web sites appeared to pose the greatest risk for piggybacked spyware, while sites that offer pirated software topped the list for drive-by attacks.

"For unsuspecting users, spyware has become the most 'popular' download on the Internet," said Hank Levy at the university's Department of Computer Science & Engineering.

There is some good news:

The study employed a Web crawler to visit sites and look for spyware. It made two crawls, in May and October of last year, and noted a 93 percent reduction in drive-by download attacks.

That may be because more people are using anti-spyware tools and employing automated patch programs such as Windows Update. Also, civil lawsuits have been brought against spyware distributors.

Most spyware is relatively benign, but can inundate a victim with pop-up advertisements. More malicious programs steal passwords and financial information. In a worst-case scenario, spyware can render a computer useless.

"You should download software only from reputable sources," said University of Washington associate professor Steven Gribble. "And it's a good idea to avoid the more shady areas of the Web."

The research is being presented today at the 13th Annual Network and Distributed System Security Symposium in San Diego.

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