CUPERTINO, Calif. – Forty-eight percent of the sites on Symantec’s “Dirtiest Websites of Summer 2009” list offer pornographic content, the company disclosed this week. However, it’s not smut that classifies the sites as “dirty” — it’s the presence of drive-by Trojans, viruses, browser exploits and phishing schemes that landed the sites on the list.How dirty are the sites? For starters, the average number of threats per malicious site rated by Norton Safe Web is 23. Compare that with the average number of threats on the Dirtiest Websites list — a staggering 18,000 per site. In fact, 40 of the 100 Dirtiest Websites have more than 20,000 threats per site. Being dirty is nothing new for 75 percent of sites on the list, which have distributed malware for more than six months.

Viruses are the most common threat represented among the websites on the list, followed by security risks and browser exploits. According to Symantec, simply visiting one of the sites on the list — even without downloading or clicking on anything in particular — could expose a surfer’s computer to infection or put his identity, personal and financial information into the hands of cybercriminals.

To find the dirt, Norton Safe Web crawls the Web and performs analysis of millions of websites. In addition, a network of more than 20 million Norton Community Watch members automatically submit suspicious URLs for analysis in real-time. Norton Safe Web analyzes websites using signature-based file scanning, intrusion detection engines, behavioral detection and install/uninstall analysis to identify security risks including phishing, malicious downloads, browser exploits and links to unsafe external sites.

Porn sites make convenient entrapment devices for malware authors and phishers who may have nothing to do with the adult entertainment industry. Symantec did not differentiate between legitimate adult Web destinations and faux-porn traps. However, the company did say the other 52 percent of sites on the Dirtiest Websites list ran the gamut of subject matter from deer hunting, catering figure skating and legal services to electronics e-tail.

“This list underscores what our research shows: There has been exponential growth in the number of online threats that are constantly evolving as cybercriminals look for new ways to target your money, identity or assets,” said Rowan Trollope, senior vice president of Symantec’s Consumer Business Unit. “In 2008, most new infections occurred while people were surfing the Web.”

Symantec’s statistics indicate the U.S. continues to harbor the most “dirty” websites, although Europe — especially the Netherlands and Denmark — is catching up. The biggest threat on the Web, in Symantec’s estimation, is a non-porn site based in Russia.

According to rival cyber-security firm McAfee, the volume of malware on the Web and the number of covert attacks has mushroomed over the past few years. In 2006, McAfee detected 125,000 unique pieces of malware online. By 2008, that number had grown to 1.5 million. During the first half of 2009, McAfee detected a whopping 1.2 million unique threats.