"The holiday season in general is a huge time for hackers ... [and] Black Friday is typically the start," says Paul Henry, vice president of strategic accounts for Secure Computing. "This year, my biggest concern for consumers is all the Web-borne malware out there."

Black Friday, the day after Thanksgiving, is followed in marketing lingo by Cyber Monday. Both are big days for retailers and online fraudsters. Consumers should watch out for e-mails advertising incredible deals that seem too good to be true.

"Freebies may be freebies in the sense that you get free malware," says Jamz Yaneza, a senior threat researcher at Trend Micro.

A common scam is to pick the hot toy of the season and send out a spam e-mail blast offering it for much less than the typical price, Henry says. Victims end up entering credit card information on malicious sites designed to look like well-known, trusted ones. They might also unknowingly download a keylogger that can steal personal information people type in when making any kind of Internet transaction.

"Be leery of sites being advertised [in e-mail that might be spam]. In all likelihood you're being directed to a malware-connected site," Henry says. "Do not click on URLs within e-mails even for well-known public sites."

In an HTML e-mail, it's a trivial task for hackers to hide the real URL a victim is clicking on.

"It might say 'ebay.com,' but you're actually clicking on something entirely different," Henry says.

Online fraudsters have been busy this year. Fraud losses related to U.S. e-commerce will top $3.6 billion in 2007, up 20% from last year, according to a report by the vendor CyberSource this month. The increase in dollar loss is due mostly to growing e-commerce sales, as the percentage of transactions that are fraudulent has held steady.

The run-up to Christmas and tax filing season are the two most dangerous times of the year for online shoppers, Yaneza says.

In addition to being wary of e-mails, be careful when searching for holiday deals or specific products on Google and other search engines. Operators of malicious sites have figured out ways to rise to the top of search listings.

"We've seen instances where the top site that is ranked actually gets there by gaming the Google search algorithm," Yaneza says.