You've heard so many warnings about phishing that you've become wary of any e-mail message purporting to come from your bank or favorite Web store. But if the link in it uses a legitimate Web domain and your phishing filter doesn't complain, the message must be okay, right?

The answer, as you can probably guess, is "Wrong." You can't even trust your eyes anymore, because online scam artists have figured out ways to turn Web sites against themselves. The technique is called cross-site scripting (or XSS), and it exploits a hole that affects hundreds of the largest Web sites. Legitimate Web sites often use a script to help direct visitors to different parts of the site. But bad guys are constantly probing these scripts as a way to bypass antiphishing filters and get you to click a link that delivers your browser--and eventually, your personal data--right into the attacker's lap.

If you were to type http://www.google.com/url?q=http://www.pcworld.net.au into your browser, you might think your destination would be Google, but that's only the first stop. This URL calls a script hosted at Google that tells the search engine you just clicked a link in a search result pointing at PC World's Web site. So Google tells your browser, "Go there instead"--and it does.

Most antiphishing tools validate only the first domain name in a URL, which can leave you vulnerable if the second one is a criminal's fake site. Experts have posted alerts about cross-site scripting problems on nearly 300 large Web sites--so far.

So what can you do to avoid the next XSS attack? Lance James, chief scientist at Secure Science, a company that tracks cybercriminals who engage in theft of financial information, recommends using the Netcraft Toolbar, which comes in versions for both Internet Explorer and Firefox. "It's very aware of phishing and has a great repository of phishing sites," he says. Netcraft's Toolbar looks for suspicious URLs anywhere within a Web address.

In general, "If a URL looks really strange, particularly with nontraditional characters on the end of it, you have to be careful," says Netcraft analyst Rich Miller. "You can't count on the fact that your financial institution's site will be secure."