SAN FRANCISCO — A giant vulnerability in the Internet's design is allowing criminals to silently redirect traffic to Web sites under their control. The problem is being fixed, but its extent remains unknown and many people are still at risk.

The gaping security hole enables a scam that targets ordinary people typing in a legitimate Web address. It happens because hackers are now able to manipulate the machines that help computers find Web sites. If the trick is done properly, computer users are unlikely to detect whether they've landed at a legitimate site or an evil double maintained by someone bent on fraud.

Security experts fear an open season for virus attacks and identity-fraud scams.

"It's kind of like saying, 'There's a bunch of money on the street. If you can get over there soon enough, you can get it,"' said Ken Silva, chief technology officer for VeriSign Inc., which manages the ".com" and ".net" directories of Internet addresses. "It's something the industry is taking seriously. You'd be in a bad place if you weren't doing something about it."

The bug's existence was revealed nearly a month ago. Since then, criminals have pulled off at least one successful attack, directing some AT&T Inc. Internet customers in Texas to a fake Google site. The phony page was accompanied by three programs that automatically clicked on ads, with the profits for those clicks flowing back to the hackers.

There are likely worse scams happening that haven't been discovered or publicly disclosed by Internet service providers. "You can bet that the (Internet providers) are going to stay tightlipped about any attacks on their networks," said HD Moore, a security researcher.

The AT&T attack probably would have stayed quiet had it not affected the Internet service of Austin, Texas-based BreakingPoint Systems Inc., which makes machines for testing networking equipment and has Moore as its labs director. He disclosed the incident in hopes it would help uncover more breaches.

The underlying flaw is in the Domain Name System (DNS), a network of millions of servers that translate words typed into Web browsers into numerical codes that computers can understand.

Getting from one place to another on the Internet typically requires a trip through several DNS servers, including some that accept incoming data and store parts of it. That opens them up for potential attack.<

What this means is that a computer user in say, San Francisco, might type and head straight to the real Yahoo site, while at the same moment, a user in New York — whose traffic is routed through different DNS servers — might type that same Web address and end up on a phony duplicate site.

Scant details have been available about how the vulnerability works.

The researcher who discovered it, Dan Kaminsky of Seattle-based computer security consultant IOActive Inc., announced July 8 that he'd found a major weakness in DNS. But he kept the rest secret because he wanted to give companies that run vulnerable servers a month to apply patches — software tweaks that cover the security hole. He coordinated with Microsoft Corp., Cisco Systems Inc., Sun Microsystems Inc. and other major vendors to simultaneously issue patches.

He got two weeks before bad guys and good guys alike accurately guessed the basics of what Kaminsky discovered.

It is this: By adding bad information to the packets of data zooming in and out of certain DNS servers, hackers can swap out the address of a legitimate Web site and insert the address of their malicious Web site instead.

A compromised server believes it's sending people to the authentic site. And if the bogus site is designed well enough, users don't know the difference, unless the site starts behaving weirdly.