IOActive Inc.

A powerful new type of Internet attack works like a telephone tap, except operates between computers and Web sites they trust.

Hackers at the Black Hat and DefCon security conferences have revealed a serious flaw in the way Web browsers weed out untrustworthy sites and block anybody from seeing them. If a criminal infiltrates a network, he can set up a secret eavesdropping post and capture credit card numbers, passwords and other sensitive data flowing between computers on that network and sites their browsers have deemed safe.

A newly discovered flaw in the Internet's core infrastructure not only permits hackers to force people to visit Web sites they didn't want to, it also allows them to intercept e-mail messages, the researcher who discovered the bug said Wednesday.

Considering the silent nature of the attack and the sensitive nature of a lot of electronic correspondence, the potential for damage from this second security flaw is high. But there's no evidence yet that this method of targeting e-mail has been used in a successful attack.

When Internet providers hire third-party companies to serve up advertisements on unused Web pages, that creative attempt to make money can open major security vulnerabilities they can't control, a researcher has found.

One such vulnerability -- described last weekend at a security conference by Dan Kaminsky, director of penetration testing for Seattle-based computer security consultant IOActive Inc. -- works like this: