Security researchers this week warned of a vulnerability in most Web browsers which could potentially allow scammers to launch phishing attacks from pop-up windows on trusted Web sites.

The vulnerability arises when an Internet user opens browser windows for both a legitimate Web site and a malicious site at the same time. Because of an old functionality that exists in most browsers, the malicious site can potentially display information in a pop-up window from the trusted site, according to Secunia Research.

The vulnerability has yet to be exploited but could present a very effective method for launching online fraud scams, often known as phishing, Secunia CTO Thomas Kristensen said Thursday.

While most users do not intentionally visit malicious Web sites, they often stumble upon them by following links, making it relatively common for 'Net surfers to have browser windows open for both legitimate and malicious sites at the same time, Kristensen said.

This could be a particularly dangerous situation if exploited to display misleading information on a pop-up window from a legitimate bank Web site, for example, he warned. Even if savvy users check for a the yellow "lock" icon on a Web site, signifying encryption, the pop-up could still display content from the malicious site, he said.

"This could be a surprisingly effective way to seduce or trick people into doing something," Kristensen said.

The vulnerability affects almost all browsers, including Internet Explorer, Mozilla, Firefox, Opera, Konqueror, Safari and Netscape, the researcher said.

Secunia, in Copenhagen, Wednesday went public with its warning, after saying that it had alerted browser vendors of the vulnerability months ago.

Microsoft Thursday said that it has investigated the report, and customers who use Windows XP SP2 and follow its advice on spoofing attacks are at a reduced risk.

The functionality described in the report allows a Web site to open or re-use a window without displaying the address bar. However, SP2 users will see a status bar in the pop-up window, allowing them to look for the yellow lock icon and confirm that the site is valid, Microsoft said.

Opera has also included measures to mitigate the vulnerability in the latest beta version of its software, Kristensen said.

The IDG News Service is a Network World affiliate.