A new spoofing flaw in Microsoft's Internet Explorer browser allows an improperly coded web link to send users to a diffferent URL than the one displayed in the status bar.

The flaw, which was posted to the Bugtraq mailing list by Benjamin Franz, is exploited by placing two URLs and a table within a single HTML href tag, producing a link that looks like this:

http://www.microsoft.com

displaying http://www.microsoft.com in the browser, but sending the user to Google. Franz says the exploit works in fully-patched versions of Internet Explorer and Outlook Express, meaning the HTML code can be used to create spoofed URLs in webpages and HTML e-mails.

The technique, which can be executed by anyone with basic knowledge of HTML, can be used to construct convicing fake URLs for use in phishing scams. The flaw is possible because Internet Explorer has difficulty processing improperly formed HTML. The attack opens one href tag, and then leaves that tag open while enclosing a second URL within a table. The browser displays the first URL in the status bar, but sends users to the second URL.

The flaw affects versions of IE up to 6.0.2800.1106 - which includes systems that haven't yet installed Windows XP SP2, but are current on all other critical updates from Windows Update - as well as the Safari browser for Macs. Users running Windows XP SP2 (IE version 6.0.2900) and the open source Firefox and Mozilla browsers are not affected.

Phishing attacks seek to trick account holders into divulging sensitive account information through the use of e-mails which appear to come from trusted financial institutions and retailers. Several previous URL spoofing weaknesses in Internet Explorer have been widely used by phishing attacks. The ability to display a fraudulent URL in the status bar is especially useful, as security-conscious users would check the status bar before clicking through. The technique does not disguise the URL displayed in the address bar upon arrival at the destination page, meaning alert users will recognize the spoof at that point. But the tactic could be used to send e-mail recipients and web surfers to pages that attempt to download malware upon loading, a common tactic used by phishers to install trojans and keyloggers.

Netcraft has developed a service to help banks and other financial organizations identify sites which may be trying to construct frauds, identity theft and phishing attacks by pretending to be the bank, or are implying that the site has a relationship with the bank when in fact there is none.