A patch Microsoft Corp. released on Monday for a dangerous Internet Explorer vulnerability that lets attackers trick Internet users into visiting malicious sites doesn't completely fix the problem.

The cumulative patch addresses, among other problems, a display bug likely to be used by phishing attacks. If the attacker uses a particular malformed URL syntax, only a portion of the actual address would appear in the address bar, creating the opportunity for the attacker to give a false impression of the site being visited.

The MS04-004 patch addresses this bug, but not a related problem. If the user visits a Web page containing such a malformed link and hovers the mouse over the link or selects it by tabbing through links in the page, the patched version of Internet Explorer will display the partial URL in the status bar.

The following link illustrates the problem:
This appears to be a link to http://www.paypal.com, but isn't.
The actual target of the link is "http://www.paypal.com%00%[email protected]." On an unpatched copy of Internet Explorer, clicking the link will open a new window and bring the browser to security.eweek.com, the eWEEK.com Security Topic Center. On a patched copy of IE the browser will go to an error page indicating illegal syntax. On either version of IE, if you hover over the link on this page, the status bar will display http://www.paypal.com

Ironically, the cumulative patch also fixed another bug in a different IE cumulative update from last year. That cumulative patch addressed several security issues in Internet Explorer, but also introduced bugs in the behavior of the IE scrollbar. The new patch fixes these bugs.