URL Parsing Bug in IE Invites Phishing Attacks

The bug, which affects fully patched versions of IE, lets malicious sites assume the privileges of more trusted zones.

A bug in fully patched versions of Microsofts Internet Explorer Web browser allows violations of the browsers security zones, with the result that an unknown malicious site could assume the privileges of more trusted zones.

Researchers on several security mailing lists have been discussing the bug since yesterday and appear still to be learning about it.

The same vulnerability could be used for phishing attacks, in which the URL in the address bar appears to be for one site but is in fact a link to a different site. Both of these problems rely on certain misconfigurations of the DNS and Web server on the malicious Web site.

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The malformed URL contains text on the left side—which may or may not be a valid address—the actual domain on the right and a particular syntax in between.

If the domain on the right-hand side of the URL is properly misconfigured, the browser will go to that site and put it in the zone belonging to the site on the left. If an arbitrary name is used on the left, the site is put in the local intranet zone, which has less restrictive privileges than the default Internet zone.

There are many mitigating factors to this problem. Attacks involving malicious Web sites are usually exploited via throwaway, free Web site services that allow a certain amount of anonymity on behalf of the attacker.

But this attack requires control of the DNS and Web server configuration, so the attacker would have more difficulty remaining anonymous. The phishing aspect of it is arguably more of a problem concerning the DNS configuration of the target site.

Microsoft said it is investigating the matter and will take appropriate actions.

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