The vulnerability was first publicised by Online Solutions, a Finnish security firm that alerted Microsoft to the hole on 1 November but released the existence and details of the exploit before Microsoft issued a patch.
Microsoft posted an advisory and recommended that users disable Active Scripts in IE to prevent their cookie data from being stolen. However, disabling active scripts also renders some Web sites unusable.
The vulnerability allows a malicious hacker to write an intentionally malformed URL in a Web page address. The action enables a hacker to see the cookies deposited by other Web sites on the user's hard drive. While proper security practice would not allow sensitive information to be stored in those cookies, some Web sites do place credit card and other personal information in cookies.
A malformed Web address link in an HTML e-mail would also expose cookie data.
The patch shuts the ability of one Web site to grab information left by another Web site. The patch also addresses three previously undisclosed problems, according to the revised bulletin.
"The first two involve how IE handles cookies across domains," the bulletin states. "Although the underlying flaws are completely unrelated, the scope is exactly the same - in each case, a malicious user could potentially craft a URL that would allow them to gain unauthorised access to a user's cookies and potentially modify the values contained in them."