Microsoft Corp. now has a patch available for the security hole in Internet Explorer Versions 5.5 and 6 that can expose cookie data to malicious hackers.
The vulnerability was first publicized last week by Online Solutions Ltd., a Finnish security firm that alerted Microsoft to the vulnerability Nov. 1 but released the existence and details of the exploit before Microsoft issued a patch.
Microsoft then 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 lies in the ability of a malicious hacker to write an intentionally malformed URL in a Web page address. The action allows a hacker to see the cookies deposited by other Web sites on the user's hard drive. While proper security practice wouldn't allow sensitive information to be stored in those cookies, some Web sites do place credit card and other personal information in the 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 unauthorized access to a user's cookies and potentially modify the values contained in them."
The bulletin goes on to say that "the third vulnerability is a new variant of a vulnerability discussed in Microsoft Security Bulletin MS01-051 affecting how IE handles URLs that include dotless IP addresses. ... [IE] would treat the site as an intranet site, and open pages on the site in the Intranet Zone rather than the correct zone. This would allow the site to run with fewer security restrictions than appropriate. This vulnerability does not affect IE 6."