Microsoft has released a patch that addresses six security vulnerabilities in its Internet Explorer browser, including a critical flaw that could allow an attacker to run code on a client machine. The patch is intended for Internet Explorer 5.01, Internet Explorer 5.5 and Internet Explorer 6.0.
Among the changes that are provided by the patch is a fix that closes a vulnerability in one of Internet Explorer's local HTML resources. One of the HTML files shipped with Internet Explorer contains a cross-site scripting vulnerability, which could allow an attacker to execute a script on a user's computer, Microsoft said in a security bulletin.
The patch also addresses two information disclosure vulnerabilities, which could allow an attacker to read, but not add, delete or change, data on a user's computer. Microsoft rated both the cross-site scripting vulnerability and the information disclosure vulnerabilities as critical flaws.
The patch also fixes a zone-spoofing vulnerability that allows a Web page to be viewed in Internet Explorer's Trusted Sites zone, allowing an attacker's Web page to be viewed with fewer security restrictions on a user's PC.
The final vulnerabilities that the patch fixes are two content-disposition vulnerabilities, which could allow an attacker to fool Internet Explorer into thinking a malicious download is safe. Microsoft rated these vulnerabilities "low" to "moderate".
But the patch doesn't fix all the problems it purports to, according to security researchers. The patch only fixes the cross-site scripting issue on one of the listed browsers, according to two security researchers who sent e-mail to the Bugtraq security e-mail list after the patch's release.
According to Microsoft's explanation of the issue, the flaw can only be exploited when a user clicks on an HTML link on a Web page or in an e-mail message. That's not true, as code embedded in an HTML file can automatically execute, according to both Thor Larholm, a security researcher who has discovered a number of Microsoft vulnerabilities and maintains a list of unpatched IE holes online, and the Israeli security group GreyMagic Software, which has also discovered a number of browser vulnerabilities. As a result, users can unwittingly launch malicious code simply by opening an infected e-mail message.
The patch doesn't completely fix the problem because the flaw resides in the dialogArguments component of IE, which is not addressed by the patch, both researchers said. Furthermore, though Microsoft claims the flaw only exists in IE 6, both researchers maintain that the problem is also found in IE 5.01 and 5.5.
"Microsoft is aware of the issues and is investigating the reports," a Microsoft spokesman said. Microsoft maintains that the patch does what the company said, but it is also investigating the researchers' claims, the spokesman said.
This isn't the first time that a Microsoft patch has caused problems for users. Another IE patch, released in February, caused the browser to crash.
The patch is available for download from Microsoft's Web site at http://www.microsoft.com/windows/ie/downloads/critical/q321232/default.asp