Microsoft Investigating Windows VBScript Security Hole

• By Jabulani Leffall
• 03/03/10

Microsoft released a security advisory Monday describing a zero-day vulnerability involving some older Windows versions and VBScript when used with Internet Explorer.

The vulnerability permits a complicated exploit only on systems using Windows 2000, Windows XP, and Windows Server 2003. Newer Windows versions aren't affected, according to the advisory. To pull off the exploit, users have to be diverted to a malicious Web site, and they have to push the F1 (Help) button on the keyboard while a script is running.

Friday, third-party security firm iSEC Security Research published a proof of concept description of the vulnerability. Microsoft responded Sunday in a blog post by Jerry Bryant, a senior manager with the Microsoft Security Response Center. Microsoft was investigating "new public claims of vulnerability involving the use of VBScript and Windows Help (HLP) files within Internet Explorer," according to Bryant.

The exploit could be triggered by "passing [a] malicious .HLP file to winhlp32," according to iSEC. It would permit an arbitrary command to be run by an attacker. iSEC also pointed to "a stack overflow vulnerability in winhlp32.exe."

The exploit allows a remote code execution attack via a malicious Web page. Microsoft's security advisory says that if "a malicious Web site displayed a specially crafted dialog box and a user pressed the F1 key," then arbitrary code could be executed during that same browser session.

Bryant said that Microsoft has not yet seen any evidence of attacks exploiting this vulnerability. Windows Vista, Windows 7, Windows Server 2008, and Windows Server 2008 R2 are not affected, according to Microsoft.

However, Bryant conceded that an inherent weakness exists involving the use of VBScript and Windows Help files with Internet Explorer.

"Windows Help files are included in a long list of what we refer to as 'unsafe file types'," he wrote in the blog post. "These are file types that are designed to invoke automatic actions during normal use of the files. While they can be very valuable productivity tools, they can also be used by attackers to try and compromise a system."

Microsoft did not appreciate how the vulnerability was reported. Bryant alluded to a need for "responsible disclosure," and the security advisory noted that it "was not responsibly disclosed." However, neither Bryant's note, nor the advisory, mentioned iSEC's proof of concept.

Microsoft may issue a security update, possibly through its monthly patch cycle. In the meantime, Microsoft suggested a workaround by changing Internet Explorer's security option.

"Setting the Internet zone security setting to High protects against this vulnerability by disabling Active Scripting, which is required in order to exploit this vulnerability," the security advisory noted. Microsoft also advised users against hitting the F1 key when prompted to do so by Web sites.