Malicious software that exploits the security flaw to download a Trojan horse to vulnerable computers has been found on the Internet, according to Microsoft. Detection and removal capabilities for the "TrojanDownloader:Win32/Delf.DH" have been added to Microsoft's recently launched online security-scanning tool.
"Customers can visit Windows Live Safety Center and are encouraged to use the Complete Scan option to check for and remove this malicious software and future variants," Microsoft said in its updated security advisory on the issue.
The security bug, exploited by the Trojan downloader, was originally reported in May. The bug was thought to only allow for a denial-of-service attack, which would cause IE to close. However,because it was discovered that it could be used to remotely run code on a vulnerable computer.
Microsoft has yet to provide a fix for the vulnerability, but is working on a patch, according to the security advisory. Security-monitoring company Secunia deems the problem "extremely critical," its rarely given highest rating.
The vulnerability puts computers running Windows 98, Windows Millennium Edition, Windows 2000 and Windows XP at risk. An attacker could gain complete control of vulnerable systems by hosting malicious code on a Web site. Once an IE user visits the site, the malicious program would run without any user interaction.
Microsoft offers several workarounds to deflect attacks. These include changing IE settings to disable active scripting or prompt the user before running such scripts.