Two new vulnerabilities have been discovered in Internet Explorer which allow a complete bypass of security and provide system access to a computer, including the installation of files on someone's hard disk without their knowledge, through a single click.

Worse, the holes have been discovered from analysis of an existing link on the Internet and a fully functional demonstration of the exploit have been produced and been shown to affect even fully patched versions of Explorer.

It has been rated "extremely critical" by security company Secunia, and the only advice is to disable Active Scripting support for all but trusted websites.

The discovery stems from Dutch researcher Jelmer who was sent an Internet link which he was warned used unknown Explorer vulnerabilities to install adware on his computer. He found it did and embarked on a detailed analysis of the link, which demonstrates an extremely sophisticated use of encrypted code to bypass the Web browser's security.

In simple terms, the link uses an unknown vulnerability to open up a local Explorer help file -- ms-its:C:\WINDOWS\Help\iexplore.chm::/iegetsrt.htm. It delays executing anything immediately but instead uses another unknown vulnerability to run another file which in turn runs some script. This script is then used to run more script. And finally that script is used to run an exploit that Microsoft Corp. has been aware of since August 2003 but hasn't patched.

That exploit -- Adodb.stream -- has not been viewed as particularly dangerous, since it only works when the file containing the code is present on the user's hard disk. The problem comes in the fact that the Help file initially opened is assumed to be safe since it is a local file and so has minimal security restrictions.

By using the unknown exploits, code is installed within the help file window, all security efforts are bypassed, and the Adodb.stream exploit is then used to download files on the Internet direct to the hard disk.

What this means in reality is that if you click on a malicious link in an email or on the Internet, a malicious user can very quickly have complete control of your PC. And there is no patch available.

With the code already available on the Net, this is effectively a security nightmare ... unless you're a Mozilla or Opera user that is.