This is a cumulative patch for Internet Explorer 5.5 & 6.0. In addition to including the functionality of all previously released patches for Internet Explorer 5.5 & 6.0, it also eliminates a newly discovered flaw in Internet Explorer's cross-domain security model. This flaw occurs because the security checks that Internet Explorer carries out when particular object caching techniques are used in web pages are incomplete. This could have the effect of allowing a website in one domain to access information in another, including the user's local system.

Affected Software:
Microsoft Internet Explorer 5.5
Microsoft Internet Explorer 6.0

Patch availability:
http://www.microsoft.com/windows/ie/downloads/critical/q324929/default.asp

As you can see here, the problems that this patch addresses have now been upgraded to critical level.