Breaking its normal security patch cycle, Microsoft has rushed out a fix for a critical bug in Internet Explorer versions from 6.0 up to but not including Windows XP Service Pack 2 (SP2). The bug in question is in the way IE handles two attributes of the "frame" and "iframe" HTML elements, which has already been exploited using overly long SRC and NAME attributes to cause IE to execute an attacker's shell code.
"Normally, one would be subject to attack by browsing a Web page that has the attack built into it, but with very old, unpatched e-mail clients, it is also possible to be compromised through HTML e-mail."
As mentioned above, XP SP2 systems and Windows Server 2003 are not affected by the bug. Patches are available for Windows XP, Windows 2000, Windows NT 4.0, Windows ME and Windows 98.