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Published: 2010-04-21,
Last Updated: 2010-04-21 21:08:19 UTC
by Guy Bruneau (Version: 3)
16 comment(s)

McAfee's "DAT" file version 5958 is causing widespread problems with Windows XP SP3. The affected systems will enter a reboot loop and loose all network access. We have individual reports of other versions of Windows being affected as well. However, only particular configurations of these versions appear affected. The bad DAT file may infect individual workstations as well as workstations connected to a domain. The use of "ePolicyOrchestrator", which is used to update virus definitions across a network, appears to have lead to a faster spread of the bad DAT file. The ePolicyOrchestrator is used to update "DAT" files throughout enterprises. It can not be used to undo this bad signature because affected system will lose network connectivity.

The problem is a false positive which identifies a regular Windows binary, "svchost.exe", as "W32/Wecorl.a", a virus. If you are affected, you will see a message like:

The file C:WINDOWS\system32\svchost.exe contains the W32/Wecorl.a Virus. 
Undetermined clean error, OAS denied access and continued. 
Detected using Scan engine version 5400.1158 DAT version 5958.0000.

McAfee released an updated DAT file, and an "EXTRA.DAT" file to fix the problem. An EXTRA.DAT file is a patch to just fix the bad signature. McAfee's support web sites currently respond slowly and are down at times, likely due to the increased load caused by this issue.

Several readers reported that this procedure worked to recover:

1 - Boot the system in "Safe Mode"
2 - copy extra.dat in c:/program files/common files/mcafee/engine
3 - reboot.

If you lost "svchost.exe", then you need to copy it back to c:/Windows/system32/svchost.exe while in safe mode. This fix has to be applied locally at the workstation. However, it may be possible to do this remotely if your workstations support Intel's "vPro" technology. We should have a link to instructions shortly.

Our reader Jim wrote in about how he managed to get some systems back remotely, as long as svchost.exe was not deleted or moved. In this case, the computer will be in a reboot loop. Here is what Jim wrote:

I created a batch file to run the following command:

echo f | xcopy.exe {server}netlogonextra.dat 
   "c:program files\common files\mcafeeengine\extra.dat" /R /Y

I put this batch file and the extra.dat in the netlogon folder.

I then set the computer configuration>windows settings>scripts>startup to run this command in a GPO that gets applied to all computers.  Then link the GPO to the domain root, or wherever is appropriate.

Upon reboot the computers process this command and so far we seem to be good to go, "mostly".  There have been a few cases where the files end up missing.


ISC reader Linnie wrote in and indicated this method works as well:

- Copy the EXTRA.dat file to c:program files\common files\mcafeeengine
- copy the svchost.exe to c:windows\system32
- Reboot, everything is back to normal.


Additional information from McAfee: http://community.mcafee.com/thread/24056?tstart=0
McAfee Knowledgebase Article: https://kc.mcafee.com/corporate/index?page=content&id=KB68780
EXTRA.DAT file: http://home.mcafee.com/VirusInfo/VirusProfile.aspx?key=265240.

THANKS TO ALL THE CONTRIBUTORS! We got too many to mention here. Please keep it coming using our contact page: http://isc.sans.org/contact.html . I haven't counted the submissions, but they may be at a hundred now. Some report about networks with thousands of down machines and organizations who had to shut down for business until this is fixed.

NOTE: We do have some journalists who would like to talk to someone whose network got affected. In particular, if the network was large. Let us know if you are willing to talk about this in public. We will not forward your contact information unless you specifically ask us to.



Guy Bruneau IPSS Inc. gbruneau at isc dot sans dot org

16 comment(s)


It deletes the svchosts.exe. So that mean no networking when the machine comes backup. So roll back is going to involve a USB stick and a new pair of shoes after walking to 1000 hosts :(
posted by Angry, Wed Apr 21 2010, 17:23
We're seeing this issue as well. Only SP3 so far. We were able to stop the spread of the bad DAT file with our ePO server and were able to push out the old DAT that helped some of our machines. We also had some laptop users get the DAT file from home or while coming into the office.

One fix is to delete the bad DAT file the client at "C:\Program Files\Common Files\McAfee\Engine". Delete any av*.dat. Then reboot and the old DAT should be grabbed.

We now have about 100 machines that are in a reboot loop. It looks like the svchosts is in quarantine.
posted by chris, Wed Apr 21 2010, 18:06

I can remember such poisonous update about 10 years ago and we had flushed McAfee right there and then for Symantec
posted by prontissimo, Wed Apr 21 2010, 18:46
Took out almost all of our 600 users in head office and call centers.. What a mess....
Having to touch each machine to get it back...
posted by trevc, Wed Apr 21 2010, 18:58
We have a fix for this issue
you can find it on my blog post at http://cosine-security.blogspot.com/2010/04/mcafee-dat-5958-fix.html
posted by TheLightcosine, Wed Apr 21 2010, 19:09
I just configured our policy to exclude "svchost.exe" for virus scan to avoid this problem.

We have not been affected yet. Do you think this will work?
posted by Brent, Wed Apr 21 2010, 19:29
I just configured our policy to exclude "svchost.exe" for virus scan to avoid this problem.

We have not been affected yet. Do you think this will work?
posted by Brent, Wed Apr 21 2010, 19:29
None of our XP SP3 machines seemed to be affected it by it yet. All but 12 are now at the 5959 level and I dropped the extra.dat in the EPO repository as well.
posted by BrianR, Wed Apr 21 2010, 19:50
We have an evaluation group of systems set-up to receive dats before rolling out to production. Model has served us well.
posted by Scooter, Wed Apr 21 2010, 21:51
Update toi the fix I originally psoted on my blog. There appears to be a 'safer' way to recover the svchost binary. While it is a less convenient method, it is probably the best way. More at:
posted by TheLightCosine, Wed Apr 21 2010, 23:03
I got a problem around 13:00. It is not like reported in thi blog, but the svchost.exe has been deleted and windows got in real problem.

No reboot loop, no message, but about nothing was working. IE not accessible. Application working with windows XP SP3 not running.

I have contacted Technical Support by chat on another machine (without McAfee) and I have never been told that there was any problem with the latest update although I asked for directly. I finally found how to fix my computer through a blog. I was lucky, but I am angry because Mc Afee never told that there was a problem, on the web site, Technical Support or other media.

An the tel center ... what an annoying thing! Sales centered not client centered.
posted by JY, Thu Apr 22 2010, 03:44
I have no idea how my firm dodged this bullet. We are currently sitting on version 5960 and use Sonicwall to push our updates out to the workstations.
When the news broke, I was cringing waiting for the phone to start ringing off the hook.....silence IS golden!
posted by sc, Thu Apr 22 2010, 19:44
I ran into a problem with one of our computers. I hadn't read this article, yet. And so, when I discovered that one of our systems was missing its network connections, I attempted to repair the operating system using a Windows setup disk. The repair made it to the Installing Devices portion and then rebooted. Now the system is stuck in an endless boot loop in the middle of the repair. I have tried going into safe mode. It does not matter what I do. it wants to continue running the repair, but it can't. Now that I know about the Mcafee problem, I could fix it if I could just get back to the desktop. Do you have any suggestions for breaking out of the Windows XP Repair loop?
posted by rallison, Thu Apr 22 2010, 20:11
we are dumping mcafee, we have over 3000 machines go down almost half had svchost.exe deleted.
posted by bob, Thu Apr 22 2010, 20:58

- http://www.symantec.com/connect/blogs/malware-authors-taking-advantage-mcafee-false-positive
April 22, 2010 - "... We have seen poisoned search results since the problem first surfaced. Search terms such as McAfee, 5958, or DAT are returning results that can lead to malicious and fake antivirus scan sites, resulting in the installation of malware... This attack by the malware creators is quite insidious since many of the people searching for information about this problem are most likely already affected by the problem and are looking for a solution using another computer..."
posted by PC.Tech, Fri Apr 23 2010, 04:54
I've written an open source fix for this McAfee problem. You can find it at: http://minjs.org/svchostfix/
posted by Maurits, Sat Apr 24 2010, 19:32
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