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Re: denial-of-service vulnerability in the Microsoft Malicious Software Removal Tool
From: webDEViL <w3bd3vil () gmail com>
Date: Sun, 23 May 2010 23:57:42 +0530

All said and done, that doesn't make it a vulnerability.

On Sun, May 23, 2010 at 11:47 PM, lsi <stuart () cyberdelix net> wrote:

On 23 May 2010 at 16:34, Thor (Hammer of God) wrote:

From:                   "Thor (Hammer of God)" <Thor () hammerofgod com>
To:                     "full-disclosure () lists grok org uk" <full-
disclosure () lists grok org uk>
Date sent:              Sun, 23 May 2010 16:34:24 +0000
Subject:                Re: [Full-disclosure] denial-of-service
vulnerability in the
       Microsoft       Malicious Software Removal Tool

And where's the part where the system was rendered unbootable?

The unbootable part comes when you replace NDIS.SYS.  Unless you know
to replace the registry keys first, which is certainly not obvious
from the MRT log.

And how did your users get infected with Cutwail?  Let me guess...
they are all still running XP and you've got them running as local
administrators right?  And they get to download codecs "willy nilly"
and are probably using Bittorrent to get illegal copies of software
pre-infected with cutwail, right?

How do I know how they got infected?  These are all third-party
companies (my customers), sometimes when they have cash problems,
they don't call me, they try and do it themselves, or do nothing. I
might not see them for months. They don't want to upgrade - they
heard about Vista (LOL) and they don't have, or don't want to spend
the money.

This is reality, not some managed datacentre in Redmond.

local administrators

Their apps needed it last I checked.  I didn't set up their machines.
They have not asked me to look at that.  I have enough trouble
getting work OK'd without putting my neck on the line suggesting a
configuration change which I cannot guarantee will not cause
instability, particularly with their legacy and unsupported software,
of which there is plenty.

Again, this is reality, not some managed datacentre in Redmond.


No, like this:

"Stuart, need your help. My computer has a virus. Yesterday night I
opened an email that I was expecting from a Bernice. It turned out
that it was the wrong Bernice and it was a virus. It loaded Security
Essentials 2010 which is a scarevirus to make the user believe that
there are virus a pay for their software which does nothing anyway.
It  has loaded a virus in the registry file. There is a lot about it
on  the net. I then found a PC tools download to remove. However when
I  turned mycomputer off it does not now allow me to log on. I have
turned it off. I am without a PC now. Can you come tomorrow to
resolve  it for me? Many thanks. Please let me know ad I need it

Regardless, let's see if we have your advisory correct.  In order to
be a victim of this "Denial of Service Vulnerability" we must first
get infected with something like Cutwail


that runs with user interaction

false.  Cutwail has no known infection vectors.  However, Cutwail is
just an example.

interaction and also requires administrator privileges (you can see
that NDIS.SYS was altered).

When I am logged in as Admin and try to replace NDIS.SYS, Windows
File Protection replaces it.  Why did WFP fail to protect the file
against Cutwail in the first place, and how can a virus replace
NDIS.SYS using Administrative privs, if I cannot do it myself when

Of course, your AV must be at least 2 years old too.

false, it was up-to-date, although I am questioning its effectiveness

 Then, once we get infected with malware, we run MRT,
and see in the logs that it was successfully removed and requires a

Actually, AV found the virus in NDIS.SYS but could not remove it.  So
I ran MRT because I thought that a Microsoft product would know this
is a Windows file that cannot simply be deleted.  MRT says it's done
and needs reboot, so I reboot... and the system is toast.

To clarify, in this particular case, the first reboot, you can login
in normal mode, but cannot use any network adapters (code 39 - driver
corrupted or missing).  Reinstalling the drivers doesn't help.  So
then you think, oh that's because NDIS was trashed by MRT, so I'll
just replace NDIS.SYS....

And thats when you get the BSOD on boot to normal mode.  So then you
need to figure out that the cause of that BSOD is a missing registry
key, then you need to figure out which keys (there are three, for
each controlset), then you need to get the correct keys from a clean
machine, then you need to figure out how to replace the keys (some of
them cannot be imported with mere Administrative permissions).

However, just last week I also fixed a problem with the userinit
registry key, also mysteriously deleted - why would a virus trash its
host?  Answer: it doesn't, I think it was MRT that trashed it.  A
missing userinit key means instant logoff on logon, even in safe mode
as Administrator.  I might be able to dig up the MRT log for that
machine (would be interesting to see whether it was in fact MRT that
did it).  Want to place bets now?

From a quick look at the web, MRT has also in the past deleted
Internet Explorer (iexplore.exe).  Oh, the poetry....

The point of my mail was that anyone can innocently run MRT and it
may trash their box.  This is due to one or more design flaws in the
MRT, and in Windows itself.  Are you saying I should just sit on this
info?  If someone had told me MRT was going to trash my customer's
machine, I would not have wasted most of last week fixing it.


-----Original Message-----
From: full-disclosure-bounces () lists grok org uk [mailto:
bounces () lists grok org uk] On Behalf Of lsi
Sent: Sunday, May 23, 2010 9:16 AM
To: full-disclosure () lists grok org uk
Subject: [Full-disclosure] denial-of-service vulnerability in the
Malicious Software Removal Tool

denial-of-service vulnerability in the Microsoft Malicious Software

platforms affected: Windows
distribution: wide
severity: high

Description of the vulnerability:

The Microsoft Malicious Software Removal Tool (MRT) is a program used to
remove malware from infected Windows systems.  However, MRT does not
always correctly repair the system.  In at least one case, the changes
made by
MRT can render the system unbootable (log below).
Repair can be time-consuming and expensive, particularly as the error
messages and log files of the software concerned are cryptic and
uninformative, or non-existent.

As MRT runs automatically in the background once a month, these changes
the system may be made without the knowledge of an Administrator (or
the user).

Suspected cause:

Missing logic in MRT to repair the system, rather than just deleting
stuff willy-


1. Do not run MRT manually.

2. Disable MRT if possible, especially on mission-critical machines.

3. Do not use Windows.

Details of notification to vendor:


Sample of the fault:

Microsoft Windows Malicious Software Removal Tool v3.7, May 2010 Started
On Tue May 18 21:24:47 2010

Threat detected: VirTool:WinNT/Cutwail.L
       SigSeq: 0x00008A78910FD971
       SHA1:   DEFB65309ABB3DD81F223ABA7CDB9EB26D66611A



Quick Scan Removal Results
Start 'remove' for
Operation succeeded !

Start 'remove' for service://NDIS
Operation was scheduled to be completed after next reboot.

Start 'remove' for
Operation succeeded !

Start 'remove' for driver://NDIS
Operation was scheduled to be completed after next reboot.

Start 'remove' for file://\\?\C:\WINDOWS\system32\drivers\NDIS.sys
Operation succeeded !

Results Summary:
For cleaning VirTool:WinNT/Cutwail.L, the system needs to be restarted.
Microsoft Windows Malicious Software Removal Tool Finished On Tue May
18 21:31:29 2010

Return code: 10 (0xa)

Stuart Udall
stuart at () cyberdelix dot net - http://www.cyberdelix.net/

 * Origin: lsi: revolution through evolution (192:168/0.2)

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Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/

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