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Re: Invalid WINS entries
From: "Fulton L. Preston Jr." <fulton () PRESTONS ORG>
Date: Wed, 17 Jan 2001 23:04:42 -0500

Interesting perl script but this method works just as well for a good
old fashioned DDoS:

About two years ago a friend of mine and I did something similar with a
Samba server.  I configured Samba to be a PDC for a Domain and pointed
it towards my friends WINS server for his network.  The Samba domain
name was the same as his.  Using snoop on a Sun box I watched the
traffic between the Linux server with Samba and the NT WINS server when
I started the Samba server.  I don't have the captures anymore but here
is a basic re-creation of the events:

Samba to WINS: I am the PDC
WINS to Samba: Uh, I don't think so.
Samba to WINS: Yes I am
WINS to Samba: Didn't you hear me the first time?
Samba to WINS (using Jedi hand waving motion):  <<I AM THE PDC>>

Elapsed time: 2 seconds.

Result:  No one in that NT Domain could login, print, or use Outlook.
It even caused the Exchange server to wig out.  All workstations and
member servers started sending their auth requests to the Samba server.
I didn't check to see if it was LanMan or NTLM, that was not the point
of the experiment at that time.

Solution: Use static IP's for PDC/BDC's and enter static entries into
WINS.  More importantly, block TCP/UDP 135-139 at firewalls (or at least
only pass trusted IP's)

Interesting side effect:  The PDC would not auth anybody even when
static entries into WINS were added and the Samba server was shutdown.
A reboot of the PDC and BDC was necessary to restore the trust of the

Lesson learned:  Large corporations or military bases with large scale
networks that let ports 135-139 through the firewalls without filtering
are subject to this type of attack.  Imagine a 25,000+ user network not
able to login, print, etc...  The overall costs are pretty staggering,
especially if it continues for days, even more sickening is how many
passwords could be snatched by doing this, even if only for a few
minutes.  Sad part of this lesson is that way too many large networks
are STILL vulnerable to this.  The only thing needed is a WINS server
address.  In the case of some NT DNS servers that information is easily
harvested since there is a value for WINS recursive lookup if it is set
by the admins (read: EVIL! Do not do this, only for internal DNS servers
behind firewalls if it must be done!)

Fulton Preston

[This is supposed to be an annoying signature.  Annoyed yet?]

-----Original Message-----
From: Byrne, David [mailto:dbyrne () TIAA-CREF ORG]
Sent: Wednesday, January 17, 2001 4:36 PM
Subject: Invalid WINS entries

After playing around with some WINS problems we were having, I
something that doesn't seem to bother very many people. WINS does
nothing to
verify the 1Ch (domain controllers) registrations sent to it. This
allows an
attacker to overwrite some or all of the Domain Controllers in the
The new entries could be pointing at a box that will participate in the
logon process long enough to capture user names and passwords. If the
passwords are only hashed with LanMan (not NTLM), they can be easily
with L0phtCrack. A less malicious problem can occur if someone brings up
server that incorrectly thinks it is a Domain Controller. Although the
server cannot participate in the domain, it will register itself with
in the 1Ch record and workstations will still send logon requests to it.

The best work around I could think of is to use static entries for
that are sensitive (there are probably more besides 1Ch). Domain
shouldn't be changed very often, so the management work would be
When I contacted Microsoft, I was told that they were aware of this, but
not consider it a significant problem. They confirmed that static
were the best solution.

Attached is a PERL script that can demonstrate the problem. Use it

David Byrne, MCSE


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