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Title: Flaw in NetBIOS Could Lead to Information Disclosure
(824105)
Date: 03 September 2003
Software:

• Microsoft Windows NT 4.0 Server
• Microsoft Windows NT 4.0, Terminal Server Edition
• Microsoft Windows 2000
• Microsoft Windows XP
• Microsoft Windows Server 2003

Impact: Information Disclosure
Max Risk: Low
Bulletin: MS03-034

Microsoft encourages customers to review the Security Bulletins at:
http://www.microsoft.com/technet/security/bulletin/MS03-034.asp
http://www.microsoft.com/security/security_bulletins/ms03-034.asp

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Issue:

Network basic input/output system (NetBIOS) is an application
programming interface (API) that can be used by programs on a local
area network (LAN). NetBIOS provides programs with a uniform set of
commands for requesting the lower-level services required to manage
names, conduct sessions, and send datagrams between nodes on a
network.

This vulnerability involves one of the NetBT (NetBIOS over TCP)
services, namely, the NetBIOS Name Service (NBNS). NBNS is
analogous to DNS in the TCP/IP world and it provides a way to find
a system's IP address given its NetBIOS name, or vice versa.

Under certain conditions, the response to a NetBT Name Service
query may, in addition to the typical reply, contain random data
from the target system's memory. This data could, for example, be a
segment of HTML if the user on the target system was using an
Internet browser, or it could contain other types of data that
exist in memory at the time that the target system responds to the
NetBT Name Service query.

An attacker could seek to exploit this vulnerability by sending a
NetBT Name Service query to the target system and then examine the
response to see if it included any random data from that system's
memory.

If best security practices have been followed and port 137 UDP has
been blocked at the firewall, Internet based attacks would not be
possible.

Mitigating Factors:

• Any information disclosure would be completely random.
• By default, the Internet Connection Firewall (ICF), which is
  available with Windows XP and Windows Server 2003, blocks the
  ports that are used by NetBT.
• To exploit this vulnerability, an attacker would have to be able
  to send a specially-crafted NetBT request to port 137 on the
  target system and then examine the response to see whether any
  random data from that system's memory is included. In intranet
  environments, these ports are usually accessible, but systems that
  are connected to the Internet usually have these ports blocked
  by a firewall.

Risk Rating:

• Low

Patch Availability:

• A patch is available to fix this vulnerability. Please read
  the Security Bulletins at

http://www.microsoft.com/technet/security/bulletin/ms03-034.asp
http://www.microsoft.com/security/security_bulletins/ms03-034.asp

for information on obtaining this patch.

Acknowledgment:

Mike Price of Foundstone Labs, http://www.foundstone.com

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