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Alert: Microsoft Security Bulletin - MS03-034


Flaw in NetBIOS Could Lead to Information Disclosure (824105)

Originally posted: September 03, 2003


Who should read this bulletin: Customers using Microsoft® Windows®

Impact of vulnerability: Information disclosure

Maximum Severity Rating: Low

Recommendation: Users should evaluate whether to apply the security patch to 
affected systems.

End User Bulletin:
An end user version of this bulletin is available at: 


Affected Software: 
- Microsoft Windows NT 4.0® Server
- Microsoft Windows NT 4.0, Terminal Server Edition
- Microsoft Windows 2000
- Microsoft Windows XP 
- Microsoft Windows Server(tm) 2003
Not Affected Software:
- Microsoft Windows Millennium EditionAn End User version of the bulletin is 
available at:

Technical description: 

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.

Vulnerability identifier: CAN-2003-0661

This email is sent to NTBugtraq automatically as a service to my subscribers. 

Russ - Surgeon General of TruSecure Corporation/NTBugtraq Editor

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