Microsoft Security Bulletin MS02-045

Technical description:

SMB (Server Message Block) is the protocol Microsoft uses to share files, printers, serial ports, and also to communicate between computers using named pipes and mail slots. In a networked environment, servers make file systems and resources available to clients. Clients make SMB requests for resources and servers make SMB responses in what described as a client server, request-response protocol.

By sending a specially crafted packet request, an attacker can mount a denial of service attack on the target server machine and crash the system. The attacker could use both a user account and anonymous access to accomplish this. Though not confirmed, it may be possible to execute arbitrary code.

Mitigating factors:

• An administrator can block access to SMB ports from untrusted networks. By blocking TCP ports 445 and 139 at the network perimeter, administrators can prevent this attack from untrusted parties. In a file and printing environment, this may not be a practical solution for legitimate users.
• An administrator can stop the Lanman server service which prevents the attack, but again may not be suitable on a file and print sharing server.

Severity Rating:

The above assessment is based on the types of systems affected by the vulnerability, their typical deployment patterns, and the effect that exploiting the vulnerability would have on them.

Vulnerability identifier: CAN-2002-0724

Tested Versions:

Microsoft tested Windows NT 4.0, Windows 2000 and Windows XP to assess whether they are affected by these vulnerabilities. Previous versions are no longer supported, and may or may not be affected by these vulnerabilities.

What's the scope of the vulnerability?
This is a denial of service attack . By sending a specially crafted packet request to a computer, an attacker can crash the system of the target machine. The attacker could use both a user account and anonymous access to accomplish this. Though not confirmed, it may be possible to execute arbitrary code.
If system administrators have turned off anonymous access, it would not be possible for a non-authenticated user to exploit this vulnerability. However, turning off anonymous access does not prevent authenticated users from this attack.
In addition, an administrator can block access to SMB on TCP ports 445 and 139 at the network perimeter. This would block access from untrusted networks. However, legitimate users could be blocked in a file and print networking environment.
Administrators could also shut down the lanman server service. However, in a file and print networking environment this may not be a viable solution because it would block legitimate users from using file and print services on a particular server where the lanman service had been stopped.

What causes the vulnerability?
The vulnerability results because of a flaw in the way Microsoft's implementation of SMB receives a packet requesting the SMB service.

What is SMB?
SMB (Server Message Block) is the protocol Microsoft uses to share files, printers, serial ports, and also to communicate between computers using named pipes and mail slots. In a networked environment, servers make file systems and resources available to clients. Clients make SMB requests for resources and servers make SMB responses in what described as a client server, request-response protocol.

What's wrong with the SMB implementation?
There is an unchecked buffer in a section of code that requests the SMB service.

What could this vulnerability enable an attacker to do?
This vulnerability could enable an attacker launch a denial of service attack against vulnerable systems either locally or remotely using either a user account or anonymous access.

But this is a buffer overrun vulnerability. Don't they usually allow the attacker to run code on the system?
Normally this is true. However, in the testing process, neither the development team nor the reporter were able to find a way for an attacker to run code on the system. This does not mean that it's not possible, just that testing so far has not yielded this result. Again, administrators are encouraged to apply the patch.

How could an attacker exploit this vulnerability?
An attacker could seek to exploit this vulnerability on machines that have anonymous access enabled by sending a malformed SMB request to a target computer and crashing it.

Could this vulnerability be exploited by a user on the Internet?
Only if TCP ports 445 and 139 were open at the firewall. This would block access from untrusted networks. Best practices say these ports should be blocked.

Would the user need to be authenticated in order to exploit the vulnerability?
If anonymous access has been disabled, only an authenticated user could exploit this vulnerability. Requiring authenticated users instead of anonymous access would make it easier for an administrator to determine who the attacker was.

What does the patch do?
The patch eliminates the vulnerability by checking for correct inputs before responding to SMB requests, thereby eliminating the vulnerability.

Download locations for this patch

• Microsoft Windows NT 4.0 – Download the update
• Microsoft Windows NT 4.0 Terminal Server Edition – Download the update
• Microsoft Windows 2000 – Download the update
• Microsoft Windows XP – Download the update
• Microsoft Windows XP 64 bit Edition – Download the update