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Title: Flaw in RPC Endpoint Mapper Could Allow Denial of
Service Attacks (331953)
Date: 26 March 2003
Software: Microsoft(r) Windows(r) NT 4.0, Windows 2000, or
Windows XP
Impact: denial of service
Max Risk: Important
Bulletin: MS03-010

Microsoft encourages customers to review the Security Bulletins
at:

http://www.microsoft.com/technet/security/bulletin/MS03-010.asp
http://www.microsoft.com/security/security_bulletins/ms03-010.asp

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Issue:

Remote Procedure Call (RPC) is a protocol used by the Windows
operating system. RPC provides an inter-process communication
mechanism that allows a program running on one computer to
seamlessly execute code on a remote system. The protocol itself
is derived from the OSF (Open Software Foundation) RPC protocol,
but with the addition of some Microsoft specific extensions.

There is a vulnerability in the part of RPC that deals with
message exchange over TCP/IP. The failure results because of
incorrect handling of malformed messages. This particular
vulnerabilty affects the RPC Endpoint Mapper process, which
listens on TCP/IP port 135. The RPC endpoint mapper allows RPC
clients to determine the port number currently assigned to a
particular RPC service.

To exploit this vulnerability, an attacker would need to
establish a TCP/IP connection to the Endpoint Mapper process on
a remote machine. Once the connection was established, the
attacker would begin the RPC connection negotiation before
transmitting a malformed message. At this point, the process on
the remote machine would fail. The RPC Endpoint Mapper process
is responsible for maintaining the connection information for
all of the processes on that machine using RPC. Because the
Endpoint Mapper runs within the RPC service itself, exploiting
this vulnerability would cause the RPC service to fail, with the
attendant loss of any RPC-based services the server offers, as
well as potential loss of some COM functions.

Microsoft has provided patches with this bulletin to correct
this vulnerability for Windows 2000 and Windows XP. Although
Windows NT 4.0 is affected by this vulnerability, Microsoft is
unable to provide a patch for this vulnerability for Windows NT
4.0. The architectural limitations of Windows NT 4.0 do not
support the changes that would be required to remove this
vulnerability. Windows NT 4.0 users are strongly encouraged to
employ the workaround discussed in the FAQ in the bulletin,
which is to protect the NT 4.0 system with a firewall that
blocks Port 135.

Mitigating Factors:

• • To exploit this vulnerability, the attacker would require the
    ability to connect to the Endpoint Mapper running on the target
    machine. For intranet environments, the Endpoint Mapper would
    normally be accessible, but for Internet connected machines, the
    port used by the Endpoint Mapper would normally be blocked by a
    firewall. In the case where this port is not blocked, or in an
    intranet configuration, the attacker would not require any
    additional privileges.
• • Best practices recommend blocking all TCP/IP ports that are not
    actually being used. For this reason, most machines attached to
    the Internet should have port 135 blocked. RPC over TCP is not
    intended to be used in hostile environments such as the internet.
• • More robust protocols such as RPC over HTTP are provided for
    hostile environments. To learn more about securing RPC for
    client and server please refer to
    http://msdn.microsoft.com/library/default.asp?url=/library/en-
    us/rpc/rpc/writing_a_secure_rpc_client_or_server.asp. To learn
    more about the ports used by RPC, please refer to
    http://www.microsoft.com/technet/prodtechnol/windows2000serv/res
    kit/tcpip/part4/tcpappc.asp
• • This vulnerability only permits a denial of service attack and
    does not provide an attacker with the ability to

Risk Rating:

Important

Patch Availability:

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

http://www.microsoft.com/technet/security/bulletin/ms03-010.asp
http://www.microsoft.com/security/security_bulletins/ms03-10.asp

for information on obtaining this patch.

Acknowledgment:

• Microsoft thanks jussi jaakonaho for reporting this issue to
  us and working with us to protect customers

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PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT
DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING
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