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Posted Aug 12, 2003
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CERT Advisory CA-2003-20 - CERT announces that the exploitation of the RPC/DCOM vulnerability on Windows has now taken the form of a worm known as the W32/Blaster worm. This worm retrieves a binary that is then used to launch further attacks and is poised to launch a denial of service against windowsupdate.com.

tags | advisory, worm, denial of service
systems | windows
SHA-256 | d0e25a4a85c54bd50ad5e142ccb2f76e9828730a435bc2804f3ba2ff79e33d2a


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CERT Advisory CA-2003-20 W32/Blaster worm

Original issue date: August 11, 2003
Last revised: --
Source: CERT/CC

A complete revision history is at the end of this file.

Systems Affected

* Microsoft Windows NT 4.0
* Microsoft Windows 2000
* Microsoft Windows XP
* Microsoft Windows Server 2003


The CERT/CC is receiving reports of widespread activity related to a
new piece of malicious code known as W32/Blaster. This worm appears to
exploit known vulnerabilities in the Microsoft Remote Procedure Call
(RPC) Interface.

I. Description

The W32/Blaster worm exploits a vulnerability in Microsoft's DCOM RPC
interface as described in VU#568148 and CA-2003-16. Upon successful
execution, the worm attempts to retrieve a copy of the file
msblast.exe from the compromising host. Once this file is retrieved,
the compromised system then runs it and begins scanning for other
vulnerable systems to compromise in the same manner. In the course of
propagation, a TCP session to port 135 is used to execute the attack.
However, access to TCP ports 139 and 445 may also provide attack
vectors and should be considered when applying mitigation strategies.
Microsoft has published information about this vulnerability in
Microsoft Security Bulletin MS03-026.

Lab testing has confirmed that the worm includes the ability to launch
a TCP SYN flood denial-of-service attack against windowsupdate.com. We
are investigating the conditions under which this attack might
manifest itself. Unusual or unexpected traffic to windowsupdate.com
may indicate an infection on your network, so you may wish to monitor
network traffic.

Sites that do not use windowsupdate.com to manage patches may wish to
block outbound traffic to windowsupdate.com. In practice, this may be
difficult to achieve, since windowsupdate.com may not resolve to the
same address every time. Correctly blocking traffic to
windowsupdate.com will require detailed understanding of your network
routing architecture, system management needs, and name resolution
environment. You should not block traffic to windowsupdate.com without
a thorough understanding of your operational needs.

We have been in contact with Microsoft regarding this possibility of
this denial-of-service attack.

II. Impact

A remote attacker could exploit these vulnerabilities to execute
arbitrary code with Local System privileges or to cause a
denial-of-service condition.

III. Solutions

Apply patches

All users are encouraged to apply the patches referred to in Microsoft
Security Bulletin MS03-026 as soon as possible in order to mitigate
the vulnerability described in VU#568148. These patches are also
available via Microsoft's Windows Update service.

Systems running Windows 2000 may still be vulnerable to at least a
denial-of-service attack via VU#326746 if their DCOM RPC service is
available via the network. Therefore, sites are encouraged to use the
packet filtering tips below in addition to applying the patches
supplied in MS03-026.

It has been reported that some affected machines are not able to stay
connected to the network long enough to download patches from
Microsoft. For hosts in this situation, the CERT/CC recommends the
1. Physically disconnecting the system from the network
2. Check the system for signs of compromise.
+ In most cases, an infection will be indicated by the presence
of the registry key
\Run\windows auto update" with a value of msblast.exe. If
this key is present, remove it using a registry editor.
3. If you're infected, terminate the running copy of msblast.exe
using the Task Manager.
4. Take one of the following steps to protect against the compromise
prior to installing the Microsoft patch:
+ Disable DCOM as described below
+ Enabling Microsoft's Internet Connection Filter (ICF), or
another host-level packet filtering program to block incoming
connections for 135/tcp
5. Reconnect the system to the network and apply the patches in the
recommended manner

Trend Micro, Inc. has published a set of steps to accomplish these
goals. Symantec has also published a set of steps to accomplish these

Disable DCOM

Depending on site requirements, you may wish to disable DCOM as
described in MS03-026. Disabling DCOM will help protect against this
vulnerability but may also cause undesirable side effects. Additional
details on disabling DCOM and possible side effects are available in
Microsoft Knowledge Base Article 825750.

Filter network traffic

Sites are encouraged to block network access to the following relevant
ports at network borders. This can minimize the potential of
denial-of-service attacks originating from outside the perimeter. The
specific services that should be blocked include
* 69/UDP
* 135/TCP
* 135/UDP
* 139/TCP
* 139/UDP
* 445/TCP
* 445/UDP
* 4444/TCP

Sites should consider blocking both inbound and outbound traffic to
these ports, depending on network requirements, at the host and
network level. Microsoft's Internet Connection Firewall can be used to
accomplish these goals.

If access cannot be blocked for all external hosts, the CERT/CC
recommends limiting access to only those hosts that require it for
normal operation. As a general rule, the CERT/CC recommends filtering
all types of network traffic that are not required for normal

Because current exploits for VU#568148 create a backdoor, which is in
some cases 4444/TCP, blocking inbound TCP sessions to ports on which
no legitimate services are provided may limit intruder access to
compromised hosts.

Recovering from a system compromise

If you believe a system under your administrative control has been
compromised, please follow the steps outlined in

Steps for Recovering from a UNIX or NT System Compromise


The CERT/CC is tracking activity related to this worm as CERT#30479.
Relevant artifacts or activity can be sent to cert@cert.org with the
appropriate CERT# in the subject line.

Appendix A. Vendor Information

This appendix contains information provided by vendors. When vendors
report new information, this section is updated and the changes are
noted in the revision history. If a vendor is not listed below, we
have not received their comments.


Please see Microsoft Security Bulletin MS03-026.

Appendix B. References

* CERT/CC Advisory CA-2003-19 -
* CERT/CC Vulnerability Note VU#561284 -
* CERT/CC Vulnerability Note VU#326746 -
* Microsoft Security Bulletin MS03-026 -
* Microsoft Knowledge Base article 823980 -


Our thanks to Microsoft Corporation for their review of and input to
this advisory.

Authors: Chad Dougherty, Jeffrey Havrilla, Shawn Hernan, and Marty

This document is available from:

CERT/CC Contact Information

Email: cert@cert.org
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from

If you prefer to use DES, please call the CERT hotline for more

Getting security information

CERT publications and other security information are available from
our web site

To subscribe to the CERT mailing list for advisories and bulletins,
send email to majordomo@cert.org. Please include in the body of your

subscribe cert-advisory

* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.

Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.

Conditions for use, disclaimers, and sponsorship information

Copyright 2003 Carnegie Mellon University.

Revision History

August 11, 2003: Initial release

Version: PGP 6.5.8

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