CERT Advisory CA-2003-20 W32/Blaster worm
From: CERT Advisory (cert-advisory_at_cert.org)
Date: Mon, 11 Aug 2003 22:21:08 -0400 To: firstname.lastname@example.org
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CERT Advisory CA-2003-20 W32/Blaster worm
Original issue date: August 11, 2003
Last revised: --
A complete revision history is at the end of this file.
* 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
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
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.
A remote attacker could exploit these vulnerabilities to execute
arbitrary code with Local System privileges or to cause a
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
Trend Micro, Inc. has published a set of steps to accomplish these
goals. Symantec has also published a set of steps to accomplish these
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
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
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 email@example.com 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
Authors: Chad Dougherty, Jeffrey Havrilla, Shawn Hernan, and Marty
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
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August 11, 2003: Initial release
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