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CERT Advisory CA-2003-20 W32/Blaster worm

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Date: Mon, 11 Aug 2003 22:21:08 -0400
From: CERT Advisory <>
Subject: CERT Advisory CA-2003-20 W32/Blaster worm


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 We
   are  investigating  the  conditions  under  which  this  attack  might
   manifest  itself.  Unusual  or unexpected traffic to
   may  indicate an infection on your network, so you may wish to monitor
   network traffic.

   Sites  that do not use to manage patches may wish to
   block  outbound traffic to In practice, this may be
   difficult  to  achieve, since may not resolve to the
   same    address    every   time.   Correctly   blocking   traffic   to  will require detailed understanding of your network
   routing  architecture,  system  management  needs, and name resolution
   environment. You should not block traffic to 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 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

          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 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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