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CERT Advisory CA-99-06-explorezip

   Original issue date: Thursday June 10, 1999
   Last Revised Date: June 14, 1999
   Added information about the program's self-propagation via networked
   shares; also updated anti-virus vendor URLs.
   Source: CERT/CC
Note: The CERT Coordination Center has discovered new information
regarding the ExploreZip worm. This re-issue of CERT Advisory CA-99-06
contains new information regarding an additional means by which the
Worm can spread, and a caution about disinfecting your systems. We
will continue to update this advisory as new information is
discovered. We encourage you to check our web site frequently for any
new information.

Systems Affected

     * Machines running Windows 95, Windows 98, or Windows NT.
     * Machines with filesystems and/or shares that are writable by a
       user of an infected system.
     * Any mail handling system could experience performance problems or
       a denial of service as a result of the propagation of this Trojan
       horse program.

   The CERT Coordination Center continues to receive reports and
   inquiries regarding various forms of malicious executable files that
   are propagated as file attachments in electronic mail.
   During the second week of June 1999, the CERT/CC began receiving
   reports of sites affected by ExploreZip, a Trojan horse/worm program
   that affects Windows systems and has propagated in email attachments.
   The number and variety of reports we have received indicate that this
   has the potential to be a widespread attack affecting a variety of
I. Description

   Our original analysis indicated that the ExploreZip program is a
   Trojan horse, since it initially requires a victim to open or run an
   email attachment in order for the program to install a copy of itself
   and enable further propagation. Further analysis has shown that, once
   installed, the program may also behave as a worm, and it may be able
   to propagate itself, without any human interaction, to other networked
   machines that have certain writable shares.
   The ExploreZip Trojan horse has been propagated between users in the
   form of email messages containing an attached file named
   zipped_files.exe. Some email programs may display this attachment
   with a "WinZip" icon. The body of the email message usually appears to
   come from a known email correspondent, and typically contains the
   following text:
   I received your email and I shall send you a reply ASAP.
          Till then, take a look at the attached zipped docs.
   The subject line of the message may not be predictable and may appear
   to be sent in reply to previous email.
   Opening the zipped_files.exe file causes the program to execute. It is
   possible under some mailer configurations that a user might
   automatically open a malicious file received in the form of an email
   attachment. When the program is run, an error message is displayed:
   Cannot open file: it does not appear to be a valid archive. If this
   file is part of a ZIP format backup set, insert the last disk
   of the backup set and try again. Please press F1 for help.
Destruction of files

     * The program searches local and networked drives (drive letters C
       through Z) for specific file types and attempts to erase the
       contents of the files, leaving a zero byte file. The targets may
       include Microsoft Office files, such as .doc, .xls, and .ppt, and
       various source code files, such as .c, .cpp, .h, and .asm.
     * The program may also be able to delete files that are writable to
       it via SMB/CIFS file sharing. The program appears to look through
       the network neighborhood and delete any files that are shared and
       writable, even if those shares are not mapped to networked drives
       on the infected computer.
     * The program appears to continually delete the contents of targeted
       files on any mapped networked drives.
       The program does not appear to delete files with the "hidden" or
       "system" attribute, regardless of their extension.
System modifications

     * The zipped_files.exe program creates a copy of itself in a file
       called explore.exe in the following location(s):
        On Windows 98 - C:\WINDOWS\SYSTEM\Explore.exe
                On Windows NT - C:\WINNT\System32\Explore.exe
       This explore.exe file is an identical copy of the zipped_files.exe
       Trojan horse, and the file size is 210432 bytes.
       MD5 (Explore.exe) = 0e10993050e5ed199e90f7372259e44b
     * On Windows 98 systems, the zipped_files.exe program creates an
       entry in the WIN.INI file:
       On Windows NT systems, an entry is made in the system registry:
                run = "C:\WINNT\System32\Explore.exe"
Propagation via file sharing

   Once explore.exe is running, it takes the following steps to propagate
   to other systems via file sharing:
     * Each time the program is executed, the program will search the
       network for all shares that contain a WIN.INI file with a valid
       "[windows]" section in the file.
     * For each such share that it finds, the program will attempt to
          + copy itself to a file named _setup.exe on that share
          + modify the WIN.INI file on that share by adding the entry
       The account running the program on the original infected machine
       needs to have permission to write to the second victim's shared
       directory. (That is, no vulnerabilities are being exploited in
       order for the program to spread in this manner.)
       The _setup.exe file is identical to the zipped_files.exe and
       explore.exe files on the original infected machine.
     * The original infected system will continue to scan shares that
       have been mapped to a local drive letter containing a valid
       WIN.INI file. For each such share that is found, the program will
       "re-infect" the victim system as described above.
   On Windows 98 systems that have a "run=_setup.exe" entry in the
   WIN.INI file (as described previously), the C:\WINDOWS\_setup.exe
   program is executed automatically whenever a user logs in. On Windows
   NT systems, a "run=_setup.exe" entry in the WIN.INI file does not
   appear to cause the program to be executed automatically.
   When run as _setup.exe, the program will attempt to
     * make another copy of itself in C:\WINDOWS\SYSTEM\Explore.exe
     * modify the WIN.INI file again by replacing the "run=_setup.exe"
       entry with "run=C:\WINDOWS\SYSTEM\Explore.exe"
   Note that when the program is run as _setup.exe, it configures the
   system to later run as explore.exe. But when run as explore.exe, it
   attempts to infect shares with valid WIN.INI files by configuring
   those files to run _setup.exe. Since this infection process includes
   local shares, affected systems may exhibit a "ping pong" behavior in
   which the infected host alternates between the two states.
Propagation via email

   The program propagates by replying to any new email that is received
   by the infected computer. The reply messages are similar to the
   original email described above, each containing another copy of the
   zipped_files.exe attachment.
   We will continue to update this advisory with more specific
   information as we are able to confirm details. Please check the
   CERT/CC web site for the current version containing a complete
   revision history.
II. Impact

     * Users who execute the zipped_files.exe Trojan horse will infect
       the host system, potentially causing targeted files to be
     * Users who execute the Trojan horse may also infect other networked
       systems that have writable shares.
     * Because of the large amount of network traffic generated by
       infected machines, network performance may suffer.
     * Indirectly, this Trojan horse could cause a denial of service on
       mail servers. Several large sites have reported performance
       problems with their mail servers as a result of the propagation of
       this Trojan horse.
III. Solution

Use virus scanners

   While many anti-virus products are able to detect and remove the
   executables locally, because of the continuous re-infection process,
   simply removing all copies of the program from an infected system may
   leave your system open to re-infection at a later time, perhaps
   immediately. To prevent re-infection, you must not serve any shares
   containing a WIN.INI file to any potentially infected machines. If you
   share files with everyone in your domain, then you must disable shares
   with WIN.INI files until every machine on your network has been
   In order to detect and clean current viruses, you must keep your
   scanning tools up to date with the latest definition files. Please see
   the following anti-virus vendor resources for more information about
   the characteristics and removal techniques for the malicious file
   known as ExploreZip.
   Aladdin Knowledge Systems, Inc.

          Central Command

          Command Software Systems, Inc

          Computer Associates

          Data Fellows

          McAfee, Inc. (a Network Associates company)

          Network Associates Incorporated

          Sophos, Incorporated


          Trend Micro Incorporated

   Additional sources of virus information are listed at
Additional suggestions

     * Blocking Netbios traffic at your network border may help prevent
       propagation via shares from outside your network perimeter.
     * Disable file serving on workstations. You will not be able to
       share your files with other computers, but you will be able to
       browse and get files from servers. This will prevent your
       workstation from being infected via file sharing propagation.
     * Maintain a regular, off-line, backup cycle.
General protection from email Trojan horses and viruses

   Some previous examples of malicious files known to have propagated
   through electronic mail include
     * False upgrade to Internet Explorer - discussed in CA-99-02
     * Melissa macro virus - discussed in CA-99-04
     * Happy99.exe Trojan Horse - discussed in IN-99-02
     * CIH/Chernobyl virus - discussed in IN-99-03
   In each of the above cases, the effects of the malicious file are
   activated only when the file in question is executed. Social
   engineering is typically employed to trick a recipient into executing
   the malicious file. Some of the social engineering techniques we have
   seen used include
     * Making false claims that a file attachment contains a software
       patch or update
     * Implying or using entertaining content to entice a user into
       executing a malicious file
     * Using email delivery techniques which cause the message to appear
       to have come from a familiar or trusted source
     * Packaging malicious files in deceptively familiar ways (e.g., use
       of familiar but deceptive program icons or file names)
   The best advice with regard to malicious files is to avoid executing
   them in the first place. CERT advisory CA-99-02 discusses Trojan
   horses and offers suggestions to avoid them (please see Section V).
   This document is available from:
CERT/CC Contact Information

   Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
          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 personnel answer the hotline 08:00-20:00 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 be added to our mailing list for advisories and bulletins, send
   email to This email address is being protected from spambots. You need JavaScript enabled to view it. and include SUBSCRIBE
   your-email-address in the subject of your message.
   Copyright 1999 Carnegie Mellon University.
   Conditions for use, disclaimers, and sponsorship information can be
   found in
   * "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.
Revision History

June 10, 1999:  Initial release
June 11, 1999:  Added information about the appearance of the attached file
                Added information from Aladdin Knowledge Systems, Inc.
June 14, 1999:  Added information about the program's self-propagation via
                networked shares; also updated anti-virus vendor URLs

Version: 2.6.2


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