CERT® Advisory CA-1999-06 ExploreZip Trojan Horse Program
Original issue date: Thursday June 10, 1999
Last revised: June 14, 1999
Added information about the program's self-propagation via networked
shares; also updated anti-virus vendor URLs.
Source: CERT/CC
A complete revision history is at the end of this file.
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.
Overview
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 sites.
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
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:
- run=C:\WINDOWS\SYSTEM\Explore.exe
On Windows NT systems, an entry is made in the system registry:
- [HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows]
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:
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 destroyed.
- 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
disinfected.
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.
www.esafe.com/vcenter/explore.html
Central Command
www.avp.com/zippedfiles/zippedfiles.html
Command Software Systems, Inc
www.commandcom.com/html/virus/explorezip.html
Computer Associates
www.cai.com/virusinfo/virusalert.htm
Data Fellows
www.datafellows.com/news/pr/eng/19990610.htm
McAfee, Inc. (a Network Associates company)
www.mcafee.com/viruses/explorezip/default.asp
Network Associates Incorporated
www.avertlabs.com/public/datafiles/valerts/vinfo/va10185.asp
Sophos, Incorporated
www.sophos.com/downloads/ide/index.html#explorez
Symantec
www.symantec.com/avcenter/venc/data/worm.explore.zip.html
Trend Micro Incorporated
www.antivirus.com/vinfo/alerts.htm
Additional sources of virus information are listed at
-
www.cert.org/other_sources/viruses.html
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
www.cert.org/advisories/CA-99-02-Trojan-Horses.html
- Melissa macro virus - discussed in CA-99-04
www.cert.org/advisories/CA-99-04-Melissa-Macro-Virus.html
- Happy99.exe Trojan Horse - discussed in IN-99-02
www.cert.org/incident_notes/IN-99-02.html
- CIH/Chernobyl virus - discussed in IN-99-03
www.cert.org/incident_notes/IN-99-03.html
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).
-
www.cert.org/advisories/CA-99-02-Trojan-Horses.html
This document is available from:
www.cert.org/advisories/CA-1999-06.html
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
U.S.A.
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
www.cert.org/CERT_PGP.key
If you prefer to use DES, please call the CERT hotline for more
information.
Getting security information
CERT publications and other security information are available from
our web site
* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.
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Copyright 1999 Carnegie Mellon University.
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