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CERT Advisory CA-2000-12 HHCtrl ActiveX Control Allows Local Files to be
Executed
Original release date: June 19, 2000
Last revised: --
Source: CERT/CC
A complete revision history is at the end of this file.
Systems Affected
* Systems running Microsoft Internet Explorer
Overview
The HHCtrl ActiveX control has a serious vulnerability that allows
remote intruders to execute arbitrary code, if the intruder can cause
a compiled help file (CHM) to be stored "locally." Microsoft has
released a security bulletin and a patch for this vulnerability, but
the patch does not address all circumstances under which the
vulnerability can be exploited. This document discusses some of the
additional ways in which this vulnerability can be exploited. Some
common circumstances under which this vulnerability can be exploited
are addressed by the Microsoft patch; others are not. Read this
document carefully with your network configuration in mind to
determine if you need to take any action. In recent discussions with
the CERT/CC, Microsoft has indicated they do not plan to alter the
patch.
I. Description
The Microsoft Windows HTML help facility (part of Internet Explorer)
is able to execute arbitrary programs through an embedded "shortcut"
in a compiled HTML file. This allows the help system to start wizards
and other programs as part of the help facility. Unfortunately, it
also makes it unsafe for users to open help files obtained from
untrusted sources.
An attacker who can construct a malicious help file and place it in a
location accessible by the victim may be able to cause this help file
to be loaded and the embedded shortcuts executed without interaction
from the victim. A malicious web site author may cause a compiled HTML
help file to be opened through the Active Scripting showHelp call in
Internet Explorer. Help files may also be opened in other environments
that support Active Scripting, such as email messages in Outlook.
The specific exploit described (and corrected) by Microsoft involves
an attacker who makes the malicious help files available via a UNC
share. The patch corrects this aspect of the problem by allowing help
files to execute shortcuts only when "located on the user's local
machine." More information about Microsoft's security bulletin and
their patch is available from
http://microsoft.com/technet/security/bulletin/ms00-037.asp
http://microsoft.com/technet/security/bulletin/fq00-037.asp
Preconditions Required for Exploitation
Unfortunately, the Microsoft patch does not address several
significant ways in which the vulnerability can be exploited. The
vulnerability can be exploited in any situation where all of the
following conditions are met:
1. The attacker must entice or compel a victim who has Active
Scripting enabled to open an email message or visit a web page.
Alternatively, the attacker could attempt to trick the victim into
opening the compiled help file, such as by sending it as an
attachment in an email message. Since it is not yet widely
recognized that help files have the potential to be just as
dangerous as an untrusted executable, this may not be difficult.
2. The attacker must be able to place a malicious help file in a
location accessible to the user when the Active Script is
executed. The attacker must also be able to predict or guess the
path to this file. If the patch described in Microsoft Security
Bulletin MS00-037 has been applied, this file may not reside on a
UNC share (\\hostname\path\file). That is, if the patch has not
been installed, an intruder must be able to place a file anywhere
that the victim can access it. If the patch has been installed,
the intruder must be able to place a file anywhere that the victim
can access it except on UNC shares.
3. The Active Script mentioned above must run in a security zone that
allows ActiveX controls to run and allows the scripting of
controls that are marked "safe for scripting." The default
security settings for the Internet Zone and the My Computer zone
allow these actions to occur without warning prompts.
4. The HHCtrl ActiveX control must be installed and be marked "safe
for scripting" and "safe for initialization." This is the default
configuration when Internet Explorer is installed.
Note that all of these conditions, some of which are default
conditions, must be met in order for an attacker to exploit this
vulnerability. Changing some of these conditions may involve
trade-offs between functionality and security.
In recent discussions with the CERT/CC, Microsoft has not indicated
any intention of changing the help system's behavior. Therefore, to be
completely protected from exploitation of this vulnerability, users
must eliminate one or more of the preconditions listed above.
It is reasonable for a user to expect that simply visiting a web page
is a safe activity, so eliminating the first precondition is
difficult. Disabling Active Scripting or the execution of ActiveX
controls prevents the vulnerability from being exploited, but it also
prevents the normal operation of these features and is likely to
affect the appearance and functionality of web pages. Removing the
"safe for initialization" or "safe for scripting" attributes of the
HHCtrl causes warning dialogs to be generated in a number of
circumstances where they may not be expected.
How an Attacker May Create "Local" Files
Although you may believe it is difficult or impossible for an intruder
to place a file in a predictable location that is accessible to you,
in fact, several common practices allow intruders to do just this.
While preventing an attacker from downloading files on the local
system without warning is a valuable security practice, it is not
sufficient as the single line of defense against the execution of
malicious code. The CERT/CC recommends adopting one of several more
conservative solutions, including disabling ActiveX controls or Active
Scripting. More information on these solutions are included in the
Solution section of this document.
If a site relies solely on limiting the attacker's ability to make
malicious code accessible to the victim, the following activities are
not safe:
* Sharing files via a network filesystem such as AFS, DFS, NFS,
Novell Netware, or Windows shares when users map these drives to
local drive letters. When the drive letter is not predictable but
the path to the file is, the attacker may be able to make multiple
exploit attempts because failed calls to showHelp generate no
error messages. Access control lists cannot be used to defend
yourself against this problem because the ACL facility allows the
intruder to give you access to malicious files they control
without your consent.
* Sharing physical disk drives in environments such as academic
labs, Internet cafes, or libraries, where an attacker may be able
to store malicious files in a writable local directory.
* Using any of several products that automatically extract
attachments from email messages and place them in predictable
locations. A notable example of this is Eudora.
* Using chat clients such as IRC-II, ICQ, or AOL Instant Messenger
in modes that allow unsolicited file transfers to be placed in a
local directory.
* Hosting an anonymous FTP site, if the upload directory is
accessible by local users.
Without other solutions, engaging in any of these activities renders a
site vulnerable to the problem described in this advisory.
Additionally, several other vulnerabilities have been discovered
recently whose impact was limited to the ability to download arbitrary
files to the victim's system. If they are exploited in conjunction
with this vulnerability, the impact is more significant, as discussed
in the next section.
II. Impact
By using the showHelp Active Scripting call in conjunction with
shortcuts embedded in a malicious help file, attackers are able to
execute programs and ActiveX controls of their choice. Since
exploitation of the vulnerability requires an attacker to place a
compiled help file (CHM) in a location accessible to the victim, it is
usually trivial to include a malicious executable as well. In this
situation, the attacker can take any action that the victim can.
The essence of the problem is this:
The ability for an intruder to make a file accessible to a victim
running Internet Explorer is equivalent to the ability to
execute arbitrary code on the victim's system if several common
preconditions are met.
III. Solution
The CERT/CC developed the information in the solution section based on
our independent tests using primarily Internet Explorer 5 on Microsoft
Windows NT 4.0 and Windows 2000. Your results will vary based on your
particular configuration.
For some sites, the patch provided by Microsoft is adequate. For
others, particularly those sites using non-Microsoft networking
products, the patch does not provide complete protection. You will
need to understand your network's configuration prior to deciding
which, if any, changes are appropriate.
Configure Outlook to read email in the Restricted Zone.
Because an email message may start Internet Explorer automatically if
Active Scripting is enabled, the CERT/CC encourages you to configure
your Outlook email client to use the Restricted Zone, and to disable
Active Scripting in this zone. This solution should be implemented in
addition to one of the changes mentioned earlier.
The steps for configuring Outlook to use the Restricted Zone are:
1. Start Outlook as you normally would.
2. From the Tools menu select Options.... The Options dialog box
appears.
3. Select the Security tab. The Security Options panel appears.
4. In the Secure content section, change the pull-down menu from
Internet to Restricted Sites.
5. Click Apply to save your changes.
6. Click OK to close the Options dialog box.
We recommend similar steps for any other mail clients that support
Active Scripting and Security Zones (or similar facilities to prevent
the unwanted execution of scripts).
Disable Active Scripting and/or ActiveX controls in the Internet Zone.
One way to prevent the exploitation of this vulnerability is to limit
the functionality available to attackers through the security zone
feature of Internet Explorer. The CERT/CC recommends this solution as
a way to protect against the vulnerability while retaining as much
functionality as possible in the help system.
A security zone is a set of security settings applied to a web page
based on the site the web paged originated from. By default, all sites
are in the Internet Zone, and disabling functionality in this zone can
protect you from attackers at all sites not associated with another
zone.
You may also need to reduce the settings in the Local Intranet Zone,
if you do not trust all web sites within your DNS domain. In fact, the
risk of exploitation by an inside attacker may be greater, since the
ability to create a file accessible by you may be easier within a
local area network.
One or more of the following options must be changed in the
appropriate zones to protect against the vulnerability:
* The Active Scripting option
Disabling Active Scripting is perhaps the best solution since it
prevents the vulnerability from being exploited and doesn't
present the user with warning dialogs. Setting this option to
"Prompt" is not recommended, because the warning dialog will
incorrectly imply that the action is safe, when in fact it is not.
* The Run ActiveX controls and plug-ins option
Disabling the execution of ActiveX controls is an option that
protects against this vulnerability, but it also prevents plug-ins
from executing normally. Since plug-ins for common applications
such as Adobe Acrobat are included in this same category, setting
the option to "Disable" results in significantly reduced
functionality. For similar reasons, setting this option to
"Prompt" is not recommended, because it is not always clear what
the safe response should be.
An excellent solution (but perhaps requiring more administrative
effort) is to set this option to "Administrator approved". In this
setting, only those ActiveX controls approved by the administrator
(using the Internet Explorer Administration Kit) will be executed.
If the administrator includes most controls but specifically
excludes the HHCtrl control, there is an attractive balance
between security and functionality. For more information regarding
this option, see
http://www.microsoft.com/Windows/ieak/en/support/faq/default.asp
* The Script ActiveX controls marked safe for scripting option
Disabling the scripting of ActiveX controls marked "safe for
scripting" protects against this vulnerability but limits the
normal operation of many controls used over the Internet. Setting
this option to "Prompt" generates a warning dialog that is not
strongly enough worded to reflect the danger inherent in the
HHCtrl control.
If all three of these options are set to "Enable", which is the
default in the Internet Zone, this vulnerability may be exploited.
Improving the security settings of any of these three options will at
least cause a warning dialog to appear and may prevent the exploit
entirely.
Steps for changing your security zone settings for Internet Explorer 5
on Windows NT 4.0 are:
1. Start Internet Explorer as you normally would.
2. From the Tools menu select Internet Options.... The Internet
Options dialog box appears.
3. Select the Security tab. The Security Options panel appears.
4. Select the zone you wish to change. For most users, this is the
Internet Zone, but depending on your circumstances, you may need
to repeat these steps for the Local Intranet Zone as well.
5. Click the Custom Level button. The Security Settings panel
appears.
6. Change one or more of the following settings based on the
information provided earlier and your desired level of security.
a. Set Run ActiveX controls and plug-ins to administrator
approved, disable, or prompt.
b. Set Script ActiveX controls marked safe for scripting to
disable or prompt.
c. Set Active scripting to disable or prompt.
7. Click OK to accept these changes. A dialog box appears asking if
you are sure you want to make these changes.
8. Click Yes.
9. Click Apply to save your changes.
10. Click OK to close the Internet Options dialog box.
Security zones can also be used to enable Active Scripting and ActiveX
controls at specific sites where you wish to retain this
functionality. To place a site in the Trusted Sites Zone using
Internet Explorer 5.0 on Windows NT 4.0,
1. Start Internet Explorer as you normally would.
2. From the Tools menu select Internet Options.... The Internet
Options dialog box appears.
3. Select the Security tab. The Security Options panel appears.
4. Select the Trusted Sites Zone.
5. Click the Sites... button.
6. Enter the name of the trusted site in the Add this Web Site to the
zone: text box.
7. Click the Add button.
8. If a dialog box appears saying "Sites added to this zone must use
the https:// prefix. This prefix assures a secure connection":
a. Click OK.
b. Add https:// to the beginning of the site name, and try to
add the site again.
c. Or uncheck the box at the bottom of the dialog box marked
Require server verification (https:) for all sites in this
zone. Making this change reduces the security of your system
by not requiring certificate based authentication, relying
instead on DNS based verification which could be misleading.
The CERT/CC encourages you not to make this change unless you
fully understand the implications. If you choose not to
require certificate based verification, you may wish to
reduce other security settings for the Trusted Sites Zone.
9. Click OK to save the new list of sites.
10. Click Apply to save your changes.
11. Click OK to close the Internet Options dialog box.
Steps for managing Security Zones in other versions of Windows and
Internet Explorer are similar.
The "My Computer" Zone
In addition to the four zones that are ordinarily visible, there is a
fifth zone called the "My Computer" zone which is not ordinarily
visible. Files on the local system are in the "My Computer" zone. You
can examine and modify the settings in the "My Computer" through the
registry. For more information, see
http://support.microsoft.com/support/kb/articles/Q182/5/69.ASP
The "My Computer" zone may also be managed through the Internet
Explorer Administration Kit (IEAK).
The CERT/CC does not recommend modifications to the "My Computer" zone
unless you have unusual security requirements and a thorough
understanding of the ramifications, including the potential for loss
of functionality.
Note, however, that if there is a vulnerability or condition that
allows an attacker to create a file locally (such as through Eudora,
for example) then this file will be subject to the security settings
of the "My Computer" zone.
Active Scripts on a web page or in a mail message will continue to be
subject to the security settings of the zone where the web page or
mail client resides. In this case, disabling Active Scripting in
untrusted locations, including the Internet Zone, provides the best
defense.
Change the attributes of the HHCtrl ActiveX control.
Because the HHCtrl control is central to the exploitation of this
vulnerability, removing either the "safe for scripting" or the "safe
for initialization" attribute in the registry corrects the problem.
Unfortunately, removing these attributes prevents some features of the
help system from operating normally, even if the help file is opened
through some other application.
Implementing this solution will allow other ActiveX controls to
function, including those referenced in Internet web pages. If you are
unable to implement one of the solutions mentioned earlier, or you are
willing to sacrifice help system features for more complete ActiveX
functionality, then you may wish to consider this solution. This
solution will provide warning dialogs when users open help files --
both malicious and benign help files.
To mark the HHCtrl ActiveX control as not "safe for scripting", remove
this registry key:
HKEY_CLASSES_ROOT\CLSID\ {ADB880A6-D8FF-11CF-9377-00AA003B7A11}\
Implemented Categories\ {7DD95801-9882-11CF-9FA9-00AA006C42C4}
To mark the HHCtrl ActiveX control as not "safe for initialization",
remove this registry key:
HKEY_CLASSES_ROOT\CLSID\ {ADB880A6-D8FF-11CF-9377-00AA003B7A11}\
Implemented Categories\ {7DD95802-9882-11CF-9FA9-00AA006C42C4}
Spaces in the keys listed above were added to improve HTML formatting
and are not in the actual registry keys.
Only one of the two changes need to be made in order to prevent the
exploitation of this vulnerability. Either of these changes will
result in additional warning dialogs when a user opens compiled help
files with references to the HHCtrl control, even if the help file is
part of legitimate locally installed software.
Avoid accessing filesystems writable by untrusted users.
Because of the difficulty in implementing this solution correctly, the
CERT/CC does not recommend relying on this solution. You may want to
consider this solution only if you can implement it easily or if you
have no other viable choices.
Care should be taken with any mechanism that might allow an untrusted
user to download or otherwise cause a file to be accessible to the
victim. This includes, but is not limited to, network-based file
sharing mechanisms (AFS, DFS, Netware, NFS, Windows shares) and mail
delivery programs that automatically extract attachments.
Also, if you choose to implement this solution, you need to be
especially vigilant in your monitoring of security resources for
information about new vulnerabilities that allow attackers to download
files to your system. The impact of these vulnerabilities will be
greater than if you had selected one of the solutions recommended
above.
Appendix A. Vendor Information
Microsoft Corporation
Microsoft recommends customers using Microsoft Internet Explorer
version 4.0, 4.01, 5.0, or 5.01 apply the patch discussed in
http://microsoft.com/technet/security/bulletin/ms00-037.asp and
routinely use the Security Zones feature.
The Security Zones feature of Internet Explorer allows you to
categorize the web sites you visit and specify what the sites in a
particular category should be allowed to do. Since most people visit a
small number of familiar, professionally-operated web sites, and it's
unlikely that such a site would pose any risk, we recommend putting
the sites that you visit frequently and trust into the Trusted Zone.
All sites that you haven't otherwise categorized will reside in the
Internet Zone. You can then configure the zones to give the
appropriate privileges to the web sites in each of these zones.
In addition Microsoft recommends Outlook users install the Outlook
Security Update
http://www.officeupdate.com/2000/downloaddetails/Out2ksec.htm to
protect against mail-borne attacks.
_________________________________________________________________
Thanks to Georgi Guninski, who originally discovered this
vulnerability and who also provided input used in the development of
this advisory.
_________________________________________________________________
Cory Cohen was the primary author of this document, with some text by
Shawn Hernan.
______________________________________________________________________
This document is available from:
http://www.cert.org/advisories/CA-2000-12.html
______________________________________________________________________
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
U.S.A.
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
http://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
http://www.cert.org/
To be added to our mailing list for advisories and bulletins, send
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* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.
______________________________________________________________________
NO WARRANTY
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
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does not make any warranty of any kind with respect to freedom from
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_________________________________________________________________
Conditions for use, disclaimers, and sponsorship information
Copyright 2000 Carnegie Mellon University.
Revision History
June 19, 2000: Initial release
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