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Advisory: Internet Explorer Drag and Drop Redeux [CVE-2005-3240] (fwd)
From: Matthew Murphy <mattmurphy () kc rr com>
Date: Mon, 13 Feb 2006 18:46:38 -0600
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My apologies to those who are receiving this late or are otherwise
inconvenienced by the staggered release. I had unexpected, last-minute
travel issues that interfered somewhat with today's release.
Of note since the initial drafting of the advisory is that Microsoft has
released a blog post on the MSRC blog about the vulnerability report,
which can be read here:
The technical/strategic points about the exploit that are raised in the
post are indeed accurate (though it references MS05-014, when I believe
the correct reference is MS05-008/MS05-013). The exploit has a greater
dependence on timing than previous, related attacks. As such,
Microsoft's decision not to include this issue in a standalone patch is
seemingly justified at this point. However, the point of disagreement
with Microsoft remains the choice of release *timeline*.
I released the information about this issue to a trusted colleague (Gadi
Evron) for publication today, after what I felt was a reasonable time,
in light of my difficulties obtaining internet access.
Though there are disagreements between myself and Microsoft about the
nature of this vulnerability, I would like to thank Brian Schafer of the
MSRC for adhering to a high level of professionalism and technical
accuracy in that post and for continuing to work with me once it was
made clear that the issue would imminently become public.
Also of note is that there was a typo in the information I provided
originally to SecuriTeam. The proper candidate is CVE-2005-3240, not
*3840* as was originally reported by me. SecurityFocus has also
informed me that my original BID reservation was a casualty of a data
migration and that the proper BID associated with this vulnerability is
now BID 16352, which is public in full detail as of this writing.
There have also been some incorrect reports made to SecuriTeam that this
issue does not affect Windows XP Service Pack 2. These reports are not
correct -- my testing during this investigation was done exclusively on
current installations of Windows 2000 and Windows XP. These systems had
all service packs applied and all updates installed when tests were
Thanks to Gadi Evron for doing some of my bidding today and taking some
of the heat for my fat-fingers.
The final advisory, corrected with the now-accurate references is
attached with an armored-format PGP signature inline.
"Social Darwinism: Try to make something idiot-proof,
nature will provide you with a better idiot."
-- Michael Holstein
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Microsoft Internet Explorer Drag-and-Drop Redeux
* Microsoft Internet Explorer 5.01
* Microsoft Internet Explorer 5.5
* Microsoft Internet Explorer 6.0
- Windows 98
- Windows 98 Second Edition
- Windows Millennium Edition
- Windows 2000
- Windows XP
- Windows Server 2003
Impact: Potential remote code execution with some user interaction
Status: Uncoordinated Release
Author: Matthew Murphy (mattmurphy () kc rr com)
II. VULNERABILITY OVERVIEW
Microsoft Internet Explorer suffers from a vulnerability in its handling of certain drag-and-drop events. As a result,
it is possible for a malicious web site to predict and exploit the timing of a drag-and-drop operation such that any
drag operation (including using scroll-bars) could potentially lead to the installation of arbitrary files in sensitive
locations that may enable further system compromise.
III. TECHNICAL DESCRIPTION
As a result of recent updates to its drag-and-drop functionality, Internet Explorer now imposes a rigid set of
restrictions on most drag-and-drop sources:
* Input to the browser from other applications is not permitted.
* Dragging an object from inside a frame is not permitted.
* Dragging an HTML element from a top-level window will produce a security warning.
However, certain objects not derived from an HTML document (specifically, file objects within a folder view) remain
draggable. This gives rise to a potential race condition in the handling of user input. If an attacker can persuade a
user to drag any object within the top-level window that his/her site is contained in, malicious script can redirect
these inputs to other top-level windows, potentially resulting in an unintended consequence such as file installation.
Proof-of-concept code has been developed that utilizes a pop-under window pointing to a malicious file share. This
window can be created using window.open() or other stealthier methods that are known to evade Internet Explorer's
built-in pop-up blocking. Focus is then returned to the opening window, where the user is encouraged to drag an object
(image, link, etc.) in a seemingly "safe" fashion.
Immediately prior to this object being dragged, a mouseOver event is triggered that enables the attacker to (with a
varying degree of success) predict the imminent drag attempt. The pop-under can then be returned to focus by way of a
window.blur() executed in the current window. If the timing of the transition is accurate to a margin of error within
a user's reaction time threshold, the user will unwittingly initiate a drag of a file from the pop-under instead of the
object originally used as a lure by the attacker.
As soon as it transfers focus, the window with the original interactive content may set a timer (via
window.setTimeout()) that returns focus to the window with a simple window.focus() call. After a split-second delay,
focus is returned to the interactive window. At this point, on-demand alteration of CSS attributes can be used to
display previously-hidden objects (such as inline frames). These objects serve as "drop target" windows and will
initiate the copying of the file dropped from the (presumably malicious) pop-under window.
While Internet Explorer blocks hiding or resizing of certain "suspect" objects (IFRAMEs, for instance), so-called
container objects (DIV, SPAN, etc.) suffer no such restrictions, even when they contain one of the objects in the
former category. The proof-of-concept code as developed simply stores a full-screen inline frame in a container
initially marked with the "hidden" visibility style.
The pop-under window, in this instance, would be a folder on a malicious server. This could be accessed via SMB
(\\HOSTILESERVER\SHARE), FTP (ftp://hostileserver/somedirectory) or even HTTP (web folders) using certain link
behaviors in combination with the click() method of a hyperlink object. In the third case, the pop-under would be
targeted to an HTML document initally, which would then open the web folder containing hostile content.
The path to the drop target (the hidden frame in the original window) requires a little more creativity. Particularly
in Windows XP Service Pack 2, Microsoft has done a fairly good job of locking down access to local resources. The most
interesting vector for the purposes of this attack is via the network redirector. By using the IP address or machine
name of the local system (typically obtainable via any number of means), such as:
It becomes possible to access resources offered by the network redirector on the local system. Of most interest is the
"Scheduled Tasks" folder:
Items dropped into this folder execute automatically at a system-determined time (3 AM local time in tests on Windows
XP Professional Service Pack 2) each day as the user dropping the file. Also of interest are common shares such as the
administrative shares (C$, D$, etc.) and typical share names like "SharedDocs" on Windows XP. In most cases, this is
at least a partial functional equivalent to local file system access and is not subject to zone restrictions, even on
Windows XP Service Pack 2.
A malicious web site, with a minimum of social engineering, may be able to compromise user systems by triggering an
unintended installation of malicious software. Typical defense-in-depth measures may mitigate this issue. For those
who run Internet Explorer with administrative privileges, the impact of any successful exploitation is complete control
of the affected system. A malicious web site could install software that would add or delete privileged user accounts,
alter, destroy or disclose the content of personal or otherwise sensitive files, record personal information or any
number of other activities.
Users who do not browse with such high levels of privilege would be at a significantly reduced risk from exploitation
of this vulnerability. In the case of a user with limited privileges, this vulnerability could only be exploited by an
attacker to install software that executes with the privileges of that user.
The following workarounds are believed at the time of this writing to be effective against the exploitation of this
vulnerability in some form:
1. Set a Kill Bit on the Shell.Explorer Control
Setting a kill bit on this control will prevent Internet Explorer from displaying the rich folder view interface that
gives rise to this attack. For more information about setting kill bits, please see Microsoft Knowledge Base Article
The CLSID of this component as deployed on Windows XP is:
Tools to automate the process of setting this kill bit have been provided at:
PGP signature: http://student.missouristate.edu/m/matthew007/tools/shellkill.zip.asc
Included in this archive are an Administrative Template (.adm) and a VBScript file (.vbs) which implement this setting.
The Administrative Template also allows an administrator to work around a specific case of functionality loss caused
by the implementation of this workaround. Instructions on using both files are contained within the readme file in the
IMPACT: This workaround will cause Internet Explorer to no longer render folder views for local directories, network
file shares, FTP directories and web folders by default. The ability to browse FTP directories in Internet Explorer
can be restored by clearing the "Enable Folder View for FTP Sites" option in Internet Explorer's "Advanced" options.
However, this countermeasure is known to expose another security vulnerability that does not appear to have been fixed
as of this writing:
For ordinary browsing purposes, the Windows Explorer tool is unaffected by this change. This defensive measure has
been successfully implemented in at least one commercial software product and tested on a significant scale prior to
the release of this advisory. Therefore, it is the belief of the author that potential loss of functionality *should*
be minimal. As with all measures, you are encouraged to test the impact of this workaround prior to making any
decision about deployment.
2. Prevent Automatic Navigation to Local Intranet Zone (Windows XP SP2, Windows Server 2003 SP1)
This workaround will prevent internet content in Internet Explorer from automatically navigating to URLs within the
Local Intranet Zone. This effectively prevents the introduction of malicious code to the local system via the network
redirector. To implement this workaround, follow these steps:
1. In Internet Explorer's Tools menu, choose "Internet Options..."
2. Select the "Security" tab and choose "Local Intranet"
3. Click the "Custom Level" button
4. Set the "Web sites in less privileged content zone can navigate into this zone" setting to "Disable" or
5. Click OK to close any dialogs and optionally, close Internet Explorer.
IMPACT: This workaround will block or prompt before allowing any navigation to LAN resources from the Internet Zone.
Direct access to LAN resources continues to function normally. As a result of this workaround, attempts to access
local intranet content (for instance, web applications on corporate intranets) from web sites outside of the LAN will
fail or produce prompts, depending upon the chosen setting.
3. Disable Active Scripting
This workaround will prevent internet content from executing script that could potentially cause the exploitation of
this vulnerability. To implement this workaround, follow these steps:
1. In Internet Explorer's Tools menu, choose "Internet Options..."
2. Select the "Security" tab and choose "Internet"
3. Click the "Custom Level" button
4. Set the "Active scripting" option to "Prompt" or "Disable".
IMPACT: This workaround will block or prompt before allowing web sites to execute any script statement. Scripting in
more-privileged zones (Local Intranet, Trusted Sites) continues to function normally. Setting this option to "Prompt"
may cause a significant increase in the number of security prompts received while browsing and may be ineffective in
closing this vulnerability for users not capable of making an assessment of a web site's relative trustworthiness.
VI. MITIGATION RECOMMENDATIONS
1. Limit Viewing to Trusted Web Sites
In some situations, browsing can be successfully limited to only trustworthy sites without significant loss of
productivity. Users should be extremely cautious while browsing unknown or untrusted web sites, as such web sites are
often able to introduce hostile code.
2. Run Exposed Applications With Reduced Privilege
Users who log on interactively without the privileges of powerful groups such as the "Administrators" or "Power Users"
groups are at a much lower risk of damage from successful exploitation of software vulnerabilities in client
applications. This mitigation step greatly reduces the likelihood of a successful malware installation if this
vulnerability is exploited.
VII. VENDOR RESPONSE
Microsoft was informed of this vulnerability on August 3, 2005. Currently, the company has no plans to issue a
security update to correct this vulnerability. Fixes for this issue are scheduled to be included in Service Pack 2 of
Windows Server 2003 and Service Pack 3 of Windows XP. Of particular note is that Windows 2000 users will *NOT* receive
an update to correct this vulnerability.
Microsoft's internal risk-assessment concluded that this issue was not sufficiently serious to be fixed in a security
bulletin. This conclusion appears fundamentally inconsistent with the way related issues were handled by Microsoft.
In particular, the drag-and-drop vulnerability patched by MS05-013 received an "Important" rating.
I disagree with the technical conclusion behind Microsoft's decision and I further find the timeframe of delivery and
deployment for maintenance releases to be largely unsuitable for security fixes of any significant magnitude. I find
the harm this decision could potentially inflict upon down-level users (most importantly, users of Windows 2000) to be
unjustified by the technical concern Microsoft has raised to me. Microsoft also rejected a request that it consider
the issue for inclusion in a later security update as a "Moderate" risk issue.
Due to Microsoft's noncommittal and generally unimpressive response to the issue, this advisory is being issued to
inform users of this vulnerability such that defensive action may be taken as desired.
The MITRE Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2005-3840 to this issue. Status
information and related references for this candidate may be found at:
The Open Source Vulnerability Database (OSVDB) project has assigned OSVDB vulnerability ID #2707 to this issue.
Information will be available shortly after the publication of this advisory at the following URL:
SecurityTracker has pre-assigned an alert number in its internal database to reference this issue. Information will be
available shortly after the publication of this advisory at the following URL:
SecurityFocus has pre-assigned BugTraq ID #15089 to reference this issue. Information will be available shortly after
the publication of this advisory at the following URL:
* The Administrative Template file supplied in the workaround ZIP was authored by Steven Platt.
The author may be contacted via e-mail at mattmurphy () kc rr com
This document is believed accurate based upon information available at the time it was written. However, the
information offered is offered in an AS-IS condition, without warranty. By acting upon this information in any way you
accept all responsibility for damage that may occur as a result.
This document may be reproduced in whole without limitation and in part provided that a full copy of the original
document is readily accessible and the author of the document is duly acknowledged.
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