PROBLEMS WITH WINDOWS SHORTCUTS

[S G Masood <sgmasood () yahoo com>]

PROBLEMS WITH WINDOWS SHORTCUTS






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Topic: Problems with Windows Shortcuts
Tested With: Windows 98, Windows 2000 Server
Author: S.G.Masood (sgmasood () yahoo com)


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DESCRIPTION:

There is a problem with the way Windows (tested with
Win98 and Win2k Server) handles shortcut (.lnk) files.

A specially crafted shortcut will crash
explorer.exe/shell32.dll.

A shortcut, say, A.lnk is created and it is made to
point to another
shortcut B.lnk. Then, B.lnk is made to point to A.lnk.
Now when the
folder containing these two files is viewed or
accessed in any way,
explorer crashes.

(Note that Windows won't allow the creation of .lnk
files in the above
format. A hex editor can be used to change the
location of the .lnk files. A zip file containing
examples for Win98 has been attached)

As an effect, a malicious user/program can hide
malware in a folder containing these .lnk files to
prevent users/programs from investigating the contents
of the folder.

This vulnerability is most damaging when the shortcuts
are placed on
the desktop. This could prevent many clueless users
from using their computer.


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VENDOR RESPONSE:

Microsoft was contacted and it responded with:

"...While this issue is certainly a bug, we believe
that it doesn't
constitute a security vulnerability.  That is, it
wouldn't enable a
malicious user to compromise data or usurp control
over the user's
machine..."


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SECURITY IMPLICATIONS OF THIS "BUG":


1. Under *most* circumstances, Explorer.exe will
restart when it crashes but in some cases, the machine
hangs and has to be restarted.  

2. When Explorer.exe crashes and restarts, it takes
all iexplore.exe instances with it, thereby crashing
them all. This scenario may not seem worthy of
attention at first glance but it may be damaging in
some cases.

3. The folder that contains these shortcuts may house
malware of other kinds. This may be exploited to hide
malware and stop users (and programs ?) from
investigating the contents of the folder. A few users
may still go ahead looking for other ways to
investigate it but, other, not-so-savvy, users will
just leave it alone thereby allowing the spread of new
types of *LAME* malware (the naivete of most users is
apparent from the wildfire type success of email
attachment viruses even after infinite warnings).

Similar vulnerabilities, harmless looking at first
glance, were used previously to devastating effect.


4. I believe this case is most serious as a DoS. If
the shortcuts or variants are placed on the Desktop,
it would keep crashing Explorer in an endless loop and
prevent users from using the machine (Oh naivete! Thou
art the most abundant quality in us mortals! ;-).


Also, this may be combined with other remote file
creation vulnerabilities to make it remotely
exploitable.


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SOLUTION:


No patch is availaible from the vendor. 
The shortcuts can be safely deleted from the
commandline.



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Regards,
S.G.Masood

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Attachment: test.zip
Description: test.zip