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Microsoft Internet Explorer JavaScript Window() Code Execution
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SUMMARY
By crafting special Javascript string, attackers can cause Internet
Explorer to execute arbitrary code.
DETAILS
Vulnerable Systems:
* Microsoft Internet Explorer version 5.5
* Microsoft Internet Explorer version 6
The JavaScript "Window()" function when used in conjunction with a <BODY
onload> event. As a result, Internet Explorer encounters an exception when
trying to call a dereferenced 32bit address located in ECX, as highlighted
by the following line of code:
CALL DWORD [ECX+8]
Due to the bug, ECX is inadvertently populated by the Unicode
representation of a text string named "OBJECT", or more specifically
0x006F005B. As offset 0x006F005B points to an invalid (or non-existent)
memory location, Internet Explorer fails to progress, and in turn the end
user experiences an application crash (DoS).
Therefore, as the bug does not yield control of any internal register
and/or points to an offset of which we have no control, the original "low"
risk classification clearly reflects the improbable scenario for remote
code execution.
If we take a closer look at the vulnerability, we actually see that the
instruction is trying to dereference an offset in the range of 0x00600000,
which, coincidently, is reserved for the facilitation of all opened Window
characteristics on the desktop.
These structures vary in both length and content, but in the main, take
the form of window titles, buttons, and any File/edit/View menus bars
attributable to a particular Window session.
Consequently, it is feasible to assume that offset 0x006F005B could be
arrived at through the invocation of several new Windows structures, for
example circa 12 new web browsing sessions, which would increment
0x00600000 into 0x006F005B.
If this were possible, it would just leave the problem of trying to
identify a means by which custom shellcode could be inserted via one of
the Window Elements, and correctly aligned against the called
[0x006F005B].
Accordingly, several methods were tested. By using a combination of
multiple open windows (expanding the memory section), and legal techniques
that allow the modification of certain Window elements (examples below),
3rd party code execution was eventually realised!
Example:
1. Long HTML <TITLE>
2. Long embedded Document File Names
3. Large Alert Boxes
Unfortunately, all methods tested suffered from one major flaw -
inconsistency.
The assumption that a potential victim has a clean desktop (no open
applications) compounded by the fact that most window elements encompasses
some form of content length restriction, results in a very small
opportunity for any realistic exploitation.
Except, for one particular approach... a JavaScript prompt box.
By employing a simple technique to invoke multiple occurrences of large
JavaScript prompt Boxes, it is possible to flood/saturate the remoteness
between 0x00600000 - 0x006F005B ++ with data of our choice, yielding very
reliable execution of arbitrary code.
Proof of Concept:
<http://www.computerterrorism.com/research/ie/poc.htm>
http://www.computerterrorism.com/research/ie/poc.htm
ADDITIONAL INFORMATION
The information has been provided by
<mailto:securityadvisory@computerterrorism.com> computerterrorism.com.
The original article can be found at:
<http://www.computerterrorism.com/research/ie/ct21-11-2005>
http://www.computerterrorism.com/research/ie/ct21-11-2005
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