ActiveX漏洞的入门读物
ActiveX - Active Exploitation
01/2008
warlord
warlord@nologin.org
http://www.nologin.org
Share what I know, learn what I don't
1) Foreword
First of all, I'd like to explain what this paper is all about, and
especially, what it is not. A few months ago I got into the technical details
of ActiveX for the first time. Prior to this point I only had some vague
ideas and a general understanding of what it is and how it works. What I did
first is probably quite obvious: I googled. To my surprise though, I could
not find a single paper discussing ActiveX and how to exploit it. My next step
was to contact some generally smart and knowledgable friends to harvest the
required information from them. I was even more surprised to find that some of
the most skilled people out there lacked the same knowledge that I did.
Perhaps it's our common background, coming from the Unix/Linux world, but
whatever the reason, I had to work to collect the information I now possess.
But still, I feel like I'm the one-eyed man explaining what the world looks
like to the blind.
The fact that there are tons of ActiveX exploits on Milw0rm which would
suggest that the knowledge is out there by now. I wonder why no one took the
time to write it all up so the less knowledgable may get into this theater as
well. It's the intention of this paper to fill this gap. If you already know
everything about ActiveX, if you've found your own 0day and exploited it
successfully, I probably can't teach you any new tricks. Everyone else I
invite to read on.
2) Introduction
ActiveX[1] is a Microsoft technology introduced in 1996 and based on
the Component Object Model (COM) and Object Linking and Embedding (OLE)
technologies. The intention of COM has been to create easily reusable pieces of
code by creating objects that offer interfaces which can be called by other
COM objects or programs. This technology is widely used for what
Microsoft calls ActiveX[2] which represents the integration of COM
into Internet Explorer. This integration offers the ability to interface
with Windows as well as third-party applications with the MS browser. This
allows for the easy extension of functionality in the Internet Explorer by
giving software developers the ability to create complex applications which
can interface with websites through the browser.
There are various ways for an ActiveX control to end up on any given machine.
Besides all the controls which are part of IE or the operating system,
programs may install and register ActiveX controls of their own to offer a
diverse set of functions in IE. Another way of installing a new control is
through web sites themselves. Depending on Internet Explorer security
settings, a website may try to instantiate a control, for example Shockwave
Flash, and failing to do so may prompt the user to install the Shockwave Flash
ActiveX control.
Security issues seems to be a constant problem with ActiveX controls.
In fact, it seems most vulnerabilities in Windows nowadays are actually due to
poorly-written third-party controls which allow malicious websites to exploit
buffer overflows or abuse command injection vulnerabilities. Quite often
these controls make the impression of their authors not having realized their
code can be instantiated from a remote website.
The following chapters will describe methods to find, analyze, and exploit
bugs in ActiveX controls will be presented to the reader.
3) Control and functionality enumeration
Any given Windows installation is likely to have a significant number of
registered COM objects. For the purpose of this paper, however, we are only
interested in controls which may be instantiated from a website. Quite a
number of the following details are taken out of the excellent "The Art
of Software Security Assessment"[3], a book I strongly recommend to
anyone interested in application security.
ActiveX controls are usually, but not always, instantiated by passing their
CLSID to CoCreateInstance. The respective class identifier (CLSID) is used as
a unique value which is associated with each control in order to distinguish
it from its peers. A list of all the existing CLSIDs on a given Windows
installation can be found in the registry in HKEY_CLASSES_ROOT\CLSID, which
actually is nothing but an alias to HKEY_LOCAL_MACHINE\Software\Classes\CLSID.
Within the CLSID key there are thousands of different class identifiers, all
of them specifying ActiveX controls. However, only a subset of those can be
instantiated by a website. Controls marked as safe for scripting are granted
this ability. To determine whether a certain control has this ability, it has
to be part of the respective category. Specifically, the category can be
found in the registry in the form: HKEY_CLASSES_ROOT\CLSID\
indicate this by having a subkey with the GUID
7DD95801-9882-11CF-9FA9-00AA06C42C4. Similarly, the 'safe for initialization'
category is listed in the same location, but with a slightly different GUID.
Its value is 7DD95802-9882-11CF-9FA9-00AA06C42C4.
In the end though, not being part of these categories doesn't necessarily mean
that a control cannot be called from IE. The component may dynamically report
itself as being safe for scripting when it is instantiated through IE. The
only surefire way is to try and instantiate a control and see if it can be
used. Axman[5] is an ActiveX fuzzer written by HD Moore which can automate this
check for all of the different CLSIDs on a system. Another tool to enumerate
the controls in question is iDefense's ComRaider[4], another ActiveX fuzzer,
which has the ability to build a database of controls that IE should be able
to instantiate.
3.1) ProgIDs
Besides the long and rather hard to memorize CLSID there is often a second
way of instantiating a certain control. This can be accomplished through the
use of a control's program ID (progID). Quite similar to IP addresses and the
domain name system(DNS), progIDs can be looked up to determine the matching
CLSID. Once the right one has been determined, Internet Explorer goes on as
if the CLSID had been provided in the first place.
For this technique to work for a given control, two requirements must be met.
First, a control must have a ProgID subkey under their CLSID key in the
register. ProgIDs are usually in the form Program.Component.Version such as
SafeWia.Script.1. Second, as there is no point for Windows to walk through up
to 2700 CLSIDs(in my example) to find the specified ProgID, the program ID
itself must have a key in HKEY_CLASSES_ROOT with a subkey named CLSID which
describes makes the association.
3.2) The Kill Bit
In some cases it is desirable to restrict a control from ever being
instantiated in IE. This can be accomplished through the use of a
kill bit. The kill bit can be defined by setting the 0x00000400 bit
in the DWORD associated with a given CLSID:
HKLM\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\
3.3) User Specific Controls
With Windows XP, Microsoft introduced support for user-specific ActiveX
controls. These do not require Administrator-level access to install because
the controls are specific to a certain user, as the name already implies.
These controls can be found under HKEY_CURRENT_USER\Software\Classes. While
this functionality exists, most ActiveX controls are installed globally.
3.4) Determining Exported Functions
ActiveX controls implement various COM interfaces in the same manner as any
other COM object. COM interfaces are well-defined definitions of what
functions and properties a COM class must implement and support. COM provides
the ability to dynamically query a COM class at runtime using QueryInterface
to see what interfaces it implements. This is how IE determines if a control
supports the safe for scripting interface (which is called IObjectSafety).
4) Examples
4.1) MW6 Technologies QRCode ActiveX 3.0
In this section the previously provided information will be demonstrated with
the help of a recent public ActiveX vulnerability and exploit. The vulnerable
control is from a company called WM6 and comes with their ``QRCode ActiveX''
version 3.0. When I downloaded the software in January 2008, several months
after the exploit was posted on Milw0rm in September, the vulnerable control
was still part of the package.
The control itself has a CLSID of 3BB56637-651D-4D1D-AFA4-C0506F57EAF8. After the
installation of the software, it can be found in the registry in:
HKEY_CLASSES_ROOT\CLSID\{3BB56637-651D-4D1D-AFA4-C0506F57EAF8}
The DLL that implements this control can be found on the harddrive in the file
that is specified in the "InprocServer32" key. In this example it is:
C:\WINDOWS\system32\MW6QRC~1.DLL
There are two interesting things to note here. For one, the ProgID key has a
default value of MW6QRCode.QRCode.1. At the ProgID's corresponding location in
the registry, namely HKCR\MW6QRCode.QRCode.1, the CLSID subkey contains the
CLSID of that control. This tells us that this control can be instantiated
using both its CLSID and ProgID. Another point of interest in the screenshot
is the absence of the "Implemented Categories" key. This means that this
control is neither part of the "safe for scripting" nor the "safe for
initialization" category. However, it appears that the control must implement
IObjectSafety since it is still possible to instantiate the control from IE.
The following simple HTML code tries to instantiate the control.
The result of this snippet of code is the appearance of a little picture in IE.
As this works just fine without Internet Explorer complaining about being
unable to load the control, the next examination step is in order.
4.1.1) Enumerating Exported Interfaces
By now it has been shown that the example control can be instantiated from IE
just fine. The question now is what kind of interfaces the control provides to
the caller. By submitting the specific CLSID of the control that is to be
examined to ComRaider, the tool lists all of the controls implemented
functions as well as the kind and number of expected parameters. An
alternative to ComRaider is the so-called OLE-COM object viewer that comes
with the platform SDK and Visual Studio.
4.1.2) Exploitation
After playing around with various functions, it soon becomes obvious that
SaveAsBMP and SaveAsWMF happily accept any path provided to save the
generated graphic in the specified location. This can make it possible to
overwrite existing files with the picture if the user running IE has
sufficient access. This is a perfect example of a program using untrusted
data and operating on it without any kind of checks. It is likely that the
control's author did not consider the security implications of what they were
doing.
A sample exploit for this vulnerability, written by shinnai, can be found on
Milw0rm: http://www.milw0rm.com/exploits/4420.
4.2) HP Info Center
On December 12th, 2007, a vulnerability in an ActiveX control which was
shipped by default with multiple series of Hewlett Packard notebooks was
disclosed. The issue itself was found in a piece of software called the HP
Info Center. The vulnerability allowed remote read and write access to the
registry as well as the execution of arbitrary commands. By instantiating
this control in Internet Explorer and calling the vulnerable functions it was
possible to run software with the same level of access as the user running IE.
Porkythepig found and disclosed this serious threat and wrote a detailed
report as well as a sample exploit covering three attack vectors.
The HP control with the CLSID 62DDEB79-15B2-41E3-8834-D3B80493887A was
responsible for the listed vulnerabilities. By default it installs itself into
C:\Program Files\Hewlett-Packard\HP Info Center. In his advisory, porky
listed three potentially insecure methods as well as the expected parameters:
- VARIANT GetRegValue(String sHKey, String sectionName, String keyName);
- void SetRegValue(String sHKey, String sSectionName, String sKeyName, String sValue);
- void LaunchApp(String appPath, String params, int cmdShow);
While the first and second method allow for remote read and write access to
the registry, the third function runs arbitrary programs. For example, an
attacker could execute cmd.exe with arbitrary arguments.
In this example the vulnerable control provided remote access to the victims
machine. Sample code to exploit all three functions can once again be found on
Milw0rm: http://www.milw0rm.com/exploits/4720.
4.3) Vantage Linguistics AnswerWorks
The third and last example of various ActiveX vulnerabilities is in the
Vantage Linguistics AnswerWorks. Advisories covering this vulnerability were
released in December, 2007. The awApi4.AnswerWorks.1 control exports several
functions which are prone to stack-based buffer overflows. The functions
GetHistory(), GetSeedQuery(), and SetSeedQuery() fail to properly handle long
strings provided by a malicious website. The resulting stack-based buffer
overflow allows for the execution of arbitrary code, as "e.b." demonstrates
with a proof of concept that binds a shell to port 4444 when the exploit
succeeds.
When the exploit is loaded from a webserver it instatiates the CLSID and links
the created object to a variable named obj. It then calls the GetHistory()
function with a carefully crafted string which consists of 214 A's to fill the
buffer followed by a return address which overwrites the one saved on the
stack. After those 4 bytes come 12 NOPs and then finally the shellcode. As
one can easily see, this exploit is based on the same techniques that can be
seen in many other stack-based exploits.
The exploit mentioned in this example can also be found on Milw0rm:
http://www.milw0rm.com/exploits/4825.
5) Summary
This paper has provided a brief introduction to ActiveX. The focus has been
on discussing some of the underlying technology and security related issues
that can manifest themselves. This was meant to equip the reader with enough
background knowledge to examine ActiveX controls from a security point of
view. The author hopes he managed to describe the big picture in enough detail
to provide readers with enough information on the matter to base further
research on the aquired knowledge.
5.1) Acknowledgements
wastedimage - For answering the first questions
deft - For providing lots of answers and examples
rjohnson - For filling in details deft forgot to mention
skape - For background knowledge on underlying functions
hdm - For knowing all the rest
References
[1] ActiveX Controls @ Wikipedia
http://en.wikipedia.org/wiki/ActiveXcontrol
[2] ActiveX Controls
http://msdn2.microsoft.com/en-us/library/aa751968.aspx
[3] The art of software security assessment
http://taossa.com
[4] ComRaider
http://labs.idefense.com/software/fuzzing.php#morecomraider
[5] Axman ActiveX Fuzzer
http://www.metasploit.com/users/hdm/tools/axman/
- 文章作者: 福州军威计算机技术有限公司
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