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Thread: Forensic Analysis of Malcode - Step by Step

  1. #1
    Master-Jedi-Pimps0r & Moderator thehorse13's Avatar
    Join Date
    Dec 2002
    Washington D.C. area

    Forensic Analysis of Malcode - Step by Step

    Overview: (Please Read)
    As many of you venture into a pervasive computing environment, it will not be long before
    you will be faced with a situation where forensics will be needed. This is an upcoming, and
    in my opinion, will be the hottest area of security. If you’re one to chase the big bucks and
    you want to stay in the technology track, then this is the route for you. Otherwise, go off
    and write documentation for all of the new regulations. That too is hot and returning hefty salaries.

    I’m going to step you through the process of dissecting a malcode sample using tools and
    techniques that are commonly used by forensic teams. Keep in mind that at the time of
    this writing there are no set standards in forensics. This means that you may find other
    forensic teams doing things differently but rest assured that the techniques I’m about to
    show you are used by the top minds in the discipline, including myself.

    Throughout this tutorial, all operations will be done in a 6 gig Windows 2000 professional
    VM environment. The OS is fully patched and there are no antivirus scanners or firewalls
    running (A virgin installation). The VM does have network connectivity and the network it
    sits on is VLANed into a network segment that only has internet access upon manual
    activation. There are no routes to production or other lab environments. In other words, it’s
    in complete isolation from areas where it could cause harm.

    Typically you want to use a VM instance to conduct your initial testing of the code or any
    other host that you can reformat or otherwise abuse for the purposes of analysis without
    affecting production hosts/environments. I use VMs because when a problem crops up I
    can simply regenerate a fresh image in seconds instead of hours if I had to do a fresh
    install. The point here is to have a controlled throw away environment to conduct your

    This tutorial is limited to one very small part of forensics. If this tutorial proves valuable
    based on feedback, I will move on to other situations/techniques such as scraping data
    from a hard drive to establish what users have done on the host including attempts to
    remove traces of their activities, what disposable media was used on the host, etc.
    Network forensics is another area I can touch on but would require me to write a book to be

    One last thing. Because I don’t want to attach images and such to the thread, I’ve decided
    to describe what I can without screenshots. I will insert text where I can but things should be relatively clear. If not, PM me
    and I will put together a screen shot library for you.

    Our Malcode: hax0r.exe
    You find a piece of malcode on a host. No antivirus company knows what it is and you want
    more information about it.

    TIP: Norman Sandbox: http://sandbox.norman.no/live_4.html Is a place where
    malcode is run in a sandbox and the results are returned to you. I use this for quick and
    dirty analysis. All you do is upload a sample and it maps out what the malcode does. This
    can be very helpful in a zero day scenario. As we will see, this method may not always
    work for you.

    Let’s begin by doing a tool inventory. Whenever you’re going to do an examination of code,
    you’re going to need a few base tools.
    a) A good hex editor. I use OllyDbg (Free) or if you have a few thousand dollars, SoftICE will
    be an excellent selection.
    b) A Strings program. Strings.exe on Win32 is not native. You’ll have to grab a third party
    tool such as strings.exe (command line) or BinText (Win32 GUI). Both are free and do the
    same thing.
    c) A Win32 Portable EXE identifier. A key part of analysis will be to identify what packer was
    used to compress an EXE. If you cannot identify the packer, you’ll be forced to actually run
    the malcode to see what it does. This isn’t necessarily a problem but for this tutorial, we’re
    going to want to identify the packer. I use PEiD for identification. Again, a free tool found
    here: http://peid.has.it/
    d) Once we ID the packer, we’ll need to unpack it. UnFSG and Procdump will be needed for
    this case. Both are free.
    e) A program to monitor changes to your environment. I use install watch pro. http://www.epsilonsquared.com/installwatch.htm

    TIP: There are hundreds of PE packers out there. You may not always be able to unpack
    them and will be forced to try to run the malcode to see what it does. For a list of PE
    packers, see: http://www.exetools.com/compressors.htm. Note that the site also
    has a bunch of unpackers as well. Depending upon successful identification of the packer,
    you may be able to use one of them to unpack the malcode.

    OK, so here we go.

    1) Using strings.exe, I first determine if the exe is compressed with a Win32 packer. Typically, you will see
    short bursts of 4 or so random characters and maybe a line or two of
    text if it is packaged/encrypted. Once you’ve seen a few packaged exe files you’ll quickly be
    able to tell when an exe is packaged or not. Even if you fail to do so, OllyDBG will throw a
    warning up if it detects that the exe is packaged/encrypted.

    First let’s look at an example of a non packaged exe: sid2user.exe. This is totally random. I
    chose it simply because I had it in the same folder as my strings executable.
    c:\temp\strings.exe sid2user.exe
    Borland C++ - Copyright 19
    Evgenii Rudnyi (C) All rig
    Chemistry Department, Mosc
    119899 Moscow, Russia, htt
    This utility is freeware a
    distribute it. Optionally,
    you may send me a bottle o
    Disclaimer of warranty:
    This utility is supplied a
    express or implied, includ
    merchantability and of fit
    no liability for damages d
    the use of this utility.
    The goal of the utility is
    sid2user [\\computer_name]
    where computer_name is opt
    sid2user 5 32 544
    By default, the search sta
    could not allocate SID
    Name is
    Domain is
    Type of SID is
    LookupSidName failed - no
    Stack Overflow!
    Error 0
    Invalid function number
    No such file or directory
    Path not found
    Too many open files
    Permission denied
    Bad file number
    As you can see, strings can be helpful in identifying potential help or avenues of
    investigation against standard output statements. Notice lots of plain text and system error

    And now, here is what a packaged exe looks like: hax0r.exe.

    c:\temp>strings.exe hax0r.exe
    =n p>+
    So as you can see, spotting a packaged exe is pretty easy. Now we have to figure out what
    the exe is packed with. Let’s open hax0r.exe in PEiD and see what it has to say.

    NOTE: This is where a screen shot would be nice but instead, I will give you simple steps to
    1) Open PEiD
    2) Browse to the hax0r.exe file
    3) Look at the bottom of the dialog box to see the packager. In our case, it shows FSG, 
    which is a acronym for Fast, Simple, Good. There is other info returned about the file such
     as entry point, offset, first bytes, etc. These values are important but will not play heavily
     in this tutorial.

    TIP: Evil mean haxors will repackage their malcode with different packers as soon as they
    discover that anti virus companies have a signature out for it. It is not uncommon to see
    the same malcode show up with 20 or 30 different hash values. This is done to beat AV

    OK, so we now know that our hax0r.exe malcode is packed with FSG. It is useless to us
    until we unpack it. This is where UnFSG comes in.
    1) Open UnFSG
    2) Select the file to “UnFSG”
    3) Tell it where to write the file.
    OK, so now we have a good unpacked copy right? Wrong. During the unpack, damage is
    done to the PE headers and tables and we need to fix that before going forward. Procdump
    will do this for you.

    1) Open procdump
    2) Select the file to fix
    3) Select a place to write the file.
    NOTE: There is a command line and GUI version of this tool. The commands vary slightly on

    OK, we now have a unpacked malcode executable that we can run. OH NO! The malcode is
    password protected. What ever will I do?

    TIP: Sometimes malcode is password protected so that only the C&C master can run it.
    This is the case in our example. Also, some malcode is very advanced and can detect when
    it’s being run in a hex editor such as OllyDBG and will crash the system. Be aware of this
    because it’s not your PC or software, it’s done by design of the malcode author.

    OK folks, time to break open this malcode to find the embedded password. Incidentally, the
    technique I’m about to show you is close to that used when stripping license info out of
    commercial software. Please don’t abuse this knowledge.
    1) In the OllyDBG main window, select F9 or select debug-run from the main menu.
    2) The malcode executes and we see a “Enter password” prompt at the command prompt.
     Since we know that “Enter password” was displayed, we can now go to the OllyDBG stack
     history window and search for this term. This window is in the bottom right of the display.
    3) A reference to the string was found at 0x00304052A71. Double click this value and you
     be taken to this address in the main thread window.
    4) Set a breakpoint (this is as much art as it is science at times) about ten lines below 
    which is where it will break right before the password is entered. To do this simply right
     click the line and choose, set break point. It will then turn red.
    5) Now, go to the command prompt and enter any old password. I’ll enter “dummy” and 
    press enter.
    6) Press F7 a few times to step through some instructions. Keep an eye on the 
    Examination of registers window in the top right side of the display. Soon you will see:
    EAX 0012FB6D ASCII “j00arenotl33t”
    ESI    0012FB8C ASCII “dummy”
    Examination of the registers have yielded the password comparison and shows that the
     password to the malcode is j00arenotl33t.
    Anyway, that’s how you can easily discover passwords in any kind of password protected
    application. Be aware that malcode writers do not want you to do this so they will go to
    great length to keep you from unpacking their malcode.

    I now turn on a free application called install watch pro. This will trace everything that the
    malcode does in my environment. To use this app, install it, open it and simply start
    the “record session” process. It’s a GUI tool and is VERY simple. Once you turn on the
    record feature (which is simply a snapshot of the current system) you open the malcode
    from within the install watch pro interface. I then records everything and exports it in
    several formats for your enjoyment.

    I now examine the output from install watch pro. I see several references in there that
    execute netcat shell commands. BINGO.

    OK, so what I see is that my malcode is a password protected NetCat shell that shovels a
    terminal back to the attacker. This is a very simple example of a malcode payload. There
    are many out there that obviously do much more complex things. This is for educational
    purposes to get the mechanics down for performing the actual analysis.

    Anyway, I hope you found this informative. Again, if feedback is good, I will do other
    forensic tutorials. As always, comments good or bad are always welcome.
    Our scars have the power to remind us that our past was real. -- Hannibal Lecter.
    Talent is God given. Be humble. Fame is man-given. Be grateful. Conceit is self-given. Be careful. -- John Wooden

  2. #2
    Senior Member
    Join Date
    Oct 2002
    Awesome tut on some forensics man.. I agree with you that in the coming years, it will become a vital and integral (sp?) part of security. Would you possibly have this in a downloadable .txt/.zip form?

    Thanks again man, definitely greenies..
    Space For Rent.. =]

  3. #3
    In And Above Man Black Cluster's Avatar
    Join Date
    Feb 2005
    Great tut. dude. One of the nicest reads!
    Filed and saved

    Keep them coming

    \"The only truly secure system is one that is powered off, cast in a block of concrete and sealed in a lead-lined room with armed guards - and even then I have my doubts\".....Spaf
    Everytime I learn a new thing, I discover how ignorant I am.- ... Black Cluster

  4. #4
    Master-Jedi-Pimps0r & Moderator thehorse13's Avatar
    Join Date
    Dec 2002
    Washington D.C. area
    Sorry about the formatting folks.

    Perhaps a little help from the moderator to fix up my FUBARed formatting?


    Fixed. No action required.

    Our scars have the power to remind us that our past was real. -- Hannibal Lecter.
    Talent is God given. Be humble. Fame is man-given. Be grateful. Conceit is self-given. Be careful. -- John Wooden

  5. #5
    I'd rather be fishing DjM's Avatar
    Join Date
    Aug 2001
    The Great White North
    Originally posted here by thehorse13
    Sorry about the formatting folks.

    Perhaps a little help from the moderator to fix up my FUBARed formatting?


    Ah.........TH13.....you are a Mod!


  6. #6
    Senior Member
    Join Date
    Jan 2003
    Hey Hey,

    TH13 -- We should have the same level of modding abilities and it's not enough ...

    Why dont' you just click edit and fix up the formatting? Add some CRs to your code sections

    Once it's fixed I'll read it and tell ya how amazing it most likely is

    IT Blog: .:Computer Defense:.
    PnCHd (Pronounced Pinched): Acronym - Point 'n Click Hacked. As in: "That website was pinched" or "The skiddie pinched my computer because I forgot to patch".

  7. #7
    Senior Member
    Join Date
    Oct 2002
    We should have the same level of modding abilities and it's not enough ...
    AFAIK, you do.
    Space For Rent.. =]

  8. #8
    Master-Jedi-Pimps0r & Moderator thehorse13's Avatar
    Join Date
    Dec 2002
    Washington D.C. area
    Formatting fixed.
    Our scars have the power to remind us that our past was real. -- Hannibal Lecter.
    Talent is God given. Be humble. Fame is man-given. Be grateful. Conceit is self-given. Be careful. -- John Wooden

  9. #9
    Senior Member
    Join Date
    Oct 2002
    Once again, excellent tut man.. probably the best I've read in some time explaning basic forensic analysis principles and means. Indeed, a simple yet explanatory example. Kudo's to you..

    Now, about that downloadable text file?
    Space For Rent.. =]

  10. #10
    Computer Forensics
    Join Date
    Jul 2001
    Not to be a bastage, but did you just happen to read real digital forensics? This is extremely similar to the analysis they run in the book.

    That said, if you want to collaborate on forensic tuts, let me know (I don't have enough time to write a full tut myself).
    Antionline in a nutshell
    \"You\'re putting the fate of the world in the hands of a bunch of idiots I wouldn\'t trust with a potato gun\"

    Trust your Technolust

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