SLAE Assignment 1 | Bind TCP Shellcode - Linux x86

March 01, 2019   assembly , bind tcp shellcode , pentester academy , security tube , shellcoding , SLAE , SLAE x86

This blog post has been created for completing the requirements of SecurityTube Linux Assembly Expert Certification:
http://securitytube-training.com/online-courses/securitytube-linux-assembly-expert/
Student ID: SLAE - 1342

Hey Shellcoders,
   I've enrolled myself in SLAE course 2 months ago maybe and last week I finally get enough motivation to start working on it. So today I'm writing this post as the part of the assignment where I was given a task to create a Bind TCP shellcode.

• Bind to a port
• Execs Shell on incoming connection
• Port number should be easily configurable

First let's see how Bind TCP shell will work theoretically. Have a look at this below image first,

So basically a port will be open in the victim machine and the attacker will connect to the victim machine from that particular open port. This is just the basic that you need to know first. Alright let's begin so first we need to check the equivalent C code for the assembly. For that I went to my favorite website Geeksforgeek [Here]

It gives us a rough idea about what syscalls we will be needing in our journey.

• socket()
• bind()
• listen()
• accept()

 Alright so these are the syscalls that we need to use, but how would we be able to call them? Well for that we will need socketcall()

Okay so " NR_Socketcall " is at 102 , in hex it's value would be 0x66.

Now in assembly we can't write like use bind() instead the particular value is assigned to each calls. Now we need call number's for the socket,bind,listen,accept and connect call's

Okay we will need 1 to 5 AF_MAX values xD amazing, now let's begin with the assembly code.

 Your assembly must have _start in the code so it will understand that the code will begin from that point. Also one interesting fact in .text section there won't be any ASCII readable text. Quite weird, isn't it? Anyways, now start writing in the _start:

 We all learned about basic Boolean XOR operation in our high school. There a very beautiful property of XOR which we are using here,

 The property says , XOR something with something will always be 0. Here I am refering something to a value that are same.

So, " xor eax, eax " will be NULL the eax register , same with “xor ebx,ebx” ebx register and same with “xor ecx,ecx” ecx register. Whereas to save our 1 byte we used cdq because the CDQ instruction copies the sign (bit 31) of the value in the EAX register into every bit position in the EDX register..

Everything is explained in the comment, let me clear some stuffs. In assembly we use " ; " semicolon to comment something.

• EAX- System call number
• EBX- First argument
• ECX- Second argument
• EDX- Third argument
• ESI- Forth argument
• EDI- Fifth argument

In the end when everything is done we use " int 0x80 " to interrupt the call. In normal language it's like calling kernel giving them all register value and using those value kernel will do the work and get it done for us. Ess Pzz?

 It will call bind() , also if you wanna change the port number then you can simply change " push word 0x3905 " with your hex value of your port number.

listen() will actually put the server socket in passive mode where it will wait for the client to send the request to make a connection.

accept() will just accept the connection and the connection will established between server and the client. Now we will need dup2() , Read more about from [Here]

First thing first, we need to find the dup2 syscall number.

So, it is 63 but it's hex value would be 0x3f ,

Here we are using loop for duplicating in , out and error with the file descriptor. Now here we are left with the final thing.

This is the basic execve() shellcode. It will just pop the " /bin/sh " shell for us xD . Read more about it. [Here]

In top most pane I've just assembled and linked the assembly file / object file. In pane 2 i.e bottom left just " netstat -l " command is running to check if the port 1337 is opened and in listening mode.

In pane 3 the bottom of the right side , I have just shown my IP address to tell I am different host (But in the same network)

In pane 4 the middle one from the right side. I'm using nc command which is short for netcat to connection to the Ubuntu box where port 1337 is running.

Thanks,