Advanced Shellcoding Techniques

Introduction

This paper assumes a working knowledge of basic shellcoding techniques, and x86 assembly, I will not rehash these in this paper. I hope to teach you some of the lesser known shellcoding techniques that I have picked up, which will allow you to write smaller and better shellcodes. I do not claim to have invented any of these techniques, except for the one that uses the div instruction.



The multiplicity of mul

This technique was originally developed by Sorbo of darkircop.net. The mul instruction may, on the surface, seem mundane, and it's purpose obvious. However, when faced with the difficult challenge of shrinking your shellcode, it proves to be quite useful. First some background information on the mul instruction itself.

mul performs an unsigned multiply of two integers. It takes only one operand, the other is implicitly specified by the %eax register. So, a common mul instruction might look something like this:

movl $0x0a,%eax
mul $0x0a

This would multiply the value stored in %eax by the operand of mul, which in this case would be 10*10. The result is then implicitly stored in EDX:EAX. The result is stored over a span of two registers because it has the potential to be considerably larger than the previous value, possibly exceeding the capacity of a single register(this is also how floating points are stored in some cases, as an interesting sidenote).

So, now comes the ever-important question. How can we use these attributes to our advantage when writing shellcode? Well, let's think for a second, the instruction takes only one operand, therefore, since it is a very common instruction, it will generate only two bytes in our final shellcode. It multiplies whatever is passed to it by the value stored in %eax, and stores the value in both %edx and %eax, completely overwriting the contents of both registers, regardless of whether it is necessary to do so, in order to store the result of the multiplication. Let's put on our mathematician hats for a second, and consider this, what is the only possible result of a multiplication by 0? The answer, as you may have guessed, is 0. I think it's about time for some example code, so here it is:

xorl %ecx,%ecx
mul %ecx

What is this shellcode doing? Well, it 0's out the %ecx register using the xor instruction, so we now know that %ecx is 0. Then it does a mul %ecx, which as we just learned, multiplies it's operand by the value in %eax, and then proceeds to store the result of this multiplication in EDX:EAX. So, regardless of %eax's previous contents, %eax must now be 0. However that's not all, %edx is 0'd now too, because, even though no overflow occurs, it still overwrites the %edx register with the sign bit(left-most bit) of %eax. Using this technique we can zero out three registers in only three bytes, whereas by any other method(that I know of) it would have taken at least six.


The div instruction

Div is very similar to mul, in that it takes only one operand and implicitly divides the operand by the value in %eax. Also like, mul it stores the result of the divide in %eax. Again, we will require the mathematical side of our brains to figure out how we can take advantage of this instruction. But first, let's think about what is normally stored in the %eax register. The %eax register holds the return value of functions and/or syscalls. Most syscalls that are used in shellcoding will return -1(on failure) or a positive value of some kind, only rarely will they return 0(though it does occur). So, if we know that after a syscall is performed, %eax will have a non-zero value, and that the instruction divl %eax will divide %eax by itself, and then store the result in %eax, we can say that executing the divl %eax instruction after a syscall will put the value 1 into %eax. So...how is this applicable to shellcoding? Well, their is another important thing that %eax is used for, and that is to pass the specific syscall that you would like to call to int $0x80. It just so happens that the syscall that corresponds to the value 1 is exit(). Now for an example:


xorl %ebx,%ebx
mul %ebx
push %edx
pushl $0x3268732f
pushl $0x6e69622f
mov %esp, %ebx
push %edx
push %ebx
mov %esp,%ecx
movb $0xb, %al #execve() syscall, doesn't return at all unless it fails, in which case it returns -1
int $0x80

divl %eax # -1 / -1 = 1
int $0x80

Now, we have a 3 byte exit function, where as before it was 5 bytes. However, there is a catch, what if a syscall does return 0? Well in the odd situation in which that could happen, you could do many different things, like inc %eax, dec %eax, not %eax anything that will make %eax non-zero. Some people say that exit's are not important in shellcode, because your code gets executed regardless of whether or not it exits cleanly. They are right too, if you really need to save 3 bytes to fit your shellcode in somewhere, the exit() isn't worth keeping. However, when your code does finish, it will try to execute whatever was after your last instruction, which will most likely produce a SIG ILL(illegal instruction) which is a rather odd error, and will be logged by the system. So, an exit() simply adds an extra layer of stealth to your exploit, so that even if it fails or you can't wipe all the logs, at least this part of your presence will be clear.



Unlocking the power of leal

The leal instruction is an often neglected instruction in shellcode, even though it is quite useful. Consider this short piece of shellcode.

xorl %ecx,%ecx
leal 0x10(%ecx),%eax

This will load the value 17 into eax, and clear all of the extraneous bits of eax. This occurs because the leal instruction loads a variable of the type long into it's desitination operand. In it's normal usage, this would load the address of a variable into a register, thus creating a pointer of sorts. However, since ecx is 0'd and 0+17=17, we load the value 17 into eax instead of any kind of actual address. In a normal shellcode we would do something like this, to accomplish the same thing:

xorl %eax,%eax
movb $0x10,%eax

I can hear you saying, but that shellcode is a byte shorter than the leal one, and you're quite right. However, in a real shellcode you may already have to 0 out a register like ecx(or any other register), so the xorl instruction in the leal shellcode isn't counted. Here's an example:

xorl %eax,%eax
xorl %ebx,%ebx
movb $0x17,%al
int $0x80

xorl %ebx,%ebx
leal 0x17(%ebx),%al
int $0x80

Both of these shellcodes call setuid(0), but one does it in 7 bytes while the other does it in 8. Again, I hear you saying but that's only one byte it doesn't make that much of a difference, and you're right, here it doesn't make much of a difference(except for in shellcode-size pissing contests =p), but when applied to much larger shellcodes, which have many function calls and need to do things like this frequently, it can save quite a bit of space.

PHP Navigation - IGameNet.Blogspot.Com - Coding


if($_SERVER['QUERY_STRING'] == "SoD")
print "owns you!";
else
print "don't front!";
?>


Description: Instead of calling files like ( index.php?str=blah ) , you could do ( index.php?SoD ) and it would print out "owns you!". You can add more strings in there, this is just an example.



$vars = explode(",", urldecode(getenv('QUERY_STRING')));
$v1 = array_shift($vars);
$v2 = array_shift($vars);
$v3 = array_shift($vars);

switch ($v1) {
case 'first.1': {
print("This is v1, first string case 'file.php?first.1'.");
break;
}
case 'first.2': {
switch ($v2) {
case 'second': {
switch($v3) {
case 'third': {
print("This is v3, the last case 'file.php?first.2,second,third'.");
break;
}
}
}
}
}
}


Description: This basically does what the 1st one does but with more strings and a different seperator rather than '&'. I don't really want to go into too much detail on the thread so if people are confused or need explaining, reply or PM me.

That's it for the navigation as I am not going to do the other due to they're everywhere else and I wanted to be different and show everyone this method.

Numbering System

Decimal System

Nowadays we use the decimal numbering system in almost everything that is related to numbers. We use it so often and in a natural way that we forget it's meaning. What is decimal system? Every decimal number, has only digits between zero and nine, making a total of 10 digits Note: how many fingers do you have? ...10. In fact the decimal system is bound to human anatomy. Ok, and what is the meaning of each digit? Consider the following numbers: 234 and 234,43

We do some transformations

-> 234 i.e. 200 + 30 + 4 i.e. 2 * 10^2 + 3 * 10^1 + 4 * 10^0

-> 234,43 = 2 * 10^2 + 3 * 10^1 + 4 * 10^0 + 0,43 = 2 * 10^2 + 3 *

10^1 + 4 * 10^0 + 4 * 10^-1 + 3 * 10^-2

Do you see the relation? Each digit appearing to the left of the decimal point represents a value between zero and nine times an increasing power of ten. Digits appearing to the right of the decimal point represent a value between zero and nine times a decreasing power of ten.

Binary system

Binary system uses only two digits, by convention the digits are 0 and 1.

This system is so widely used in computers... By coincidence or not this system adjusts perfectly to computers... Computers operate using binary logic. The computer represents values using two different voltage levels, in this way we can represent 0 and 1. Like I said before the same applies to binary system, it is well adjust to computer anatomy!


1.2.1. Converting a binary number to a decimal number

Apply the same rule we saw in 1.1, but with powers of two.

Example: 1010 -> 1 * 2^3 + 0 * 2^2 + 1 * 2^1 + 0 * 2^0 = 10

1.2.2. Converting a decimal number to a binary number

We have two ways to do it:

1.2.2.1 We consecutively divide the decimal value by a power

two(keeping the remainder), while the result of the division is different than zero. The binary representation is obtained by the sequence of remainders in the inverse order of the divisions.

Consider the number 10(in decimal):



10 / 2

0 5 / 2

1 2 / 2

0 1 / 2

1 0



So in binary we write 1010



1.2.2.2 You can try to find out the number by adding powers of two, that added will produce the decimal result.



Consider for example number 123... hmmm it's a number not less than 2^0 and not greater then 2^7. Cool…



2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0

0 1 1 1 1 0 1 1 because 1 * 2^6 + 1 * 2^5 + 1 *

2^4 + 1 * 2^3 + 0 * 2^2 + 1*2^1 + 1 * 2^0

= 123

Our result is 1111011.

Hexadecimal system

You saw how many digits took to represent the number 123 in binary. 7 digits! Imagine 1200, 10000,... it hurts. So programmers had to choose another numbering system, just to "talk" to the machine... and no... it's not the decimal system!! You saw the trouble we had to convert one simple number like 10 between decimal and binary... I think you don't want to spend half of your life doing that. Engineers thought on that and they elected the hexadecimal system... Hexadecimals is the "english" for computers. They have two special features:

- They're very compact

- it's simple to convert them to binary and vice-versa. A hexadecimal number has digits with a value between 0 and 15 times a certain power of sixteen. Because we only know digits between 0-9 we have to use six more digits! We can use the 6 first letters of the alphabet.

Let's see a example: FF = 15 * 16^1 + 15 * 16^0 = 255 (16) (10)

Converting between binary and hexadecimal is very easy! To convert binary to hexadecimal remember that every four digits correspond to a single hexadecimal digit... to convert back to binary just apply the inverse rule! Let's take a look at the next example:

110 1011 = 0110 1011 =6B

(2) (16)

It's very easy!! To make things easier, take a look at the following table:

################
# D # H # B #
################
# 0 # 0 # 0000 #
# 1 # 1 # 0001 #
# 2 # 2 # 0010 #
# 3 # 3 # 0011 #
# 4 # 4 # 0100 #
# 5 # 5 # 0101 #
# 6 # 6 # 0110 #
# 7 # 7 # 0111 #
# 8 # 8 # 1000 #
# 9 # 9 # 1001 #
#10 # A # 1010 #
#11 # B # 1011 #
#12 # C # 1100 #
#13 # D # 1101 #
#14 # E # 1110 #
#15 # F # 1111 #
################

Conventions

Programming in assembly, requires you to obey some rules when using numbers, because you can use three different numbering systems.

When writing a number:

- all numbers have to start with a decimal digit

- all numbers end with a letter, indicating the type of number:

. for hexadecimals the letter is h

. binary numbers end with b

. decimals end with t or d We will use the following notation:

Xn Xn-1 ... X2 X1 -> Xi represents a bit, and i<-[0,1,...,n] represents it's position.