Cheats with ELF: Code Injecting into ELF Headers

EDB-ID:

14087

CVE:

N/A




Platform:

Linux

Date:

2010-06-28


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|=-----------------------------------------------------------------------=|
|=---------------------------[ HELLCODE RESEARCH ]-----------------------=|
|=-----------------------------------------------------------------------=|
|=--------=[ Cheats with ELF: Code Injecting into ELF Headers  ]=--------=|
|=-----------------------------------------------------------------------=|
|=-----------------------=[ murderkey@hellcode.net ]=--------------------=|
|=-----------------------------------------------------------------------=|




--[ Index


	0x0 - Introduction
	0x1 - Requirements
	0x2 - Basic ELF Structure
	0x3 - Last Wordz
	0x4 - Greetz



--[ 0x0 - Introduction

	Hi Reader, In this paper, I will explain uncommon/unknown techniques 
to affect ELF headers directly or helping of compiler and assembly linkers. 
I've discovered this technique while thinking about injecting code to elf32 
headers and i became succesful in my research 3 years ago..you can use 
this technique for different ideas, anyway i've tested it for IDS by-passing
and as an anti-anti-debug technique. 



--[ 0x1 - Requirements
	
	You should know the topics which are below to understand this paper, 
because probably, you can't understand this paper if you don't have high-level
knowledge about topics.


	-> Unix Assembly (AT&T)
	-> C/C++
	-> ELF Structure
	-> Time and Brain

if you know
	



--[ 0x2 - Basic ELF Structure

 	An elf32 file structure always include header, data and instruction 
segments.In this part, we will create executable files, play/change their sizes 
and createfiles with minimum sizes.


Lets start to show that on "merv.asm" (Seni seviyorum Merve :);

 
------------merv.asm-----------------

	BITS 32
	section .data
	merv  db    " murderkey ownz jo !"
	
	GLOBAL MAIN
	SECTION .text
	main:
;write()
	mov eax, 4                                                                     
	mov ebx, 1                                                                     
	mov ecx, merv                                                                   
	mov edx, 28                                                                    
	int 0x80   

; exit()
	mov eax, 1                                                                     
	mov ebx, 0                                                                     
	int 0x80           

-------------merv.asm-----------------


As you see above, that is a simple assembly code. It prints " murderkey ownz jo !" 
to screen and ending via "exit call".Compiling it with "nasm" and giving link 
with "ld".



h4x0r elf32 # nasm -felf merv.asm -o merv.o
h4x0r elf32 # ld merv.o -o merv
ld: warning: cannot find entry symbol _start; defaulting to 00000000080480a0


yeah, the warning is so important here! dont forget that! 
the program gave us "_start sectioun is not defined for elf32 in .text segment" 
error.even so , we are running the binary.

h4x0r elf32 # ./merv 
 murderkey ownz jo !

h4x0r elf32 #

Good! there is not any problem for now so lets look at the size of program.

h4x0r elf32 # wc -c merv
892 merv

It is 892, we will match this value with the next code that we will write.

--------------merv2.asm-------------------



        BITS 32
        section .data
        merv  db    " murderkey ownz jo !"
        
        SECTION .text
        GLOBAL _start
        _start
        
        mov eax, 4
        mov ebx, 1  
        mov ecx, merv
        mov edx, 28
        int 0x80

        mov eax, 1
        mov ebx, 0
        int 0x80

----------------merv2.asm------------------------

Now, compiling, linking and running the program again.

h4x0r elf32 # nasm -felf merv2.asm
h4x0r elf32 # ld merv2.o -o merv2
h4x0r elf32 # ./merv2
 murderkey ownz jo !



and lets look at the size of file



h4x0r elf32 # wc -c merv2
848 merv2
h4x0r elf32 # 


848! There is not a clear changing at the size. We will see this in a better 
way with details now.


compiling "merv2.asm";


h4x0r elf32 # nasm -f elf merv2.asm
h4x0r elf32 # gcc -Wall -s merv2.o
merv2.o(.data+0x13): In function `_start':
: multiple definition of `_start'
/usr/lib/gcc-lib/i386-pc-linux-gnu/3.3.5/../../../crt1.o(.text+0x0): first defined here
/usr/lib/gcc-lib/i386-pc-linux-gnu/3.3.5/../../../crt1.o(.text+0x18): In function `_start':
: undefined reference to `main'
collect2: ld returned 1 exit status
h4x0r elf32 #


The error is so interesting at the above. "/crt1.o(.text+0x0): first defined here." 
This error means that _start symbol is defined in "crt1.o elf32 startup script" and
also we defined a _start symbol too. 

So these overlaps and it causes this error :)For blocking this, We are giving a command 
to gcc compiler to not running "startup" script when our code is compiled.We are doing 
it with (gcc) "-nostartfiles" option. 

h4x0r elf32 # gcc -Wall -s -nostartfiles merv2.o -o merv2
h4x0r elf32 # ./merv2
Segmentation fault
h4x0r elf32 # 

UPPSSS we comiled our code but there is a (segmentation fault) memory crash!


What is the problem here? Let's find! When we call _start image symbol in our assembly 
code ,it must be ended via _exit() call but we gave "-nostartfiles" option to it so it 
can not find _exit()How can we defeat it? Of course, defining the _exit as EXTERN and 
calling it in our code.


Lets write the code again...

----------------- merv2.asm ------------------


        BITS 32
        section .data
        merv  db    " murderkey ownz jo !"

        EXTERN _exit
        section.text
        global _start
        _start

        mov eax, 4
        mov ebx, 1
        mov ecx, merv
        mov edx, 28
        int 0x80

       call _exit

------------------- merv2.asm ----------------------


Now, compiling again and linking...


h4x0r elf32 # nasm -f elf merv2.asm 
h4x0r elf32 # gcc -Wall -s -nostartfiles merv2.o -o merv2
h4x0r elf32 # ./merv2
 murderkey ownz jo !

WE ARE THE CHAMPION !!  As you see, our function was run!



Now , looking the size of our code;

h4x0r elf32 # wc -c merv2
1392 merv2
h4x0r elf3

1392 <----- Now you understand well the difference of sizes.



Now, we will practice a deceit with a different option. Watch here carefully!

---------------- merv3.asm---------------------

        BITS 32
        section .data
        merv  db    " murderkey ownz jo !"

        GLOBAL _start
        SECTION .text
        _start

        mov eax, 4
        mov ebx, 1
        mov ecx, merv
        mov edx, 28
        int 0x80

        mov eax, 1
        mov ebx, 0
        int 0x80

------------------ merv3.asm -----------------------

Compiling and linking the code;

h4x0r elf32 # nasm -f elf merv3.asm 
h4x0r elf32 # gcc -Wall -s -nostdlib merv3.o -o merv3

Run it;

h4x0r elf32 # ./merv3
 murderkey ownz jo !

So nice, we are wondering of size.

h4x0r elf32 # wc -c merv3
524 merv3
h4x0r elf32 #

524, what did we do here? we gave -nostdlib option to compiler and canceled _exit().
We used this code instead of it;
 	mov eax, 1
        mov ebx, 0
        int 0x80

and runned "_exit call" directly...


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

Shortly I want to say that we can cancel elf32 file headers, we can inject sections 
and can change size of fileshow we want.. or we can bypass anti-debug protections. 
That provide convenience us while developing unix worms.because any ids or any 
antivirus can not check elf32 file headers in this way.You can exceed every obstacle 
via this method. It's up to your imagination and creativity.


Also I want to refer an issue here.

While an elf32 executable image is being mapped to memory, it is mapped at 
"0x00000000080480a0" 32-bit hexadecimal address as a default in unix systems.
If you dont define _start in elf32 , linker will give an error like below;

...
/usr/lib/gcc-lib/i386-pc-linux-gnu/3.3.5/../../../../i386-pc-linux-gnu/bin/ld: warning: 
cannot find entry 
symbol _start; defaulting to 00000000080480a0
...

so kernel maps image to memory as default. If you run your head! , you can map this 
image to another place!I am changing this address and making it "0x08048000" !!

Lets compile the code which is below as a bin format look its' starting place..

P.S: You should know details of elf32 file structure for this part. 


------------------- lame-elf32.asm----------------------------------

 BITS 32

                org     0x08048000
  ehdr:                                                 ; Elf32_Ehdr
                db      0x7F, "ELF", 1, 1, 1            ;   e_ident
        times 9 db      0
                dw      2                               ;   e_type
                dw      3                               ;   e_machine
                dd      1                               ;   e_version
                dd      _start                          ;   e_entry
                dd      phdr - $$                       ;   e_phoff
                dd      0                               ;   e_shoff
                dd      0                               ;   e_flags
                dw      ehdrsize                        ;   e_ehsize
                dw      phdrsize                        ;   e_phentsize
                dw      1                               ;   e_phnum
                dw      0                               ;   e_shentsize
                dw      0                               ;   e_shnum
                dw      0                               ;   e_shstrndx

  ehdrsize      equ     $ - ehdr

  phdr:                                                 ; Elf32_Phdr
                dd      1                               ;   p_type
                dd      0                               ;   p_offset
                dd      $$                              ;   p_vaddr
                dd      $$                              ;   p_paddr
                dd      filesize                        ;   p_filesz
                dd      filesize                        ;   p_memsz
                dd      5                               ;   p_flags
                dd      0x1000                          ;   p_align

  phdrsize      equ     $ - phdr

  section .data
  merv db "murderkey own jo!"

  _start:

mov eax, 4
mov ebx, 1
mov ecx, merv
mov edx, 28
int 0x80

; exit() call

mov eax, 1
mov ebx, 0
int 0x80
                
filesize      equ     $ - $$
                
----------------------------lame-elf32.asm-------------------------------




Now lets link our code in bin format and run it..


h4x0r elf32 # nasm -f bin lame-elf32.asm -o lame-elf32     
h4x0r elf32 # chmod +x lame-elf32
h4x0r elf32 # ./lame-elf32 

murderkey own jo!




--[ 0x3 - Last Wordz

	I want to finish my paper now. After learning basic things in paper, we can 
play with elf32 headers how we want..you can distribute elf32 worm to systems via 
bypassing all IDS with this oh-day technique! Thats up to only your creativity.You 
should have deep assembly and elf32 file system knowledge to do that.. This paper has 
been written to be a basic for this bypassing methods etc..

Used linux source codes and elf headers as references in this paper..


/usr/include/linux/elf.h      <--------- this header gives us all details about elf structure.



--[ 0x4 - Greetz

	
	karak0rsan, l4m3r, n00b (dont forget about misdirection lol'd), GOBBLES, PHC and
all blackhat community...hey man, you should contact me because someone is watchin' u, i 
know you hate "contributors" and still im waiting you ! someone offered me money to give 
information about you but i didn't accept coz im not like the others...dont forget !