Self modifying PE code

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OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

Hi people! I know how to make some self-modifying code under DOS executables. But how to do it with windows PE files? Do you have some example code?

Thanks!

26 Aug 2005, 14:30

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

mark your code section is writable and write to it

26 Aug 2005, 14:42

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

Ok. I know how to change the code in memory, but how to change it in disk? Like a encrypted program that decrypts itself, run the code, use GetTickCount() to make a new random number and encrypt itself with this random number, saving this number to a variable and write everything to disk?
The problem is I can get the code and write to disk, but the PE header won't be written, just the code.

26 Aug 2005, 14:50

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

do you want to overwrite your own executable?

26 Aug 2005, 17:18

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

yes... i did it with DOS .com files... but I don't know how to do with PE because of the headers and maybe it's needed to alloc some memory...
any ideas?

26 Aug 2005, 18:18

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

no, it is not because of PE headers

it is because while the program is running, its EXE file is locked by the OS, and you cannot write to it

26 Aug 2005, 19:02

r22

Joined: 27 Dec 2004
Posts: 805

r22

The easiest way is to.

Have EXE1 create a new version of itself as a separate file, run that file and terminate itself.
Now EXE2 deletes EXE1's file and renames itself to whatever the proper name should be.

26 Aug 2005, 19:07

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

The problem with PE headers is that:

Code:

format PE GUI 4.0
include '%fasminc5\win32a.inc'
start:
invoke MessageBox,0,msg,msg,0
ret
msg db 'Hi!',0
eof:

see? if I try to reading beggining at start and with size of eof-start it will read only the code, not the header. That's my problem, not if the exe is able to overwrite itself. With .com files it was easy:

Code:

org 100h
start:
mov ah,09h
mov dx,msg
int 21h
ret
msg db 'Hi!',0
eof:

here the full code would be written to the EXE2, cause there's no header.
even if i try to do what r22 said i'll write to EXE2 only the code without the PE header. How can I change the code from EXE1 in memory, read the entire code in a buffer and write this changed version to EXE2 in disk without missing header?

26 Aug 2005, 20:30

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend

Maybe you meant to modify one PE with another program? Then you must have a basic knowledge of how the PE is structured. You'd have to do as follows:

Code:

  filename                      db '1.exe', 0

  hfile                         dd ?
  hmapping                      dd ?
  hmap                          dd ?

        CODE_SECTION_INDEX      = 1 ;which section contains code (counting from 1)

        invoke  CreateFile, filename, GENERIC_READ, 0, 0, OPEN_EXISTING, 0, 0
        mov     [hfile], eax
        invoke  CreateFileMapping, eax, 0, PAGE_READONLY, 0, 0, 0
        mov     [hmapping], eax
        invoke  MapViewOfFile, eax, FILE_MAP_READ, 0, 0, 0
        mov     [hmap], eax

;       mov     eax, [hmap]
        add     eax, [eax+IMAGE_DOS_HEADER.e_lfanew]
        movzx   ecx, [eax+IMAGE_NT_HEADERS.FileHeader.SizeOfOptionalHeader]
        lea     eax, [eax+ecx+sizeof.IMAGE_FILE_HEADER+4]
        lea     eax, [(sizeof.IMAGE_SECTION_HEADER-1)*CODE_SECTION_INDEX]
;       comment the following line if you loaded the file via LoadLibrary
;       in all other cases don't change it
        stdcall rva2off, [eax+IMAGE_SECTION_HEADER.VirtualAddress]
        add     eax, [hmap]
;       here eax contains pointer to the code exactly
        ret

;================================
proc    rva2off                 _rva
        mov     eax, [hmap]
        add     eax, [eax+IMAGE_DOS_HEADER.e_lfanew]
        movzx   ecx, [eax+IMAGE_NT_HEADERS.FileHeader.NumberOfSections]
        movzx   edx, [eax+IMAGE_NT_HEADERS.FileHeader.SizeOfOptionalHeader]
        lea     eax, [eax+edx+sizeof.IMAGE_FILE_HEADER+4]
    rva2off_loop:
        mov     edx, [eax+IMAGE_SECTION_HEADER.PointerToRawData]
        cmp     [_rva], edx
        jl      @F
        mov     edx, [eax+IMAGE_SECTION_HEADER.VirtualAddress]
        add     edx, [eax+IMAGE_SECTION_HEADER.SizeOfRawData]
        cmp     [_rva], edx
        jae     @F
        mov     edx, [_rva]
        sub     edx, [eax+IMAGE_SECTION_HEADER.VirtualAddress]
        add     edx, [eax+IMAGE_SECTION_HEADER.PointerToRawData]
        xchg    eax, edx
        ret
    @@:
        add     eax, sizeof.IMAGE_SECTION_HEADER
        loop    rva2off_loop
        xor     eax, eax
        ret
endp

And you need my 'pe.inc' include.

If you don't know which section contains code you must add some kind of loop to check current section attributes. Hope it is what you meant Smile

Description:	Include for PE files	Download
Filename:	PE.inc
Filesize:	3.26 KB
Downloaded:	132 Time(s)

26 Aug 2005, 21:56

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

Yes! It was what I meant, but modifying itself and overwriting in disk... but too hard to do... i give up... Sad

26 Aug 2005, 22:35

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

you can check SelfSafe sample on my page (check my signature)

26 Aug 2005, 23:30

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY

comrade, where's your page? I can't see.

27 Aug 2005, 00:38

f0dder

Joined: 19 Feb 2004
Posts: 3170
Location: Denmark

f0dder

Ozzy, click the "www" button? Smile

http://comrade64.cjb.net/

27 Aug 2005, 01:35

comrade

Joined: 16 Jun 2003
Posts: 1137
Location: Russian Federation

comrade

My bureau is the greatest page of all time. How dare you disrespect?

27 Aug 2005, 01:41

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