Didier Stevens

Congratulations to the winners of the BlueHat Prize contest.

My entry was CounterHeapSpray:

CounterHeapSpray monitors the private memory usage of an application to guard against heap  sprays. When the private memory usage of the application exceeds a predefined threshold,  CounterHeapSpray assumes that a heap spray is ongoing and will pre-allocate virtual memory pages  and populate these pages with its own shellcode. When the heap spray terminates and the exploit  executes, code execution will transfer to CounterHeapSpray’s own shellcode. This shellcode will  suspend all threads and display a warning message for the user. When the user clicks OK,  CounterHeapSpray’s shellcode terminates the application.
By planting its own shellcode before the heap spray can fill the heap with malicious shellcode,  CounterHeapSpray not only prevents execution of this malicious shellcode but is able to suspend the process and to inform the user of the attack.

CounterHeapSpray.zip (https)
MD5: 1947380F935AE0B1A8828DE79621F82F
SHA256: CA0BF635655EE05ABED117C858BC86ECDF3EBB4C39544D7D0C396D7C457F1BBC

I wrote shellcode that calls ExitProcess for my TaskManager.xls spreadsheet.

Now I’ve added the asm files (sc-ep.asm for 32-bit and sc-64-ep.asm for 64-bit) for this shellcode to my library.

Remark that the 32-bit version assembler code, that was generated with my simple shellcode generator, has a ret instruction after the call to ExitProcess. This instruction will never be executed, as a call to ExitProcess does not return.

You can find this shellcode on my shellcode page.

Last year I found great x64 shellcode for Windows on McDermott’s site. Not only is it dynamic (lookup API addresses), but it even handles forwarded functions.

But it’s written for MASM, and I prefer to use NASM. Hence I translated it, but also normalized it to adhere to the x64 calling convention and fixed a bug in the error handling.

And I modularized it so you can use it like my 32-bit shellcode.

Here’s the classic MessageBox example:

; x64 shellcode to display a "Hello from injected shell code!" MessageBox, then return to caller
; Written for NASM assembler (http://www.nasm.us) by Didier Stevens
; Source code put in public domain by Didier Stevens, no Copyright
; https://DidierStevens.com
; Use at your own risk
;
; History:
;   2011/12/27: Refactored functions to include file sc-x64-api-functions.asm

%include "sc-x64-macros.asm"

INDEX_KERNEL32_LOADLIBRARYA        equ 0 * POINTERSIZE + STACKSPACE
INDEX_MESSAGEBOXA                            equ 1 * POINTERSIZE + STACKSPACE
APIFUNCTIONCOUNT                            equ 2

segment .text

; Setup environment
sub rsp, STACKSPACE + ROUND_EVEN(APIFUNCTIONCOUNT) * POINTERSIZE        ;reserve stack space for called functions and for API addresses

LOOKUP_API KERNEL32DLL, KERNEL32_LOADLIBRARYA, INDEX_KERNEL32_LOADLIBRARYA

lea rcx, [rel USER32DLL]
call [rsp + INDEX_KERNEL32_LOADLIBRARYA]

LOOKUP_API USER32DLL, USER32_MESSAGEBOXA, INDEX_MESSAGEBOXA, INDEX_KERNEL32_LOADLIBRARYA

; Display MessageBox
xor r9, r9
lea r8, [rel TITLE]
lea rdx, [rel HELLO]
xor rcx, rcx
call [rsp + INDEX_MESSAGEBOXA]

add rsp, STACKSPACE + ROUND_EVEN(APIFUNCTIONCOUNT) * POINTERSIZE
ret

%include "sc-x64-api-functions.asm"

KERNEL32DLL                            db    "KERNEL32.DLL", 0
KERNEL32_LOADLIBRARYA        db    "LoadLibraryA", 0

USER32DLL                                db    "USER32.DLL", 0
USER32_MESSAGEBOXA            db    "MessageBoxA", 0

HELLO                                        db    "Hello from injected shell code!", 0
TITLE                                        db    "Message", 0

Here’s what I changed exactly from the original MASM code:
1) non-volatile registers are preserved (by storing them on the stack)
2) building the DLL name for forwarded functions is done with a variable on the stack frame of lookup_api, and not of the caller
3) the address of LoadLibraryA is passed via r9, and no longer r15
4) lookup_api not only returns the function address in rax, but also stores it in memory at an address provided in r8
5) fixed the error handling bug (stack restoration)
6) added some EQUs to make it easier to use this code as a “library” (include)

You can get the code from my shellcode page. Look for filenames starting with sc-x64 in the zip file.

I wrote an article in Hakin9 magazine how to write shellcode with a C-compiler.

People before me have worked out methods to do this; the advantage of my method is that you can debug your shellcode inside the Visual Studio IDE.

The template can be found here.

As its name implies: shellcode to send a Twitter update.