What is the smallest possible Windows (PE) executable?

As quoted from source (Creating the smallest possible PE executable): ¹

Smallest possible PE file: 97 bytes

Smallest possible PE file on Windows 2000: 133 bytes

Smallest PE file that downloads a file over WebDAV and executes it: 133 bytes

The files above are the smallest possible PE files due to requirements of the PE file format and cannot be improved further.

This result was achieved with some clever NASM tricks, such as removing the step that links to C stdlib and removing a number of header fields and data directories.

The full source code is below. It is effectively the same as the article with these modification:

Removal of blank lines
sectalign label renamed to sect_align. Since the time this assembly code was written sectalign became a NASM keyword. Rename it to avoid warnings and errors.

The code is as follows:

; tiny97.asm, copyright Alexander Sotirov

BITS 32
;
; MZ header
; The only two fields that matter are e_magic and e_lfanew

mzhdr:
    dw "MZ"       ; e_magic
    dw 0          ; e_cblp UNUSED

; PE signature
pesig:
    dd "PE"       ; e_cp, e_crlc UNUSED       ; PE signature

; PE header
pehdr:
    dw 0x014C     ; e_cparhdr UNUSED          ; Machine (Intel 386)
    dw 1          ; e_minalloc UNUSED         ; NumberOfSections

;   dd 0xC3582A6A ; e_maxalloc, e_ss UNUSED   ; TimeDateStamp UNUSED

; Entry point
start:
    push byte 42
    pop eax
    ret

codesize equ $ - start

    dd 0          ; e_sp, e_csum UNUSED       ; PointerToSymbolTable UNUSED
    dd 0          ; e_ip, e_cs UNUSED         ; NumberOfSymbols UNUSED
    dw sections-opthdr ; e_lsarlc UNUSED      ; SizeOfOptionalHeader
    dw 0x103      ; e_ovno UNUSED             ; Characteristics

; PE optional header
; The debug directory size at offset 0x94 from here must be 0

filealign equ 4
sect_align equ 4  ; must be 4 because of e_lfanew

%define round(n, r) (((n+(r-1))/r)*r)

opthdr:
    dw 0x10B      ; e_res UNUSED              ; Magic (PE32)
    db 8                                      ; MajorLinkerVersion UNUSED
    db 0                                      ; MinorLinkerVersion UNUSED

; PE code section
sections:
    dd round(codesize, filealign)  ; SizeOfCode UNUSED  ; Name UNUSED
    dd 0  ; e_oemid, e_oeminfo UNUSED ; SizeOfInitializedData UNUSED
    dd codesize  ; e_res2 UNUSED  ; SizeOfUninitializedData UNUSED  ; VirtualSize
    dd start  ; AddressOfEntryPoint  ; VirtualAddress
    dd codesize  ; BaseOfCode UNUSED  ; SizeOfRawData
    dd start  ; BaseOfData UNUSED  ; PointerToRawData
    dd 0x400000  ; ImageBase  ; PointerToRelocations UNUSED
    dd sect_align ; e_lfanew  ; SectionAlignment  ; PointerToLinenumbers UNUSED
    dd filealign  ; FileAlignment  ; NumberOfRelocations, NumberOfLinenumbers UNUSED
    dw 4  ; MajorOperatingSystemVersion UNUSED ; Characteristics UNUSED
    dw 0  ; MinorOperatingSystemVersion UNUSED
    dw 0  ; MajorImageVersion UNUSED
    dw 0  ; MinorImageVersion UNUSED
    dw 4  ; MajorSubsystemVersion
    dw 0  ; MinorSubsystemVersion UNUSED
    dd 0  ; Win32VersionValue UNUSED
    dd round(hdrsize, sect_align)+round(codesize,sect_align) ; SizeOfImage
    dd round(hdrsize, filealign)  ; SizeOfHeaders
    dd 0  ; CheckSum UNUSED
    db 2  ; Subsystem (Win32 GUI)

hdrsize equ $ - $$
filesize equ $ - $$

To build into an executable use:

nasm -f bin tiny97.asm -o tiny97.exe

For GNU/Linux ELF executables, See the article “Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux”. TL;DR: 1340 bytes, using NASM

Note: This answer is an expansion of J…’s comment on Dec 3 ’16 at 17:31, in order to preserve the information found in the link (in case that too goes dead).

Tiny PE; Alexander Sotirov; viewed 15/11/2017 @ 17:50 SAST

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