# [ ASM ] Chain-Loader *Theoretical Assembly Based Boot-Sector RootKit* By [Rek0n](https://paragraph.com/@rek0n) · 2025-03-24 code, programmer, asm, hacker --- * * * ![](https://storage.googleapis.com/papyrus_images/7b1ad8bc423850d59215af52c5e22b72.png) * * * Halt & Catch Fire: ------------------ ### /x00/ --> NullByte Processing > Since the dawn of computing in the '80s and '90s, hackers—once simply known as developers and coders—have pursued the ultimate bootkit/rootkit subversion method. The approach proposed here represents only a fragment of the broader concept. **Can we craft specialized shellcode capable of embedding itself into the OS layer, then tunneling indirectly down into the BIOS, ultimately resurfacing within the kernel?** Theoretically sound, this hypothesis aims to commandeer the entire boot process, traversing through BIOS and returning seamlessly into the system kernel, thereby enabling a persistent, continuous, and advanced shell environment. The described method s below are an advanced, multi-stage bootkit-style technique intended for safely placing a file (`bootres.dll`) into the `\Windows\system32` directory upon the next reboot in Legacy BIOS/MBR Windows systems. It leverages multiple boot stages to bypass the complexities of directly interacting with Windows file systems. ### High-Level Soft Layers: High-Level Software Layers +-----------------------------------------------------+ | User Applications | | (Browsers, Editors, Games, etc.) | +-----------------------------------------------------+ | Operating System (OS) | | (System APIs, Libraries, User Interfaces) | +-----------------------------------------------------+ | OS Kernel | | (Process Scheduling, Memory Management, Device I/O) | +-----------------------------------------------------+ | Drivers & Kernel Modules | | (Device Drivers, File Systems, etc.) | +-----------------------------------------------------+ | Bootloader | | (Stage 2 Bootkit Core, Patched Snippet) | +-----------------------------------------------------+ | Master Boot Record (MBR) | | (Stage 1 - Minimal Loader) | +-----------------------------------------------------+ | BIOS Firmware | | (POST, Hardware Initialization) | +-----------------------------------------------------+ | Physical Memory | | (RAM, ROM, Cache, etc.) | +-----------------------------------------------------+ | Hardware | | (CPU, Disk, Peripherals, Interfaces) | +-----------------------------------------------------+ Custom MBR – Minimal "Hello World" Chainloader with Conceptual Hook Method -------------------------------------------------------------------------- This document outlines a minimal "Hello World" chainloader example alongside an advanced conceptual bootkit-style hook method, intended to demonstrate safely placing a file (`bootres.dll`) into the `\Windows\system32` directory upon reboot in Legacy BIOS/MBR Windows systems. Advanced Hook Method Breakdown ------------------------------ ### Stage 1 (MBR) * Minimal code footprint (≤466 bytes). * Prints a simple message for feedback. * Loads the next stage from hidden sectors. ### Stage 2 (Bootkit Core) * Identifies and patches early Windows boot files (`bootmgr`, `winload.exe`, or older loaders). * Injects custom code snippet ("hook") directly into the identified bootloader. * Transfers control seamlessly to Windows' original Volume Boot Record. ### Patched Bootloader Snippet * Executes during early Windows boot. * Safely places or replaces `bootres.dll` in `\Windows\system32`. * Optionally cleans itself or remains persistent. Considerations & Limitations ---------------------------- * Intended for educational or controlled environments only. * Requires careful handling due to ethical, security, and technical complexities. * Modern protections like Secure Boot and NTFS complexity present significant implementation challenges. This combined approach demonstrates both a basic understanding of bootloader operations and an advanced conceptual grasp of boot processes, filesystem interactions, and persistence strategies. * * * ### Minimal "Hello World" MBR Chain-loader * * * **Warning:** This is _educational_ code. Writing this to your real hard drive MBR could corrupt your system if something goes wrong. Use a VM or backup first. * * * ### Assemble and Write to Disk: nasm -f bin custom_mbr.asm -o custom_mbr.bin Then (**VERY DANGEROUS**): dd if=custom_mbr.bin of=/dev/sdX bs=512 count=1 _(Replace_ `sdX` _with correct disk; do_ **_NOT_** _do this on a live system unless you know exactly what you’re doing.)_ * * * ### Functionality: 1. Displays a short text line via BIOS interrupt `0x10`. 2. Locates the active partition in the partition table. 3. Loads the first sector of that partition (the Volume Boot Record) to memory address `0x7E00:0000`. 4. Jumps there, allowing Windows (or any OS) to continue booting. **Note:** * The last two bytes **must** be `0x55`, `0xAA` to mark the sector as bootable for a BIOS system. * **Key Offsets:** * Partition table starts at `0x1BE` * Boot signature at `0x1FE` (the last two bytes of the 512-byte sector) * * * Assembly Code: -------------- [org 0x7C00] ; BIOS loads the MBR at 0x0000:0x7C00 bits 16 start: xor ax, ax mov ss, ax mov sp, 0x7C00 mov ds, ax mov si, hello_message call print_string mov bx, PARTITION_TABLE_OFFSET mov cx, 4 find_active_partition: push cx mov al, [bx] cmp al, 0x80 je partition_found add bx, 16 pop cx loop find_active_partition no_active_partition: call print_line mov si, error_no_active call print_string jmp halt partition_found: mov dl, [BOOT_DRIVE] mov dh, [bx+1] mov cl, [bx+2] mov ch, [bx+3] mov [part_head], dh mov [part_sector], cl mov [part_cyl], ch call read_vbr jmp 0x7E0:0x0000 read_vbr: mov ax, 0x7E0 mov es, ax xor bx, bx mov ah, 0x02 mov al, 1 mov ch, [part_cyl] mov cl, [part_sector] mov dh, [part_head] int 0x13 ret print_string: .next_char: lodsb or al, al jz .done mov ah, 0x0E int 0x10 jmp .next_char .done: ret print_line: mov ah, 0x0E mov al, 0x0D int 0x10 mov al, 0x0A int 0x10 ret halt: cli hlt jmp halt hello_message db "Custom MBR: Hello from the boot sector!", 0 error_no_active db "No active partition found! System halted.", 0 part_head db 0 part_sector db 0 part_cyl db 0 BOOT_DRIVE db 0x80 PARTITION_TABLE_OFFSET equ 0x1BE ; Fill up to 510 bytes with zeros times 510 - ($-$$) db 0 ; Bootable sector signature dw 0xAA55 * * * Conceptual Hook Method for File Placement in `\Windows\system32` ---------------------------------------------------------------- * * * **Important Security & Ethical Note:** Modifying Windows’ boot sequence or hooking OS components can be malicious if done without authorization. Always conduct such experiments in controlled environments (e.g., VMs). * * * Overview -------- Rather than placing `bootres.dll` directly into the root (`C:\`), use a multi-stage bootkit-style hook to ensure the file ends up in `\Windows\system32` during the next boot on Legacy BIOS/MBR systems. * * * Detailed Steps -------------- ### Stage 1 (Minimal MBR Code) * Print message. * Load Stage 2 using INT 0x13 (AH=0x42 for large disks or AH=0x02 for CHS addressing). * Maintain partition table integrity (offset 0x1BE). * Mark bootable with `0x55AA` at offset `0x1FE`. ### Stage 2 (Loader/Bootkit Core) * Parses active partition to locate `\bootmgr`, `\Boot\BCD`, or older loaders (NTLDR). * Implements minimal filesystem parsing (FAT32 easier; NTFS complex). * Patches identified Windows bootloader file with injected code: * Allocates space or utilizes existing code cave. * Modifies bootloader to run custom snippet. * Chains to original Windows VBR. ### Hook Snippet (in Patched Bootloader) * Runs within Windows' early loader environment, enabling proper filesystem access. * Creates or replaces `\Windows\system32\bootres.dll` with desired content (e.g., `copy /b bootres.dll+newlogo-decoded.bmp patched-bootres.dll`). * Closes file, returning execution to Windows boot. * * * Alternate Hook Methods ---------------------- * **INT 13h Hook**: Effective temporarily; limited by Windows drivers. * **Early Driver Injection**: Modify registry or BCD to load custom driver at boot. * * * Challenges & Considerations --------------------------- * **Digital Signatures/Secure Boot**: Modern systems may block modifications. * **Filesystem Complexity (NTFS vs. FAT32)**: NTFS demands sophisticated parsing. * **Persistence & Space Constraints**: Limited MBR size (466 bytes); Stage 2 stored separately. * * * Hook Method Flowchart --------------------- [Power On / BIOS] ↓ [MBR (Stage 1)] - Message display - Load Stage 2 ↓ [Stage 2] - Locate & patch bootloader - Chain-load Windows VBR ↓ [Windows Bootloader] - Hook executes - Creates `bootres.dll` in `\Windows\system32` - Continues booting Windows * * * Final Thoughts -------------- * Always test in a controlled VM environment. * Modern protections (UEFI/Secure Boot) significantly complicate or prevent this approach. * Even partial implementation demonstrates advanced understanding of bootkit concepts. // ============================================================================= // // _____ __ _____ _____ __ __ // / ___/____ ___ ____ ______/ /_ /__ / / ___// /_____ ______/ /__ // \__ \/ __ `__ \/ __ `/ ___/ __ \______/ / ______\__ \/ __/ __ `/ ___/ //_/ // ___/ / / / / / / /_/ (__ ) / / /_____/ /_/_____/__/ / /_/ /_/ / /__/ ,< // ____/_/ /_/ /_/\__,_/____/_/ /_/ /____/ /____/\__/\__,_/\___/_/|_| // ==================================================== // // // Tribute to "Smashing the Stack" // and the original hackers who paved the way. // // [ Aleph One ] [ Levy ] [ Condor ] // [ L0pht ] [ CDC ] [ Phrack ] [ 2600 ] // // ================================================= // // // "Because security is an illusion, // and knowledge is power." // // ================================================= // --- *Originally published on [Rek0n](https://paragraph.com/@rek0n/%5B-asm-%5D-chain-loader)*