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efi_fuzz

A simple, coverage-guided fuzzer for UEFI NVRAM variables. Based on Qiling and AFL++.
Written by Itai Liba (@liba2k) and Assaf Carlsbad (@assaf_carlsbad).

Usage

Using Docker environment

1. Build image:
   docker build -t efi_fuzz .

2. Test environment:
   docker run -v $PWD:/efi_fuzz -it efi_fuzz sh -c "cd efi_fuzz/tests/ && pytest -s -v -W ignore::DeprecationWarning"

3. Use the environment:
   docker run -v $PWD:/efi_fuzz -it efi_fuzz sh -c "cd /efi_fuzz ; bash"

4. Prepare the emulated NVRAM environment (You will have to provide the rom image):
   python3 scripts/prepare_nvram.py rom.bin nvram.pickle

5. Prepare the initial corpus for the NVRAM variables:
   python3 scripts/prepare_afl_corpus.py rom.bin afl_inputs

6. Perform a dry run of the fuzzer:
   python3 efi_fuzz.py <target> <nvram> <varname> <seed>

7. If successful, move on to full-fledged fuzzing:
   afl-fuzz -i afl_inputs/<varname> -o afl_outputs/ -U -- python3 efi_fuzz.py <target> <nvram> <varname> @@

Install Environment locally

1. If running on Windows, install WSL. We recommend WSL2 as opposed to the original WSL, which tends to be slow sometimes. The full installation instructions for Windows 10 can be found here: https://docs.microsoft.com/en-us/windows/wsl/install-win10

2. Inside the WSL distribution, install some necessary packages that will allow us to compile C source code:
   sudo apt install build-essential automake

3. Install AFL++ with Unicorn mode support.
   3.1. Clone the repository:
   git clone https://github.com/AFLplusplus/AFLplusplus
   3.2. Build core AFL++ binaries:
   make
   3.3. Build the Unicorn support feature:
   cd unicorn_mode
./build_unicorn_support.sh
   3.4. Install everything:
   make install

4. Acquire and unpack the UEFI firmware you wish to fuzz. For the full technical details on how this is done is practice, see here or here.

5. Clone the fuzzer and install required dependencies:
   git clone https://github.com/Sentinel-One/efi_fuzz
pip install -r efi_fuzz/requirements.txt

Triton is required for taint propagation.
Instructions for installing Triton can be found here:
https://triton.quarkslab.com/documentation/doxygen/#install_sec

6. Prepare the emulated NVRAM environment:
   python scripts/prepare_nvram.py rom.bin nvram.pickle

7. Prepare the initial corpus for the NVRAM variables:
   python scripts/prepare_afl_corpus.py rom.bin afl_inputs

8. Perform a dry run of the fuzzer:
   python efi_fuzz.py <target> <nvram> <varname> <seed>

9. If successful, move on to full-fledged fuzzing:
   afl-fuzz -i afl_inputs/<varname> -o afl_outputs/ -U -- python efi_fuzz.py <target> <nvram> <varname> @@

Command-line options

• -e, --end: Specify an end address for the emulation.
• -t, --timeout: Specify a new timeout value for the emulation, in ms.
• -o, --output: Specify output format for debugging purposes. Valid values are: trace, disasm, debug and off (defaults to off).
• -n, --no-sanitize: Disable the memory sanitizer (defaults to False).
• -u, --track-uninitialized: Keeps track of uninitialized memory via Triton and taint propagation (EXPERIMENTAL!).
• -x, --extra-modules: A list of extra modules to load to satisfy the dependencies of the target.

Further reading

Further information regarding UEFI firmware in general and efi_fuzz in particular can be found here:

• Moving From Common-Sense Knowledge About UEFI To Actually Dumping UEFI Firmware
• Moving From Manual Reverse Engineering of UEFI Modules To Dynamic Emulation of UEFI Firmware
• Moving From Dynamic Emulation of UEFI Modules To Coverage-Guided Fuzzing of UEFI Firmware