Offensive Fuzzing

Offensive Fuzzing

Fuzzer Types

GreyBox sub-variants: Directed (AFLGo, UAFuzz), Grammar (AFLSmart, Tlspuffin), Concolic (QSYM, Driller), Kernel (syzkaller, kAFL, wtf).

Core Workflow

Research target → Choose analyses → Build harness → Seed corpus → Instrument → Fuzz → Triage crashes → Report

1. Research Target

• Map all input surfaces (files, network, IPC, syscalls, IOCTL)
• Identify high-value areas: previously patched code, complex parsers, newly added code, input ingestion points
• For kernel modules: look beyond copy_from_user — DMA-BUF ops, page fault handlers, VM operation structs, allocation callbacks

2. Instrument and Build

# AFL++ (preferred for GreyBox)
CC=afl-clang-fast CXX=afl-clang-fast++ cmake -DCMAKE_BUILD_TYPE=Release .. && make -j

# libFuzzer + ASan/UBSan (C/C++)
cmake -DCMAKE_CXX_FLAGS="-fsanitize=fuzzer,address,undefined -O1 -g" ..

# CmpLog build for hard compares
AFL_LLVM_CMPLOG=1 CC=afl-clang-fast CXX=afl-clang-fast++ make clean all

Windows (MSVC): Project Properties → C/C++ → Address Sanitizer: Yes (/fsanitize=address)

3. Write Harness

libFuzzer (C++):

#include <cstdint>
#include <cstddef>
extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
    parse_or_process(data, size);
    return 0;
}

Honggfuzz HF_ITER (persistent mode — preferred for large targets):

#include "honggfuzz.h"
int main(int argc, char** argv) {
    initialize_target(); // runs once
    for (;;) {
        size_t len; uint8_t *buf;
        HF_ITER(&buf, &len);
        FILE* s = fmemopen(buf, len, "r");
        target_function(s);
        fclose(s);
        reset_target_state();
    }
}

AFL++ persistent mode (__AFL_LOOP):

while (__AFL_LOOP(10000)) {
    // re-read input and process
}

macOS IPC (Mach message fuzzing):

void *lib_handle = dlopen("libexample.dylib", RTLD_LAZY);
pFunction = dlsym(lib_handle, "DesiredFunction");

4. Build Seed Corpus

• Pull from target's test suite, bug reports, and real-world samples
• Web-crawl (Common Crawl) for file formats; filter by MIME type
• Minimize: afl-cmin -i raw_corpus -o seeds -- ./target @@
• Trim inputs: afl-tmin -i crash -o crash.min -- ./target @@

5. Launch Fuzzing

AFL++ parallel (primary + secondary with cmplog):

afl-fuzz -M f1 -i seeds -o findings -x dict.txt -- ./target @@
afl-fuzz -S s1 -i seeds -o findings -c 0 -- ./target @@

libFuzzer:

./target_libfuzzer corpus/ -max_total_time=3600 -workers=4

Binary-only (QEMU):

afl-fuzz -Q -i seeds -o findings -- target.exe @@

Snapshot (AFL++ Nyx):

NYX_MODE=1 AFL_MAP_SIZE=1048576 afl-fuzz -i seeds -o findings -- ./target_nyx @@

Ensemble (AFL++ + Honggfuzz sharing corpus):

# Terminal 1
afl-fuzz -M fuzzer1 -i seeds -o sync_dir -- ./target @@
# Terminal 2
../honggfuzz/honggfuzz -i sync_dir/fuzzer1/queue -W sync_dir/hfuzz \
  --linux_perf_ipt_block -t 10 -- ./target ___FILE___

6. Monitor and Unstick

If progress stalls:

• Enable CmpLog: -c 0 on AFL++ secondaries
• Add dictionary: -x dict.txt or AFL_TOKEN_FILE
• Switch to directed fuzzing (AFLGo) targeting specific BBs/functions
• Use concolic assistance (QSYM, Driller) on hard branches
• Snapshot the target to increase exec/s
• AFL_MAP_SIZE=1048576, -L 0 for MOpt scheduler

7. Triage Crashes

# 1. Minimize
afl-tmin -i crash -o crash.min -- ./target @@
# 2. Symbolize
ASAN_OPTIONS=abort_on_error=1:symbolize=1 ./target crash.min 2>asan.log
# 3. Hash + bucket
./cov-tool --bbids ./target crash.min > cov.hash
./bucket.py --key "$(cat cov.hash)" --log asan.log --out triage/

Sanitizer env quick reference:

ASAN_OPTIONS=abort_on_error=1:symbolize=1:detect_stack_use_after_return=1
UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1
TSAN_OPTIONS=halt_on_error=1:history_size=7
MSAN_OPTIONS=poison_in_dtor=1:track_origins=2

Oracle Selection

Property oracle patterns:

• Idempotency: f(x) == f(f(x))
• Differential: compare two impls, bucket on output mismatch
• Invariants: monotonic lengths, checksum equality, schema validation post-parse

Specialized Targets

Kernel (Linux) — syzkaller

{
  "target": "linux/arm64",
  "http": ":56700",
  "workdir": "/path/to/workdir",
  "kernel_obj": "/path/to/kernel",
  "image": "/path/to/rootfs.ext3",
  "sshkey": "/path/to/id_rsa",
  "procs": 8,
  "enable_syscalls": ["openat$module_name", "ioctl$IOCTL_CMD", "mmap"],
  "type": "qemu",
  "vm": { "count": 4, "cpu": 2, "mem": 2048 }
}

• Limit enable_syscalls to deepen coverage on specific subsystems
• Use syz-extract to pull constants for custom modules
• Enable CONFIG_KASAN=y, CONFIG_KCFI=y, CONFIG_DEBUG_INFO_BTF=y
• Use kcov filters and syz_cover_filter to direct coverage
• Network fuzzing: inject via TUN/TAP + pseudo-syscalls (syz_emit_ethernet)
• Crash decode: ./scripts/decode_stacktrace.sh vmlinux ... < dmesg.log

syzkaller repro:

syz-execprog -repeat=0 -procs=1 -cover=0 -debug target.repro

EDR / Windows Scanning Engines

WTF snapshot harness skeleton (mpengine.dll / mini-filter):

g_Backend->SetBreakpoint("nt!KeBugCheck2", [](Backend_t *Backend) {
    const uint64_t BCode = Backend->GetArg(0);
    Backend->Stop(Crash_t(fmt::format("crash-{:#x}", BCode)));
});

FilterConnectionPort fuzzing:

HANDLE hPort;
FilterConnectCommunicationPort(L"\\PortName", 0, NULL, 0, NULL, &hPort);
FilterSendMessage(hPort, fuzzData, sizeof(fuzzData), NULL, 0, &bytesReturned);

IOCTL fuzzing pattern:

HANDLE hDev = CreateFile(L"\\\\.\\DeviceName", GENERIC_READ|GENERIC_WRITE, ...);
DeviceIoControl(hDev, ioctlCode, inputBuf, inputLen, outBuf, outLen, &ret, NULL);

• Take snapshots after initialization, right before parse/dispatch loop
• Use IDA Lighthouse for coverage visualization
• Monitor: DRIVER_VERIFIER_DETECTED_VIOLATION (0xc4), IRQL_NOT_LESS_OR_EQUAL (0xa)
• WinDbg: .symfix; !analyze -v; k; !heap -p -a @rax

Cross-platform mpengine.dll on Linux (loadlibrary + HF_ITER + Intel PT):

// Bypass Lua VM to avoid stability issues
insert_function_redirect((void*)luaV_execute_address, my_lua_exec, HOOK_REPLACE_FUNCTION);
for (;;) {
    HF_ITER(&buf, &len);
    ScanDescriptor.UserPtr = fmemopen(buf, len, "r");
    __rsignal(&KernelHandle, RSIG_SCAN_STREAMBUFFER, &ScanParams, sizeof ScanParams);
}

Rust

# Full Rust fuzzing pipeline
cargo test                                         # 1. property tests
cargo +nightly miri test                           # 2. UB via interpreter
cargo +nightly careful test                        # 3. runtime bounds checks
cargo fuzz run fuzz_target_1 -- -max_total_time=3600  # 4. libFuzzer crashes
RUSTFLAGS="--cfg loom" cargo test --release        # 5. concurrency (if needed)
cargo fuzz coverage fuzz_target_1                  # 6. coverage report

Focus unsafe blocks on: Vec::from_raw_parts, unchecked indexing, transmute size mismatches, pointer arithmetic, FFI integer truncation.

Embedded / Binary-Only

• LibAFL: Modular Rust framework; Unicorn engine, snapshot module, LBRFeedback (zero-instrumentation on Intel), SAND decoupled sanitization
• Retrowrite / QASAN: Binary rewriting for coverage + ASan without source
• Nautilus: Grammar-based fuzzing for structured formats

Language Ecosystems

• Go 1.18+: go test -fuzz=Fuzz -run=^$ ./...
• Python: Atheris (CPython native extension fuzzing)
• Rust: cargo-fuzz or honggfuzz-rs
• JS engines: Fuzzilli with extended instrumentation (__builtin_return_address(0) for PC tracking)
• Wasm runtimes: wasmtime-fuzz, wafl for differential fuzzing across V8/Wasmer/Wasmtime
• Smart contracts: Echidna, Foundry-fuzz (Solidity); Move-Fuzz (Aptos/Sui)

CI/CD Integration

- name: Build with afl-clang-fast
  run: CC=afl-clang-fast make -j
- name: Fuzz (smoke, 15 min)
  run: timeout 15m afl-fuzz -i seeds -o findings -- ./target @@ || true
- name: Upload crashes
  if: always()
  uses: actions/upload-artifact@v4
  with:
    path: findings/**/crashes/*

Use ClusterFuzzLite for persistent continuous fuzzing; cache corpora between runs.

Crash Analysis Quick Reference

Linux:

ulimit -c unlimited && sysctl -w kernel.core_pattern=core.%e.%p
gdb -q ./target core.* -ex 'bt' -ex 'info reg' -ex q
addr2line -e ./target 0xDEADBEEF

Windows:

# Enable local dumps
New-Item 'HKLM:\SOFTWARE\Microsoft\Windows\Windows Error Reporting\LocalDumps' -Force
# PageHeap
gflags /p /enable target.exe /full

Kernel KASAN/KMSAN:

dmesg -T | egrep -i 'kasan|kmsan' -A 60
./scripts/decode_stacktrace.sh vmlinux /lib/modules/$(uname -r)/build < dmesg.log

Reproducibility: pin CPU governor, disable ASLR only where safe, fix RNG seeds, save input sequences in persistent mode, record binary hashes and sanitizer options with every crash.

Tool Index

Source: SnailSploit/Claude-Red → Skills/fuzzing/offensive-fuzzing/SKILL.md