Fuzzer Skill

When to Use

Invoke when asked to fuzz a target, find memory/integer bugs via fuzzing, write a fuzz harness, or run a fuzzing campaign on a codebase.

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Workflow

Step 1 — Audit (MANDATORY — always run first, no exceptions)

You MUST invoke the audit-context-building skill before writing any harness. Do not skip this step even if you think you already understand the code.

Goal: read the entire codebase — all source files — before ranking anything. Do not stop after finding the first suspicious location in one directory. Cover all modules.

Look specifically for:

• size_t → int or size_t → uint32_t narrowing casts
• Unchecked length arithmetic (additions, multiplications on sizes)
• Public API functions that accept attacker-controlled size_t / length parameters

Output: a ranked list of suspicious locations with file:line drawn from the full codebase. Pick the top candidate for the harness.

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Step 2 — Write the Harness

You MUST target the exact file:line ranked #1 by the audit. Do not target a different function, code path, or "more interesting" area based on your own judgment. The audit decides the target. You implement it.

• Read the call path the audit provided
• Call the exact function in that call path
• Use the bug class it identified to pick the pattern below

Pick the pattern based on the bug class from Step 1.

Arithmetic overflow (size_t→int accumulation, unchecked addition on sizes): The bug is in the arithmetic — not the buffer contents. Extract a uint32_t claimed size from fuzz bytes and pass it directly. Use a 1-byte static stub as the data pointer. Never derive the size from the actual buffer you hand over.

#include <stddef.h>
#include <stdint.h>
#include <string.h>

static const uint8_t kStub[1] = {0};

/* Required by libAFLDriver.a — must be present or link fails */
int LLVMFuzzerInitialize(int *argc, char ***argv) {
    (void)argc; (void)argv;
    return 0;
}
void LLVMFuzzerCleanup(void) {}

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    uint8_t n = data[0] % MAX_CALLS;
    if (n == 0 || size < 1 + (size_t)n * 4) return 0;

    TargetState *s = TargetState_Create();
    for (uint8_t i = 0; i < n; i++) {
        uint32_t claimed;
        memcpy(&claimed, data + 1 + i * 4, 4);
        target_fn(s, (size_t)claimed, kStub);  /* claimed drives the arithmetic */
    }
    TargetState_Destroy(s);
    return 0;
}

Split-input (API takes a config/size parameter + a data payload):

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    if (size < N) return 0;
    /* bytes [0..N-1] → config / mode / count */
    /* bytes [N..]    → payload */
    target_fn(config, data + N, size - N);
    return 0;
}

Direct call (simple buffer + length):

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    if (size < 1) return 0;
    target_fn(data, size);
    return 0;
}

Rust:

fuzz_target!(|data: &[u8]| { target_function(data); });

Go:

func FuzzTarget(f *testing.F) {
    f.Fuzz(func(t *testing.T, data []byte) { target_function(data) })
}

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Step 3 — Build

Before building, locate AFL++:

which afl-clang-fast || find /usr/local /opt/homebrew /tmp -name afl-clang-fast 2>/dev/null

If not found, install it:

brew install afl++       # macOS
apt-get install afl++    # Debian/Ubuntu

C / C++ with AFL++:

AFL=$(which afl-clang-fast)
AFLDRIVER=$(dirname $(dirname $AFL))/lib/afl/libAFLDriver.a

# Compile target sources into instrumented static library
mkdir -p build
find <src-dir> -name "*.c" | while read f; do
  $AFL \
    -g -O1 -fsanitize=address,undefined -fno-omit-frame-pointer \
    <include-flags> -c "$f" -o "build/$(basename $f .c).o"
done
ar rcs build/libtarget.a build/*.o

# Compile harness + link
$AFL \
  -g -O1 -fsanitize=address,undefined -fno-omit-frame-pointer \
  harness.c build/libtarget.a $AFLDRIVER \
  -o build/fuzzer

Rust:

cargo fuzz build <target_name>

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Step 4 — Run

C / C++ with AFL++:

ASAN_OPTIONS=abort_on_error=1:detect_leaks=0:symbolize=0 \
AFL_SKIP_CPUFREQ=1 AFL_I_DONT_CARE_ABOUT_MISSING_CRASHES=1 \
$(which afl-fuzz) \
  -i corpus/ \
  -o findings/ \
  -- build/fuzzer

Rust:

cargo fuzz run <target_name> corpus/

Go:

go test -fuzz=FuzzTarget -fuzztime=12h ./...

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Step 5 — Report

When findings/default/crashes/ contains files, report:

CRASH FOUND
  Input:   findings/default/crashes/id:000000,...
  Signal:  sig:06 (SIGABRT = sanitizer) or sig:11 (SIGSEGV)

Reproduce (with symbolized output):
  UBSAN_OPTIONS=print_stacktrace=1 \
  ASAN_OPTIONS=detect_leaks=0:print_stacktrace=1 \
  ./build/fuzzer findings/default/crashes/id:000000,...

Sanitizer output:
  <paste UBSan/ASan stacktrace here>

Root cause: <file>:<line> — <one sentence description>

If no crashes after the time budget: report paths found and unique inputs in corpus.

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Harness Rules

Rule	Why
Return 0 always	Never abort from the harness itself
Never call exit()	Kills the fuzzer process
Handle all input sizes	Fuzzer generates empty / tiny / huge inputs
Be fast — no logging	Target 100–1000+ exec/sec
Same input = same output	Determinism required for crash reproduction
Free all resources each call	Prevents memory exhaustion over millions of runs
Reset global state	Isolates each iteration

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Tool Selection

Target	Fuzzer	Notes
C / C++	AFL++ (afl-clang-fast)	Best coverage instrumentation
Rust	cargo-fuzz	Uses libFuzzer API under the hood
Go	go test -fuzz	Native, no extra tooling
Any binary	AFL++ black-box (afl-fuzz @@)	No source needed
Custom / research	LibAFL	Modular Rust fuzzing library