covfuzz

Reader-robustness fuzzing for Dart parsers — code that reads untrusted input (binary formats, text protocols, JSON from the network). One library, two tiers, plus command-line tools for the surrounding workflow.

The contract it checks: a parser must never crash or hang on malformed input. It either parses leniently or rejects with a documented exception (FormatException, or its own reject type). A leaked RangeError / StateError / TypeError, an out-of-memory, an infinite loop, or a multi-second parse is a defect. Any thrown exception outside your allow-list is recorded and reduced to a minimal reproducer.

Install

dev_dependencies:
  covfuzz: ^0.1.0

How-to: harden a parser (the optimal loop)

The order matters — each step feeds the next, and doing them out of order wastes time (e.g. coverage-guided fuzzing a target that hard-hangs will just freeze).

1. Find the target. List a tree's parse entry points:

dart pub global activate covfuzz
covfuzz_discover ~/projects

2. Check it fuzzes standalone.

covfuzz_probe ~/projects/my_pkg my_pkg src/foo_parser.dart

FUZZABLE → continue. FFI-BLOCKED → the import chain reaches a NativeCallable; copy the pure parse functions into a standalone file (stub their return types) and fuzz that.

3. Blind first — it's ~1000× faster and catches most bugs.

covfuzz_scaffold my_pkg src/foo_parser.dart foo --bytes   # writes tool/fuzz_foo.dart
# edit tool/fuzz_foo.dart: real seeds, the entry call, your clean-reject list
dart run tool/fuzz_foo.dart

Fix everything it reports before going further: escapes (with their minimized reproducers) and the slow-parse signal (exit 2 = a size-driven allocation bomb) and any hang (blind fuzzing surfaces these; coverage-guided cannot, because a hang freezes it). Seed from valid samples so mutations reach deep code — random bytes bounce off the magic check.

4. Coverage-guided — for the paths behind a magic check or precondition. Only once blind is clean of hangs. Write a covFuzz harness and run it in its own process with the VM service on:

dart run --enable-vm-service=0 --no-pause-isolates-on-exit tool/covfuzz_foo.dart

It evolves a corpus toward new coverage and finds what blind can't reach (in practice: a real RangeError behind a valid-protobuf/valid-header prefix that blind never produces). Persist the corpus with corpusDir so the next run continues from the coverage already reached.

5. Fix, wrap the guard, prove it. Wrap each fix in markers:

// GUARD:foolen >>>
if (n > _max) throw const FormatException('...');
// GUARD:foolen <<<

Add the minimized reproducer as a regression test, then prove the test actually exercises the guard:

covfuzz_mutverify --file lib/src/foo_parser.dart --guard foolen \
  --test 'dart test test/foo_parser_test.dart'

It reverts the guard, asserts the test now fails, and restores the file. A guard the test can't detect has no regression protection.

Library

Blind fuzzing — fuzz

Zero setup, ~1M execs/sec. The fast first pass: it shakes out crashes and the slow-parse (bomb) signal, and minimizes each escape.

import 'dart:typed_data';
import 'package:covfuzz/covfuzz.dart';
import 'package:my_pkg/src/foo_parser.dart';

void main() {
  final report = fuzz<Uint8List>(
    seeds: [validSample],               // valid input(s) — mutations start here
    entry: (b) => parseFoo(b),
    mutate: mutateBytes,                // or mutateString
    isClean: (e) => e is FormatException, // your clean-reject types
    stressors: [Uint8List(0), hugeInput], // structural cases mutation misses
  );
  report.report(); // prints; returns 0 clean & fast, 1 escapes, 2 clean-but-slow
}

Coverage-guided fuzzing — covFuzz

For paths behind a magic check or a multi-field precondition that blind mutation can't reach. It reads the target library's coverage from the VM service after each input and keeps any input that reaches new code as a corpus seed, so the corpus evolves toward deep code.

Must run with the VM service enabled:

dart run --enable-vm-service=0 --no-pause-isolates-on-exit tool/covfuzz_foo.dart

final r = await covFuzz<Uint8List>(
  seeds: [validSample],
  entry: (b) => parseFoo(b),
  mutate: mutateBytes,
  targetLib: 'package:my_pkg/src/foo_parser.dart', // library to score coverage on
  isClean: (e) => e is FormatException,
  corpusDir: '.corpus/foo',  // optional: persist the evolved corpus across runs
  crashDir: '.crashes/foo',  // optional: save minimized crashes
  log: true,                 // print the coverage climb
);
r.report();

On the bundled demo (example/, a bug behind a 4-byte FUZZ magic), blind mutation finds nothing in 500k tries while covFuzz climbs the four conditions (coverage 8→12→16→20) and reports the minimized 5-byte trigger [70,85,90,90,8].

Command-line tools

dart pub global activate covfuzz (or dart run covfuzz:<tool>):

  • covfuzz_discover [root] — list a tree's packages and their parse entry points (byte readers, parseXxx(String), fromJson); flags dart:ffi users.
  • covfuzz_probe <repo> <pkg> <import-path> — does the file fuzz under bare dart run, or is it FFI-blocked (extract the pure logic first)?
  • covfuzz_scaffold <pkg> <import-path> <name> [--bytes] — write a harness stub in tool/.
  • covfuzz_mutverify --file F --guard NAME --test 'CMD' — prove a hardening guard is load-bearing: revert a marker-wrapped guard, run the test expecting failure, restore the file byte-identically. Wrap guards as: 
    // GUARD:maxdepth >>>
if (depth > _maxDepth) throw const FormatException('nesting too deep');
// GUARD:maxdepth <<<

Seeds

Start from valid encodings so mutations keep enough structure (magic bytes, signatures, a valid JSON envelope) to reach deep parse paths. Random bytes mostly bounce off the format's magic check. Seed binary formats from real sample files; JSON from a valid document.

Prior art and limits

Coverage-guided fuzzers — libFuzzer, AFL++, OSS-Fuzz — are the state of the art for C/C++/Rust, with native per-execution coverage hitmaps. Dart has no such in-process signal, so covFuzz reads coverage from the VM service (one query per input, ~100–1000 execs/sec — orders below a native fuzzer, but it evolves a corpus blind mutation cannot). Crash minimization borrows libFuzzer's -minimize_crash; covfuzz_mutverify is a guard-targeted form of what package:mutation_test does across a file; for property testing with shrinking, package:glados is the established Dart tool.

  • The target runs in-process, so a hard hang freezes covFuzz — shake hangs out with blind fuzz first.
  • Coverage is process-global and cumulative — run each covFuzz session in its own process.
  • No structure-aware / dictionary mutation yet; mutateBytes is biased toward in-place edits so positions stay stable enough to match magic bytes.

Requirements

Dart SDK 3.x. Native platforms only (dart:io + VM service).

License

MIT — see LICENSE.

Libraries

covfuzz
Reader-robustness fuzzing for Dart parsers, in two tiers: