Ruzzy

Ruzzy

Ruzzy is a coverage-guided fuzzer for Ruby built on libFuzzer. It enables fuzzing both pure Ruby code and Ruby C extensions with sanitizer support for detecting memory corruption and undefined behavior.

When to Use

Ruzzy is currently the only production-ready coverage-guided fuzzer for Ruby.

Choose Ruzzy when:

• Fuzzing Ruby applications or libraries
• Testing Ruby C extensions for memory safety issues
• You need coverage-guided fuzzing for Ruby code
• Working with Ruby gems that have native extensions

Quick Start

Set up environment:

export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"

Test with the included toy example:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby -e 'require "ruzzy"; Ruzzy.dummy'

This should quickly find a crash demonstrating that Ruzzy is working correctly.

Installation

Platform Support

Ruzzy supports Linux x86-64 and AArch64/ARM64. For macOS or Windows, use the Dockerfile or development environment.

Prerequisites

• Linux x86-64 or AArch64/ARM64
• Recent version of clang (tested back to 14.0.0, latest release recommended)
• Ruby with gem installed

Installation Command

Install Ruzzy with clang compiler flags:

MAKE="make --environment-overrides V=1" \
CC="/path/to/clang" \
CXX="/path/to/clang++" \
LDSHARED="/path/to/clang -shared" \
LDSHAREDXX="/path/to/clang++ -shared" \
    gem install ruzzy

Environment variables explained:

• MAKE: Overrides make to respect subsequent environment variables
• CC, CXX, LDSHARED, LDSHAREDXX: Ensure proper clang binaries are used for latest features

Troubleshooting Installation

If installation fails, enable debug output:

RUZZY_DEBUG=1 gem install --verbose ruzzy

Verification

Verify installation by running the toy example (see Quick Start section).

Writing a Harness

Fuzzing Pure Ruby Code

Pure Ruby fuzzing requires two scripts due to Ruby interpreter implementation details.

Tracer script (test_tracer.rb):

# frozen_string_literal: true

require 'ruzzy'

Ruzzy.trace('test_harness.rb')

Harness script (test_harness.rb):

# frozen_string_literal: true

require 'ruzzy'

def fuzzing_target(input)
  # Your code to fuzz here
  if input.length == 4
    if input[0] == 'F'
      if input[1] == 'U'
        if input[2] == 'Z'
          if input[3] == 'Z'
            raise
          end
        end
      end
    end
  end
end

test_one_input = lambda do |data|
  fuzzing_target(data)
  return 0
end

Ruzzy.fuzz(test_one_input)

Run with:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby test_tracer.rb

Fuzzing Ruby C Extensions

C extensions can be fuzzed with a single harness file, no tracer needed.

Example harness for msgpack (fuzz_msgpack.rb):

# frozen_string_literal: true

require 'msgpack'
require 'ruzzy'

test_one_input = lambda do |data|
  begin
    MessagePack.unpack(data)
  rescue Exception
    # We're looking for memory corruption, not Ruby exceptions
  end
  return 0
end

Ruzzy.fuzz(test_one_input)

Run with:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby fuzz_msgpack.rb

Harness Rules

See Also: For detailed harness writing techniques, patterns for handling complex inputs, and advanced strategies, see the fuzz-harness-writing technique skill.

Compilation

Installing Gems with Sanitizers

When installing Ruby gems with C extensions for fuzzing, compile with sanitizer flags:

MAKE="make --environment-overrides V=1" \
CC="/path/to/clang" \
CXX="/path/to/clang++" \
LDSHARED="/path/to/clang -shared" \
LDSHAREDXX="/path/to/clang++ -shared" \
CFLAGS="-fsanitize=address,fuzzer-no-link -fno-omit-frame-pointer -fno-common -fPIC -g" \
CXXFLAGS="-fsanitize=address,fuzzer-no-link -fno-omit-frame-pointer -fno-common -fPIC -g" \
    gem install <gem-name>

Build Flags

Running Campaigns

Environment Setup

Before running any fuzzing campaign, set ASAN_OPTIONS:

export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"

Options explained:

1. allocator_may_return_null=1: Skip common low-impact allocation failures (DoS)
2. detect_leaks=0: Ruby interpreter leaks data, ignore these for now
3. use_sigaltstack=0: Ruby recommends disabling sigaltstack with ASan

Basic Run

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby harness.rb

Note: LD_PRELOAD is required for sanitizer injection. Unlike ASAN_OPTIONS, do not export it as it may interfere with other programs.

With Corpus

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby harness.rb /path/to/corpus

Passing libFuzzer Options

All libFuzzer options can be passed as arguments:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby harness.rb /path/to/corpus -max_len=1024 -timeout=10

See libFuzzer options for full reference.

Reproducing Crashes

Re-run a crash case by passing the crash file:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby harness.rb ./crash-253420c1158bc6382093d409ce2e9cff5806e980

Interpreting Output

Sanitizer Integration

AddressSanitizer (ASan)

Ruzzy includes a pre-compiled AddressSanitizer library:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby harness.rb

Use ASan for detecting:

• Heap buffer overflows
• Stack buffer overflows
• Use-after-free
• Double-free
• Memory leaks (disabled by default in Ruzzy)