Imported from trailofbits/skills (plugins/testing-handbook-skills/skills/ruzzy/SKILL.md) via skills.sh. Install upstream with npx skills add trailofbits/skills --skill ruzzy. Copyright stays with the author.

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)

UndefinedBehaviorSanitizer (UBSan)

Ruzzy also includes UBSan:

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

Use UBSan for detecting:

• Signed integer overflow
• Null pointer dereferences
• Misaligned memory access
• Division by zero

Common Sanitizer Issues

See Also: For detailed sanitizer configuration, common issues, and advanced flags, see the address-sanitizer and undefined-behavior-sanitizer technique skills.

Real-World Examples

Example: msgpack-ruby

Fuzzing the msgpack MessagePack parser for memory corruption.

Install with sanitizers:

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 msgpack

Harness (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:

export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"
LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby fuzz_msgpack.rb

Example: Pure Ruby Target

Fuzzing pure Ruby code with a custom parser.

Tracer (test_tracer.rb):

# frozen_string_literal: true

require 'ruzzy'

Ruzzy.trace('test_harness.rb')

Harness (test_harness.rb):

# frozen_string_literal: true

require 'ruzzy'
require_relative 'my_parser'

test_one_input = lambda do |data|
  begin
    MyParser.parse(data)
  rescue StandardError
    # Expected exceptions from malformed input
  end
  return 0
end

Ruzzy.fuzz(test_one_input)

Run:

export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"
LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby test_tracer.rb

Troubleshooting

Related Skills

Technique Skills

Related Fuzzers

Resources

Key External Resources

Introducing Ruzzy, a coverage-guided Ruby fuzzer Official Trail of Bits blog post announcing Ruzzy, covering motivation, architecture, and initial results.

Ruzzy GitHub Repository Source code, additional examples, and development instructions.

libFuzzer Documentation Since Ruzzy is built on libFuzzer, understanding libFuzzer options and behavior is valuable.

Fuzzing Ruby C extensions Detailed guide on fuzzing C extensions with compilation flags and examples.

Fuzzing pure Ruby code Detailed guide on the tracer pattern required for pure Ruby fuzzing.