---

name: ruzzy type: fuzzer description: "Sets up and runs Ruzzy, Trail of Bits' coverage-guided Ruby fuzzer and the only production-ready one for the language. Covers harness structure, fuzzing pure Ruby and the native C extensions in gems, and sanitizer builds. Use when fuzzing a Ruby library or gem, testing a Ruby C extension for memory safety, or asking how to fuzz Ruby at all."

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

| Do | Don't | |----|-------| | Catch Ruby exceptions if testing C extensions | Let Ruby exceptions crash the fuzzer | | Return 0 from test_one_input lambda | Return other values | | Keep harness deterministic | Use randomness or time-based logic | | Use tracer script for pure Ruby | Skip tracer for pure Ruby code |

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

| Flag | Purpose | |------|---------| | -fsanitize=address,fuzzer-no-link | Enable AddressSanitizer and fuzzer instrumentation | | -fno-omit-frame-pointer | Improve stack trace quality | | -fno-common | Better compatibility with sanitizers | | -fPIC | Position-independent code for shared libraries | | -g | Include debug symbols |

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

| Output | Meaning | |--------|---------| | INFO: Running with entropic power schedule | Fuzzing campaign started | | ERROR: AddressSanitizer: heap-use-after-free | Memory corruption detected | | SUMMARY: libFuzzer: fuzz target exited | Ruby exception occurred | | artifact_prefix='./'; Test unit written to ./crash-* | Crash input saved | | Base64: ... | Base64 encoding of crash input |

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

| Issue | Solution | |-------|----------| | Ruby interpreter leak warnings | Use ASAN_OPTIONS=detect_leaks=0 | | Sigaltstack conflicts | Use ASAN_OPTIONS=use_sigaltstack=0 | | Allocation failure spam | Use ASAN_OPTIONS=allocator_may_return_null=1 | | LD_PRELOAD interferes with tools | Don't export it; set inline with ruby command |

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

| Problem | Cause | Solution | |---------|-------|----------| | Installation fails | Wrong clang version or path | Verify clang path, use clang 14.0.0+ | | cannot open shared object file | LD_PRELOAD not set | Set LD_PRELOAD inline with ruby command | | Fuzzer immediately exits | Missing corpus directory | Create corpus directory or pass as argument | | No coverage progress | Pure Ruby needs tracer | Use tracer script for pure Ruby code | | Leak detection spam | Ruby interpreter leaks | Set ASAN_OPTIONS=detect_leaks=0 | | Installation debug needed | Compilation errors | Use RUZZY_DEBUG=1 gem install --verbose ruzzy |

Related Skills

Technique Skills

| Skill | Use Case | |-------|----------| | fuzz-harness-writing | Detailed guidance on writing effective harnesses | | address-sanitizer | Memory error detection during fuzzing | | undefined-behavior-sanitizer | Detecting undefined behavior in C extensions | | libfuzzer | Understanding libFuzzer options (Ruzzy is built on libFuzzer) |

Related Fuzzers

| Skill | When to Consider | |-------|------------------| | libfuzzer | When fuzzing Ruby C extension code directly in C/C++ | | aflpp | Alternative approach for fuzzing Ruby by instrumenting Ruby interpreter |

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 exampl