Ruby 4.0: Universal Deserialization Gadget Chain Enables Critical RCE

Researcher elttam has identified a critical vulnerability in Ruby 4.0 involving a universal Remote Code Execution (RCE) gadget chain within the Marshal.load method. Unlike previous deserialization attacks that relied on specific third-party gems or frameworks to build exploit chains, this discovery utilizes a sequence of Ruby core class instantiations to trigger arbitrary command execution. By passing a specifically crafted binary payload to any Marshal.load sink processing untrusted input, attackers can achieve full system compromise. This finding shifts the threat model from application-specific risk to a systemic vulnerability inherent in the Ruby 4.0 core serialization mechanism, necessitating immediate transition to safer serialization formats like JSON.

• Introduction/Overview: Systemic Risk in Ruby 4.0
  • Identifies a "universal" gadget chain affecting all Ruby 4.0 environments.
  • Transitions the threat from application-specific (gem-dependent) to systemic (core-dependent).
  • Targets the Marshal.load function, a known vulnerable sink for binary deserialization.
• Vulnerability Mechanics: Universal Gadget Chain Deep Dive
  • Leverages internal Ruby object hooks, such as initialize or custom marshal_load methods.
  • Employs a sequence of core class instantiations to pivot from object creation to method execution.
  • Uses a crafted binary blob payload to trigger arbitrary shell commands on the host system.
• Impact/Exploitation Status: Critical RCE Capabilities
  • Severity: Critical; provides unauthenticated Remote Code Execution (RCE).
  • Attack Vector: Network-based, targeting any exposed Marshal.load endpoint.
  • Potential Impact: Full host takeover via arbitrary command execution with process privileges.
• Detection/Mitigation: Remediation and Defense
  • Primary Mitigation: Immediate replacement of Marshal.load with JSON.parse or other data-only formats.
  • Defensive Layering: Implement strict cryptographic signing, such as HMAC, for all serialized data.
  • Input Validation: Apply rigorous validation to reject non-standard or unexpected binary input.
• Conclusion: Securing Deserialization Workflows
  • Reinforces the long-standing security mandate to avoid Marshal for untrusted input.
  • Highlights the inherent risks of binary serialization within core language modules.