A newly disclosed universal deserialization gadget chain shows how a single unsafe Marshal.load operation can reportedly produce remote command execution in Ruby 4.0.6, underscoring the persistent danger of exposing Ruby’s native serialization mechanism to attacker-controlled data.
According to research published by elttam, the chain also works unchanged on Ruby versions as far back as 3.3. The finding renews concerns surrounding Marshal.load, which reconstructs Ruby objects from serialized data and may invoke application and language behavior as it restores object state.
The technique builds on a long history of Ruby deserialization research involving Ruby on Rails, YAML.load, RubyGems, and standard-library components. Earlier gadget chains were disrupted by changes introduced in Ruby 3.4.
Ruby 4.0 Marshal.load RCE Gadget Chain
The latest approach, however, reportedly avoids dependency and environmental requirements that constrained previous proofs of concept.
The new chain combines RubyGems behavior with core Ruby object reconstruction features. It does not require application-specific dependencies, pre-existing files, or non-default gems on the target system.
At a high level, the attack begins by referencing Gem::SpecFetcher. This causes RubyGems autoloading to expose additional classes, even in an otherwise minimal Ruby process. The chain then connects two dangerous capabilities: retrieving attacker-controlled content and evaluating that content as Ruby code.
A surviving RubyGems gadget can retrieve compressed data from an attacker-controlled HTTPS server and write the inflated result to a predictable writable location, such as /tmp.
A separate Gem::StubSpecification object then causes RubyGems to invoke Gem::Specification.load against that downloaded file.
The loading routine reads the target file and evaluates its contents using Ruby’s eval functionality. If an attacker controls both the remote payload and its destination path, the deserialization workflow can execute arbitrary Ruby statements with the permissions of the affected application.
Two features make the reported gadget chain particularly difficult to mitigate with narrowly targeted RubyGems changes. First, the chain uses Time deserialization to trigger the download action while suppressing an expected exception.
A crafted value moves through a conversion path that eventually invokes methods controlled by the attacker’s serialized object. Second, execution is initiated via standard hash reconstruction.
During Marshal.load, Ruby rebuilds a Hash and calculates hash values for restored keys. A crafted Gem::StubSpecification object used as a hash key can therefore cause Ruby to invoke its hash method, ultimately reaching the file-loading and evaluation path.
Previous RubyGems mitigations blocked known chains by adding type checks and removing directly attacker-controlled executable paths. However, researcher Luke Jahnke stated that core behaviors such as hash-key processing and tolerant Time restoration are hard to remove without potentially breaking expected language semantics.
The practical risk is severe wherever a Ruby service accepts, processes, or indirectly consumes attacker-influenced Marshal data. Exploitation may require only a reachable attacker-controlled HTTPS server and access to a writable directory.
It does not necessarily rely on third-party gems, application-specific classes, or pre-positioned files. Organizations should treat Marshal.load on untrusted input as equivalent to direct code execution.
Developers should remove it from network-facing and user-controlled workflows, including cookies, cache entries, background-job payloads, message queues, uploaded files, and database fields that can be modified by lower-trust systems.
Where serialization remains necessary, teams should migrate to data-only formats such as JSON and enforce strict schemas, explicit type validation, and integrity protections.
Existing Ruby deployments should be audited for Marshal.load, Marshal.restore, YAML.load, and comparable unsafe deserialization paths.
Security teams should monitor Ruby workloads for unexpected outbound HTTPS traffic, unusual file writes beneath temporary directories, and Ruby processes spawning commands. These events can provide important early-warning signals of attempted or successful unsafe deserialization exploitation.
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