bd89c6704a
Chain registry modelled on PHPGGC: the class is the chain identity, metadata carries the CVE and its affected version ranges, generate returns an object rather than bytes, and serialization is a separate step. Ships the ErbDefMethod chain for CVE-2026-41316. Ruby 2.7.0 added an @_init guard to stop Marshal.load code execution on ERB objects, and def_method never checked it. def_module and def_class delegate to def_method, so the single missing check exposed all three entry points for six years. The payload is an ERB built by allocate with @src, @filename and @lineno set and @_init deliberately absent. @src opens with a comment line and a bare end so that the def wrapper def_method injects is closed before the payload runs, which puts execution at eval time rather than at call time. Gate proves both halves and neither alone is sufficient: 4.0.2-slim erb=6.0.1 outcome=FIRED predicted=FIRED 4.0-slim erb=6.0.1.1 outcome=BLOCKED predicted=BLOCKED The prediction column is the load-bearing one. affects? evaluates the CVE ranges encoded in the chain metadata against the erb version present in the image, before the payload runs. Observed behaviour matched on both, so the registry is making falsifiable claims rather than carrying documentation. Exploit containers run with no network, a read-only root filesystem, a 1MB noexec tmpfs and an unprivileged user. The parser also inspects the payload and reports ERB without deserializing it, so the offensive and defensive halves meet on the same artifact. 70 tests, 151 assertions across four suites. |
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| .. | ||
| lib | ||
| scripts | ||
| test | ||
| .gitignore | ||
| .rubocop.yml | ||
| CHANGELOG.md | ||
| Gemfile | ||
| LICENSE | ||
| README.md | ||
| Rakefile | ||
| justfile | ||
| rube.gemspec | ||
README.md
rube
A Ruby object-deserialization security lab.
A gadget chain is a Rube Goldberg machine. One untrusted blob goes in, a dozen unrelated standard-library methods knock each other over, and code execution falls out the far end. This project builds the machine, then builds the thing that stops it.
Why this exists
Marshal.load on untrusted input is arbitrary code execution. So is YAML.unsafe_load,
JSON.load with additions enabled, and Oj.load in its default mode. This is not a Ruby
quirk. It is the same class of bug as Java deserialization, PHP POP chains, and Python
pickle, and it sits at CWE-502 in the CISA Known Exploited Vulnerabilities catalog with a
34.8% known-ransomware rate against a 20.1% baseline across the catalog as a whole.
Most write-ups on this topic teach the exploit. Fewer teach why the obvious defense does not work. This one does both, because the second half is where the actual lesson lives:
You cannot make Marshal.load safe with an allowlist. The proc you pass runs in
r_post_proc, which marshal.c invokes after load_funcall(... s_mload ...). By the
time your allowlist sees the object, marshal_load has already run. The pattern widely
copied off Stack Overflow is a post-mortem, not a veto.
Psych's allowlist genuinely is a veto — for exactly one reason. It checks the tag before revival, where Marshal checks the object after construction. Identical intent, opposite outcome, decided entirely by where the check sits.
Status
Under construction. What exists and is tested:
- Marshal stream parser — parses the binary format, extracts referenced class names
and gadget sinks, and validates structure, all without ever calling
Marshal.load. Rejects truncated streams, unsupported versions, unknown tags, out-of-bounds object links and symlinks, oversized fixnum widths, trailing bytes, and excessive nesting.
Planned: version-compatibility matrix, reflection-based gadget scanner, payload builder, a deliberately vulnerable containerized target, and the defensive layer.
Usage
require "rube"
payload = Marshal.dump(Gem::Requirement.new(">= 0"))
result = Rube::Marshal::Parser.new(payload).parse
result.class_names
# => ["Gem::Requirement", "Gem::Version"]
result.sinks.map { |s| "#{s.class_name}##{s.sink_method}" }
# => ["Gem::Requirement#marshal_load", "Gem::Version#marshal_load"]
Nothing above instantiates a class, calls a constructor, or invokes Marshal.load.
Development
Everything runs in Docker against a pinned Ruby.
just test run the parser suite
just control run the negative controls
just check both
just build build the gem with --strict
just manifest list exactly what would ship in the .gem
A note on the object-link index
Ruby's Marshal format documentation states that object links are one-indexed. They are
zero-indexed. A self-referential array dumps as 04 08 5b 06 40 00, where the trailing
00 is a link to the outermost object at index 0. The parser is written against the
observed bytes, not the documentation.
License
AGPL-3.0-or-later. See LICENSE.