43ea87dffc
Duplicate ivar names deleted subtrees and five of six readers threw away the class-name node, so a gadget in either position was invisible to the detector while Marshal.load still fired it. Both had working proofs; the suite was green the whole time. The parse graph now has exactly one traversal owner. read_class_name and read_instance_variables push every class-name node, ivar name and ivar value into auxiliary, and Node#each no longer walks instance_variables_map. The map stays as a lookup convenience with last-write-wins semantics, it just is not load-bearing for security any more. Walking both would have double-counted every ivar value. Corpus entries take an optional allowlist. Without one every case ran through an empty strict allowlist where any class name rejects, which is why the corpus could not express B2 at all. The 40 existing cases default to [] and are unchanged. At the tag level there are seven class-name slots, not six: o S u U d C e, and only o retained its node. u, U and d are themselves sink tags so a corpus case there can never fail; those three are asserted at the parser level instead and a test pins the exclusion as deliberate rather than an oversight. Verified by mutation, since green means nothing on this project. Dropping the class-name push, dropping the ivar value push, and restoring the map walk each now fail 2, 4 and 3 tests. The first two previously survived the entire suite. 119 tests from 110, 48 corpus cases from 40. test, control, exploit, detector, target and matrix all pass. |
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|---|---|---|
| .. | ||
| lib | ||
| scripts | ||
| target | ||
| 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.