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.
Forty explicit Marshal streams built from tag bytes rather than Marshal.dump,
so the corpus stays valid when Ruby changes what it emits. Thirteen must be
accepted, twenty-seven must be rejected across eight distinct error types.
The accept half is what gives the reject half meaning. A detector that
rejected everything would fail thirteen cases here, including legitimate
cycles, shared object references, symlink reuse, and fixnums at every encoding
width.
Provenance note, stated because it weakens the evidence: Codex was asked to
build this corpus and the request was blocked by its provider's content filter,
which reads constructing hostile streams as offensive tooling. The request was
not reworded to get around that. The corpus was therefore written by the same
model that wrote the detector, which is exactly the arrangement the split
exists to avoid, and it should be treated as weaker than the negative-count
findings Codex produced independently.
The boundary of what Codex can contribute here is now mapped. Defensive
research, packaging research, defensive design and acceptance criteria, and
probing the parser during a design task all succeeded. Researching gadget
chains and building a hostile corpus were both refused.
Also fixes a bug in the corpus fixnum encoder, which computed the width marker
as width plus 256 and raised RangeError for any positive multibyte value.
110 tests, 244 assertions across five suites.
Implements the detector Codex specified while it had no implementation to look
at, so neither model defined and graded the same thing.
Parser-level budgets rather than post-parse checks. Codex's architectural point
was that inspecting after parsing is too late because the allocation already
happened, so Limits and Budget enforce byte size, depth, node count, registered
objects, symbol definitions, collection entries, scalar bytes and object links
DURING recursive descent. The size ceiling is checked before the parser is
constructed and non-String input is rejected without ever calling to_s.
Three policies, STRICT_ALLOWLIST as the default, on the reasoning that people
keep defaults far longer than they intend. No enforcing mode accepts an
allowed_sinks option, because permitting a class-and-sink pair still authorizes
a callback during load. OBSERVE_AND_LOG refuses to construct without a
reporter. Allowlisting a class does NOT exempt its sinks, and that is a test.
No method is named safe?, trusted?, sanitized? or safe_load, and a test asserts
their absence. Those names claim a guarantee this cannot make.
The gate demonstrates the documented bypass rather than asserting it. Under
DENY_SINKS_ONLY the detector ACCEPTS the CVE-2026-41316 payload, and the gate
then loads that accepted snapshot on vulnerable erb and confirms the canary
fires. Our own detector, in a shipped mode, admits a payload that achieves code
execution. That is the limitation notice being true rather than decorative, and
if it ever stops being demonstrable the gate fails.
The notice ships verbatim and names the bypass concretely: a payload carrying
no sink tag can still reach dangerous code, the published chain produces zero
sink tags because ERB defines no marshal_load, and an application that
allowlists ERB will accept it.
106 tests, 235 assertions across four suites. Five gates: check, matrix,
exploit, detector, target.
Codex, working the defensive half without having written the parser, probed M1
with adversarial input and found defects my own tests missed. Verified
independently before fixing, and two more were found while confirming:
negative_array_count ACCEPTED as an empty array
negative_hash_count ACCEPTED as an empty hash
negative_bignum_words NoMethodError leaked outside StreamError
negative_ivar_count ACCEPTED
negative_string_len cursor moved BACKWARD, wrong error raised
The last is the worst. take(-5) does not trip the count > remaining guard, so
byteslice returns nil and @position decreases. A parser whose cursor can rewind
on attacker input is a loop primitive, not merely a wrong error.
The M1 gate claimed bignum length confusion was covered. It was not. Oversized
widths were tested and negative counts never were, because the same author
chose both the implementation and the cases it would face. That is the
negative-control failure one level up, and it is exactly what an author cannot
catch alone.
Fixes: every count and length now flows through read_count with a role label
and a nonnegative check, take rejects negative byte counts outright, and
MalformedCountError joins the StreamError hierarchy so nothing leaks a raw
NoMethodError. Negative link indices were already guarded; regression tests now
pin that.
Also closes a detection blind spot Codex identified. read_ivar and read_object
discarded the parsed name nodes after taking their values, so a sink tag placed
in an instance-variable-name position vanished from Result#sinks. Node now
carries an auxiliary collection that Node#each traverses, and name and class
nodes are retained. Proven: a stream with a userdef tag in the name position
now reports Evil#_load where it previously reported nothing.
46 parser tests, 80 assertions. All controls pass, exploit gate still passes.
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.
Explanatory prose in puts statements is commentary living in code. Removed
from control_check.rb and render_matrix.rb, leaving facts and verdicts.
Controls now report as a uniform PASS/FAIL table with a single exit status,
which also makes them usable as a gate rather than something a human reads.
Records the resolution on the two third-party findings: both dropped, no
disclosure, no further investigation. Neither was independently verified, no
obligation attaches to unpublished observations, and the lab has a stronger
flagship in CVE-2026-41316. Kept as unverified leads in gitignored docs.
Walks the live class graph and classifies auto-invoked methods along the
gated/ungated dispatch axis. Marshal calls respond_to? before invoking
marshal_load and _load, while hash, eql?, <=>, []= and to_s are dispatched
blind, so a class can be dead as a Marshal entry point and live as a #hash
entry point.
Rediscovers Gem::Requirement#marshal_load, Gem::Version#marshal_load and
Gem::Specification._load with source locations, from reflection alone, with no
class names hardcoded anywhere in the scanner.
A new control asserts the M1 parser and the M3 scanner agree. The parser reads
bytes off a payload, the scanner walks the class graph, neither consults the
other, and every sink the parser finds in a real Gem::Requirement payload is
one the scanner independently located. Two routes to the same fact.
Raw output was unusable at 163 ungated candidates dominated by to_s and hash,
which nearly every class defines. The reachability predicate from the research
narrows that to methods that are zero-arity AND touch instance state, cutting
163 to 12 and keeping 7.4 percent. Gated sinks are always reachable.
Two corrections found while building the predicate:
RubyVM::AbstractSyntaxTree.of does not work on Ruby 4.0. It raises
'cannot get AST for ISEQ compiled by prism' because prism is now the default
parser. Any tool reaching for that API on modern Ruby is broken. Replaced with
Prism, which ships as a default gem.
The literal reads-its-own-ivars predicate is too narrow. Gem::Requirement#hash
calls the requirements attr_reader rather than @requirements, so an instance
variable check reports false on a method that plainly operates on instance
state. Widened to instance variable reads OR implicit-self calls.
55 tests, 112 assertions, 0 failures across both suites.
Probes six pinned Ruby images with no network and renders a compatibility
matrix for the deserialization gadget surface. Reproducible with just matrix.
The matrix carries three controls, because a table that reports one value
everywhere cannot be distinguished from a probe that always returns the same
answer. Two axes must show more than one state, and the ERB guard column is
cross-checked against the published CVE-2026-41316 affected ranges. That third
control is the load-bearing one: the probe reads source and knows nothing about
NVD, and it agrees with the advisory on 6 of 6 images.
Findings recorded in the research docs:
The gadget surface moved rather than shrank. Net::WriteAdapter is reachable at
baseline on Ruby 3.1 and 3.2 and gone from 3.3 onward, which is why vakzz-era
chains needed no preloaded net/http on old Ruby. Gem::URI appears in the same
release that took it away, and Gem::URI reached through an autoloaded
Gem::SpecFetcher is exactly the bootstrap the 2024 chain relies on. One door
closed and another opened in the same version, so a defense reasoning about
the known gadget classes is reasoning about a moving target.
The ERB @_init guard sits only on def_method. def_module and def_class delegate
to it in both vulnerable and patched releases, so one check covers all three.
The first probe measured all three independently and reported the delegates as
unguarded even on patched erb 6.0.1.1. That was the probe being wrong, not the
patch being incomplete, and it is corrected here.
Marshal stream format is 4.8 on every image, so the M1 parser applies across
the whole range unchanged. make and git are absent from every slim image, so
rake is the only exec binary present on that family.
Scaffolds the Ruby deserialization security lab and lands its defensive core
first: a parser that extracts structure, referenced class names, and gadget
sinks from a Marshal stream without ever calling Marshal.load.
Sinks are classified along the gated/ungated dispatch axis. Marshal checks
respond_to? before invoking marshal_load and _load, while hash, eql?, <=> and
[]= are dispatched blind, so the same class can be dead as a Marshal entry
point and live as a #hash entry point.
Object links are ZERO-indexed. Ruby's Marshal format documentation says
one-indexed and is wrong: a self-referential array dumps as 04 08 5b 06 40 00
with the trailing 00 linking to the outermost object. Written against observed
bytes rather than the docs.
Validation rejects truncated streams, unsupported version bytes, unknown type
tags, out-of-bounds object links and symlinks, oversized fixnum widths,
trailing bytes, and nesting past a configurable depth limit.
A negative-control script accompanies the suite and caught a test that was
passing vacuously: the TracePoint oracle watched :c_call, but Marshal.load is
a Ruby-level method in Ruby 4.0 (<internal:marshal>:33) and fires :call, so
the test could never have failed. The suite now asserts the oracle observes a
real Marshal.load before the negative assertion is allowed to mean anything.
Gem manifest is an explicit allowlist rather than git ls-files, so the
deliberately vulnerable target cannot be swept into a published gem later.
34 tests, 62 assertions, 0 failures. 52/52 corpus round-trip. gem build
--strict clean. All execution in ruby:4.0-slim with --network none.
CarterPerez-dev