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.
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.
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.
CarterPerez-dev