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.
Sinatra on webrick in a container, storing session state as a base64 Marshal
blob in a cookie. Three endpoints: /render deserializes and compiles the
session template, /render/safe inspects the stream first, /canary reports
execution. Runs read-only, unprivileged, with a 1MB noexec tmpfs, on a high
configurable host port.
Gate proves three things and the third is what stops the defense being a brick:
PASS HTTP request achieved code execution through Marshal.load
PASS defended endpoint rejected the identical payload
PASS defended endpoint still serves a legitimate session
Two findings that change the defensive design.
Sink tags do not catch this chain. The working payload produces ZERO sink-tag
hits. ERB defines no marshal_load, so it serializes as a plain object with
instance variables and carries no u, U or d tag. The defended endpoint rejected
it on the class allowlist, and an application that allowlisted ERB as a
legitimate template class would have passed it through untouched. A gadget does
not need a marshal_load hook, it needs an object whose ivars the application
later feeds to a dangerous method. The dangerous call site lives in the
application, not in the serialized class. Any policy treating absence of sink
tags as safe is defeated by this exact public payload.
A legitimately initialized ERB cannot be serialized at all. @_init holds
self.class.singleton_class and Marshal raises TypeError: singleton class can't
be dumped. So the guard is not a flag an attacker might satisfy, it is anchored
to a value the serializer physically cannot reproduce. Any ERB an attacker can
serialize necessarily lacks a valid @_init. The generalized pattern for learn/:
do not validate the untrusted object, anchor trust to something unreachable
through the channel.
Also fixes a gate that skipped a control silently. The benign-session check
produced no output because POST with no body returns WEBrick LengthRequired,
and the script treated an empty result as nothing to test rather than as a
failure. It now fails loudly.
70 tests, 151 assertions, 0 failures. Target app excluded from the gem
manifest, verified at 0 files.
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.
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