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obligation attaches to unpublished observations, and the lab has a stronger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