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.
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