rube has been on rubygems.org since 2009-08-05: Richard LeBer, 12,305 downloads,
and it is an ERB front-end, which is funny given the flagship CVE here is an ERB
gadget. The name was never publishable, so publishing required a rename first.
marshalsea. The Marshalsea was a London debtors' prison, 1373 to 1842, and the
name is the job description: hold untrusted objects at the gate and decide what
gets through before Marshal.load turns bytes into behaviour. It also carries
"Marshal", so the gem reads as on-topic without a subtitle.
module Rube is module Marshalsea, lib/rube/ is lib/marshalsea/, require "rube" is
require "marshalsea", RUBE_TARGET_PORT is MARSHALSEA_TARGET_PORT, and the canary
moved to /tmp/marshalsea-canary. 31 files, roughly 163 occurrences, every one a
hand edit. The single deliberate survivor is the README's "a Rube Goldberg
machine", which describes the gadget chain and not the gem.
Publishing is trusted publishing over OIDC, so no long-lived API key exists in
this repository to leak. A marshalsea-v* tag runs the five suites and the
standalone controls on Ruby 3.4 and 4.0, refuses to continue if the tag disagrees
with Marshalsea::VERSION or if the gemspec floor stops matching the tested
matrix, and then publishes with a Sigstore attestation. The attestation is
recorded as an auditable record and explicitly NOT as an install-time protection,
because neither gem install nor bundle install verifies one today.
Two things the primary source settled that the docs did not. rubygems/release-gem
does accept working-directory, which neither its README nor the RubyGems guide
mentions, so a monorepo subdirectory works. And it runs bundle exec rake release,
which this Rakefile had no task for at all.
Adding bundler/gem_tasks exposed a monorepo trap: Bundler::GemHelper tags a bare
v0.1.0, which says nothing about which of sixty projects it belongs to. Fixed
with tag_prefix. The catch is that rake -T still PRINTS "Create tag v0.1.0",
because that description is built when gem_tasks is required and the prefix is
assigned after. The tag actually created is marshalsea-v0.1.0. The label is
wrong and the behaviour is right, so the gate asserts the runtime value and
carries a control proving a Rakefile without the prefix line really does produce
the bare tag.
Full gate: 58 PASS, 0 FAIL across six stages, package now 25 of 25. 194 tests.
Lint 0 across 30 files.
B17 through B30, fourteen items. Every one reproduced before it was touched
and mutation-proven after. 194 tests from 119, rubocop 903 offenses to 0,
all six gate stages green.
One rule runs through all of it: nothing may present a guess, a default, or
a swallowed error as a verdict.
Decision states (B19). Under POLICY_OBSERVE_AND_LOG a payload carrying a live
Gem::Requirement#marshal_load snapshot reported accepted? true AND rejected?
false, so both obvious caller shapes loaded it. The predicate pair could not
express the third outcome, so there was no safe branch to pick - the prior
note claiming target/app.rb sidestepped this by branching on rejected? was
wrong, and both forms were byte-for-byte equivalent in outcome. accepted?,
rejected? and would_reject? are removed rather than redefined, so copying
`if d.accepted?` now raises NoMethodError instead of silently changing
meaning. One state validated in the constructor, three exclusive predicates,
and proceed? is the only one that may gate a Marshal.load. Observe-and-log
stays non-blocking and still hands back its snapshot; the monitoring caller
writes `proceed? || observed?` and names the state out loud.
Scanner error accounting (B18). Five rescues returned nil, [] or false and
told nobody. They now record site, subject and error class, and Report
exposes suppressed_count, suppressions_by_site, complete? and
candidates_lost?. Wiring the counter immediately surfaced 3 suppressions on
a stock image that had always been invisible: <internal:symbol>,
<internal:pathname_builtin> and <internal:ractor> all fail Prism.parse_file
with ENOENT because Ruby hands out those paths but they are not files.
The fourth state (B29). Those 3 suppressions were also 7 wrong answers -
candidates scored "does not touch state", indistinguishable from analysed
and inert, and 4 zero-arity ungated ones silently dropped from reachable.
touches_state is now four-state. A source that was given and could not be
parsed fails OPEN and stays reachable, because a scanner that discards what
it failed to analyse is the exact failure mode B18 names. A C-defined method
with no Ruby source at all is reported as unanalysable instead: 132 of 173
candidates, and failing open there would take reachable from 25 to 74 of 165
ungated and stop the filter filtering. Report#unanalysable and
#fully_analysed? state the real coverage - 33 of 173 - rather than implying
the filter saw everything. The 5 recovered candidates were verified by
executing them, not by reading source we could not read; one of them,
ERB::Compiler::PercentLine#to_s, is an alias of an attr_reader, which is a
second and distinct analysis gap.
Reason escaping (B20). Reject reasons interpolated raw attacker bytes into a
caller-supplied reporter. A class name carrying CR, LF, ESC and NUL turned
one reporter call into three log lines, the middle one forging a successful
authentication. All three interpolation sites now truncate at 96 bytes and
inspect the binary form, so no byte below 0x20 survives and the value is
quote-delimited.
Target hardening (B21, B22). The defended endpoint returned HTTP 500 with a
source line for three roots the detector had just accepted, leaking paths the
same way the B3 mutant did. show_exceptions is off, the shape is checked, and
the gate now greps every error body for source paths. ALLOWED_CLASSES could
never match anything, measured: a benign session cookie carries zero class
names. It is PERMITTED_CLASS_NAMES = [] now, which is what the app actually
requires and is strictly tighter - the old list admitted a C-wrapped String,
and under a mutant restoring it only the new shape check stopped that payload.
Gated agreement (B23). GATED_SINK_TAGS omitted TAG_DATA while the scanner
listed _load_data. The scanner was right, and this is now execution evidence
rather than a reading of marshal.c: a hand-built d stream naming Thread::Mutex,
a real C-level T_DATA, raises TypeError naming the missing _load_data, while
the same stream naming String dies earlier at "dump format error" - which is
why the previous attempt could not see it. A test compares both definitions
directly so they cannot drift again.
Float fidelity (B17). read_float returned nil for seven body forms
Marshal.load accepts, two more than the finding listed. Ruby uses its own
ruby_strtod, so "INF" is 0.0 while "inf" is Infinity, and String#to_f turns
out to be that same function. Ruby's legacy binary mantissa is NOT decoded:
a model fitted to four oracle points passed a 25-case table and then failed
930 of 5000 randomised cases, and marshal.c is not available in these images.
Since 0 of 209 Marshal.dump outputs contain a NUL, no living Ruby emits that
form, so the parser records the strtod prefix and flags Node#undecoded_tail
instead. A plausible wrong number is worse than a labelled non-answer.
Final differential: 2919 agreed exactly, 2081 flagged, 0 claimed-and-wrong.
Hygiene (B24, B25, B27, B28). PRIMITIVE_CLASS_NAMES and NAMESPACE_SEPARATOR
had one reference each, their own definition. BIGNUM_SIGN_POSITIVE is live
and stays. The width > FIXNUM_MAX_WIDTH guard is unreachable for all 256
possible marker bytes, checked exhaustively, and raised the wrong error class;
it is replaced by a test that derives widths from real Marshal.dump output.
The symlink and object-link bounds checks no longer lean on negative-index
wraparound. The parse graph is sealed before it is returned - every node, its
collections and its scalars frozen - and the whole suite stayed green first
try, which proves nothing downstream was mutating it. exploit-gate.sh pins
both sides now, 4.0.2-slim erb 6.0.1 FIRED against 4.0.6-slim erb 6.0.1.1
BLOCKED; the finding's claim about detector-gate.sh was wrong, it never had
a patched side.
Lint (B30). just lint used a --network none runner, so gem install could
never reach RubyGems, the && short-circuited, and the recipe exited 2 while
printing absolutely nothing. That is the fourth instance of a dropped return
value hiding a failure in this project. It is loud now. The config had never
been validated against a real run: 903 offenses, dominated by a quote style
the codebase does not use. frozen_string_literal was verified safe by running
the whole suite under RUBYOPT=--enable-frozen-string-literal BEFORE the change,
so ~247 offenses were retired by fixing code rather than silencing a cop.
Every remaining disabled cop carries a reason.
Every item contracted to clear before M7 is closed. 151 tests from 119,
58 corpus cases from 48, all six gates green.
Depth accounting (B3, B4). TAG_IVAR charged no depth at all, so an I-chain
of any length parsed under any ceiling. Proven end to end against a rebuilt
target image: a 12,936-byte cookie returned HTTP 500 with a SystemStackError
that no rescue StreamError can catch, and a 724,287-byte response body
leaking absolute container paths for every file in lib/. Fixed, the same
cookie returns 400 DepthLimitError, and so does a 53,340-byte one.
read_userdef also hard-coded a depth of 1 for its class-name slot.
Budget axes (B12, B13, B14). Bignum magnitude bypassed the scalar budget
entirely and the sign byte accepted anything as positive where Marshal.load
raises ArgumentError. Added max_symbol_references, max_symbol_name_bytes,
max_class_name_bytes, max_instance_variables and max_struct_members.
Parser.new now enforces Limits.new instead of resolving to an unbounded
config; Limits.permissive became a class method.
Hash-key dispatch (B11). Nothing rejected an allowlisted class used as a
hash KEY, where #hash and #eql? run during load before any allowlist can
act. Measured against real Marshal.load: a key dispatches iff it carries a
class name and its underlying value is not a T_STRING. So TAG_REGEXP is not
a key-position risk and TAG_USERCLASS only conditionally - rejecting either
outright would have been a false positive. No opt-out allowlist was added,
because the dispatch happens before any check could run.
Fidelity (B15, B16, B9). The parser already matched Marshal.load on header
versions, so the 4.8 contradiction was resolved by giving the detector the
policy check and leaving the forensic parser permissive. Class-name slots
now accept only a symbol, an ivar-wrapped symbol, or a symlink. Wrapper
tags C and e no longer take an object-table slot, which Ruby does not give
them - link index 3 resolved to "bbb" for us and "ccc" for Ruby.
Gate soundness (B6, B7). Three discarded check() return values now register
as failures; section 6 no longer reports a vacuous 0/0; section 7 requires
reachable > 0, and prism absence is a named failure rather than a silent
zero. version-matrix.sh exits non-zero when any image produces no probe
result. control_check.rb no longer pulls in minitest, which was printing
"0 runs, 0 assertions" directly under ALL CONTROLS PASSED. Rewrote the
vacuous tests: the regexp options byte had zero minitest coverage and its
mutant survived the whole suite, and read_count's negative guard was
alibied by take's own guard.
The target app (B5) lost its hand-rolled copy of the sink-plus-allowlist
policy and now runs one BoundaryDetector with real limits, branching on
rejected? rather than accepted?.
Everything here is mutation-proven. Notable misses that mutation caught:
B14 had no test at all until reverting it stayed green, and a struct-member
test was vacuous on the first attempt because struct member names are always
symbols.
lib/rube/marshal/parser.rb carries eight backlog items at once and cannot be
split without interactive hunk staging, so this is one commit rather than
eight.
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