docs(security): document the ByteStorage decompression bound (LAB-2504) - #75
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The threat-model bullet claimed "size limits + ratio validation" with no numbers and no statement of reach, which left three things a reader has to rediscover from source: what the limits actually are, that original_size is attacker-controlled and deliberately not trusted, and that xxHash3-64 is unkeyed and therefore not a control against forgery at all. Also states the ceiling's blast radius honestly. 512 MiB assumes a host that can absorb a 512 MiB allocation; a Workers isolate has ~128 MiB, so a payload well inside these limits can still OOM it. Sizing is LAB-2505's call, so this records the constraint rather than changing it. Records that compression_bomb runs in the quick-fuzz matrix on every push and PR, not as an ad hoc script — the guard is only worth citing if a reader can tell it gates merges.
…-2504) Expert-panel findings on the previous commit. Three claims were wrong or overstated and a security doc that overstates its own guarantees is worse than one that says nothing. CI coverage: 'runs on every push and pull request' was false for pushes -- security.yml is on: push: branches: [main], so feature-branch pushes never run quick-fuzz. Worse, fuzz/.gitignore excludes corpus/*/, so on a fresh CI checkout the corpus is empty and 'cargo fuzz run -runs=0' generates nothing: at PR time the job proves the target BUILDS and exercises no input. The real input coverage is the weekly deep-fuzz run plus the unit tests and Kani proofs, so those are what the section now points at. The 120 s figure is gone; it was a timeout, not a measure of coverage. Fuzz-target contract: the target also accepts DecompressionFailed, and its compressed_size is a u16 -- so it caps compressed input at 64 KiB and cannot reach the 512 MiB boundary the old text cited. It exercises the ratio bound, not the absolute one. 'original_size is not trusted' was too strong and papered over the interesting part. It is not trusted as a BOUND, but it does size the allocation within that bound, so a forged envelope can still make a reader allocate up to 1000x its wire size before the LZ4 stream is validated -- an eager memory.grow on wasm32 needing no valid stream behind it. That is the LAB-2505 sizing question, now named as such instead of hidden behind a reassuring phrase.
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Estimated code review effort: 1 (Trivial) | ~2 minutes Merge Risk: 🔵 Low · up to The change documents decompression safeguards without changing runtime behavior, but two descriptions of verification coverage remain inaccurate. The PR is mergeable with explicit owner awareness and follow-up to correct those bounded security-documentation issues. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@SECURITY.md`:
- Around line 74-75: Update the SECURITY.md description of zero-length
compressed data to state that StorageEnvelope::extract rejects every input with
compressed_data.len() == 0, including when original_size == 0, while preserving
the existing DecompressionBomb behavior.
- Around line 103-104: Update the SECURITY.md fuzz-coverage statement to say
compression_bomb.rs does not cover the 512 MiB compressed-size boundary, while
noting that its u16 compressed_size exercises the ratio bound and its u32
original_size can test values above the 512 MiB uncompressed limit.
- Around line 112-113: Update the Kani coverage statement in SECURITY.md to
describe Kani proofs as bounded formal checks of selected arithmetic predicates,
not execution or input coverage for StorageEnvelope::extract; reserve “input
coverage” claims for unit-test and fuzz executions.
- Around line 109-110: Update the SECURITY.md description of cargo fuzz run
-runs=0 to state that the empty-corpus path executes the initialization callback
and newline seed before the run-limit check, while no mutation-based fuzzing
occurs; avoid claiming that no inputs execute or implying a fixed libFuzzer
version, since the workflow uses the rolling nightly toolchain.
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…e decompression-limits section All four CodeRabbit findings verified against source and applied; an expert-panel pass on the result found five more of the same class, also applied. Corrected here: - zero-length compressed data is rejected unconditionally, not only when original_size is non-zero (byte_storage.rs:122 has no original_size term) - the compression_bomb target reaches MAX_UNCOMPRESSED_SIZE's rejection branch (u32 original_size) but never MAX_COMPRESSED_SIZE (u16 compressed_size) - its 512 MiB output assertion is vacuous for the same reason, and its error-variant assertions are guarded, not universal - -runs=0 executes libFuzzer's newline seed before the run-limit check; it does no mutation rather than no execution - the Kani harnesses are tautologies (assert_eq!(P, P)) and run only on schedule/dispatch, so they are neither verification of the bound nor a merge-time gate Also stated: deep-fuzz does not persist its corpus, validate() reaches the full extract allocation, the pre-deserialization bound lives in ByteStorage rather than on the public StorageEnvelope, the OutputTooSmall bound depends on lz4_flex's default safe-decode, and wasm32 linear memory never shrinks. CodeRabbit-Resolved: SECURITY.md:75:Document the unconditional zero-length rej CodeRabbit-Resolved: SECURITY.md:104:Correct the fuzz coverage boundary claim CodeRabbit-Resolved: SECURITY.md:110:Describe the -runs=0 coverage accurately CodeRabbit-Resolved: SECURITY.md:113:Qualify the Kani coverage statement
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Inline comments:
In `@SECURITY.md`:
- Around line 159-164: Update the SECURITY.md discussion of the size/ratio
harnesses to identify only verify_input_size_limits and
verify_compressed_size_limits as containing literal self-comparisons; describe
verify_decompression_bomb_protection as comparing checked and direct
multiplication, and verify_compression_ratio_calculation_safety as deriving
is_bomb and checking its branches.
- Around line 147-148: Update the quick-fuzz job description in SECURITY.md to
characterize it as single-seed smoke coverage: state that it can catch failures
on the initial seed but does not perform mutation-based fuzzing or generate
boundary coverage, and identify unit tests as providing merge-time coverage for
boundary cases.
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…riptions Both from CodeRabbit's re-review of a5f051a, both correct. quick-fuzz: "proves little beyond building" undersold it. The seed does execute, so the job is single-seed smoke coverage — it catches a build break or a panic on that one input. Says so, and points at the unit tests as the merge-time boundary coverage. Kani: my previous text said three of four harnesses assign the same predicate to two bindings. Only two do that literally. The other two are equally vacuous by different routes — checked_mul compared against the same product under an assume that makes overflow impossible, and a branch that restates its own definition. All four are now described by their actual pattern rather than lumped under one. CodeRabbit-Resolved: SECURITY.md:148:Describe the pull-request fuzz job as sin CodeRabbit-Resolved: SECURITY.md:164:Correct the Kani harness count and patter
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…ne (LAB-2504) (#275) Docs-only. Part of the LAB-2504 cross-SDK decompression-bomb audit. **Depends on [cachekit-core#75](cachekit-io/cachekit-core#75 — the new `[core-decompress]` link targets an anchor that PR adds, so merge core first. ## Two findings, opposite verdicts **The default read path is sound.** `ByteStorage.retrieve` in `rust/src/python_bindings.rs` is a pass-through to cachekit-core's `extract()`, which caps output at `min(512 MiB, 1000 × compressed_len)` before decompressing. Worth stating because the MessagePack size caps sit on already-decompressed bytes — a reader can easily assume those caps are the protection when they are in fact downstream of it. **`ArrowSerializer` is not.** `deserialize()` hands the body to `pa.ipc.open_file(...).read_all()`, which decompresses zstd with no size or ratio limit and never touches `extract()`. Reproduced end-to-end through the real serializer: ``` stored envelope : 2,570 bytes -> 67,108,864 bytes (26,112:1) stored envelope : 8,714 bytes -> 268,435,456 bytes (30,805:1) ``` cachekit-core rejects at 1000:1. Neither existing control covers it: the `[8-byte xxh3][Arrow IPC]` prefix is unkeyed, and `max_value_size` is enforced on the **write** path only (`cache_handler.py`), making it a producer-side quota rather than a check on bytes coming back off the wire. Documented rather than patched — the fix is a wire-size/L1-footprint decision, filed as LAB-2730 with the PoC and the ruled-out approaches. Mitigations that work today are named in the doc. ## What this PR corrects about its own first draft The expert panel caught three errors in the initial commit: - **The listed mitigation "use `compression=None`" was false and actively harmful.** `deserialize()` never reads `self.compression`; the reader decompresses according to the *stored stream's* own `BodyCompression` metadata, so an attacker's forged envelope declares zstd regardless of the reader's setting. Anyone who followed that advice would have believed they were protected. Now stated explicitly as a non-mitigation. - **Exposure was wrong in both directions.** Narrower: `@cache.io` is the CachekitIO *backend* preset and does not select Arrow — that needs an explicit `serializer="arrow"` plus the `[data]` extra. Wider: `deserialize()` also accepts raw `ARROW1` bodies with **no checksum at all** via the legacy integrity-off branch, so an attacker need not even recompute the unkeyed prefix. - **"A sound bound requires a Flatbuffers walk" was wrong.** Uncompressed Arrow IPC allocates in proportion to its own length (measured ratio 1.000), so refusing bodies that declare `BodyCompression` makes `len(body)` a real pre-decompression bound in three lines. It costs the compression feature, which is why it is still an owner call — but the doc now says a bound *exists* and what it costs, rather than implying none does. The Flatbuffers walk is only needed to *keep* compression. ## Scope No code. The Arrow fix is LAB-2730; core's constants are LAB-2505. Closes LAB-2504 for this repo.
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) Docs-only. Part of the LAB-2504 cross-SDK decompression-bomb audit. **Depends on [cachekit-core#75](cachekit-io/cachekit-core#75 — the inline link targets an anchor that PR adds, so merge core first. ## Why `SECURITY.md` covered reporting and scope but said nothing about the read pipeline, so a reader had no way to tell whether this SDK decompresses anything itself. ## Audit outcome for this repo: no bypass, but a misleading ceiling It does not decompress anything itself. Both bindings — `cachekit-core-ts` (NAPI, `src/lib.rs:118`) and `cachekit-core-wasm` (Workers, `src/lib.rs:98`) — are one-line pass-throughs to cachekit-core's `retrieve` → `extract()`, which bounds output at `min(512 MiB, 1000 × compressed_len)` before decompressing. No SDK-side LZ4, no pre-`extract` allocation. The audit did surface a real mismatch, which this PR documents and [LAB-2732](https://github.com/cachekit-io/cachekit-ts) tracks for a code fix: `serializer.maxDecodedSize` defaults to 10 MiB but is only applied inside `serializer.decode`, i.e. **after** `unpack` returns. Core may have materialized up to 512 MiB by then, so the two ceilings differ by ~51×, and `cache-core.ts:574` currently claims the blast radius *is* bounded by `maxDecodedSize`. Turning that knob down to harden a Workers deployment changes nothing about what `unpack` may allocate. ## What this PR corrects about its own first draft The panel found the Workers callout said "bound payload size at the caller" without naming a lever, while the only knob the section mentions is precisely the one that does not work — so a reader would reasonably reach for `maxDecodedSize` and stay exposed. It now names the two levers that exist: check the fetched value's byte length before handing it to the cache, or cap value size at the backend. ## Scope No code. The `maxDecodedSize` mismatch is LAB-2732; core's constants are LAB-2505. Closes LAB-2504 for this repo.
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…rage note (LAB-2504) Drop the checked-decode claim: lz4_flex 0.12.2 declares the feature but never gates on it, and its OutputTooSmall checks run unconditionally. Condense the fuzz/Kani section to what a security reader needs, state the real retrieve() peak memory, and fix validate()'s doc comment, which claimed it does not extract.
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Docs, plus one corrected doc comment. Part of the LAB-2504 cross-SDK decompression-bomb audit. Merge this before the py and ts PRs — both link the
#decompression-limitsanchor this adds.Why
SECURITY.mdclaimed "Decompression bombs: Size limits + ratio validation" with no numbers, which left a reader to rediscover from source what the limits are, thatoriginal_sizeis attacker-controlled and not used as a bound, and that xxHash3-64 is unkeyed and therefore not a control against forgery.Audit outcome for this repo: the bound is sound
StorageEnvelope::extractbounds decompression before callinglz4_flex::decompress, and the published 0.5.0 that all three SDKs pin is byte-identical tomainon this file. Verified during the audit:u64 checked_mulratio → decompress → checksum → post-decompress length re-check.u64, so there is nousizewrap onwasm32.lz4_flexreturnsOutputTooSmallrather than writing past the allocation, and the post-decompress length re-check catches a short decode even when an attacker recomputes the unkeyed checksum. A decompressor's size argument sizes a buffer; it never asserts the decoded length.rmp_serde::from_slicedoes not pre-allocate from a declaredbin32length, so there is no envelope-deserialization bomb upstream ofextract.What the section documents
original_sizesizes the allocation within the bound, so a forged envelope can make a reader allocate up to 1000× its compressed size before the LZ4 stream is validated.validate()reaches the same allocation.retrieve/validate, not on the publicStorageEnvelope.retrievecan peak around 1.5 GiB at the limits, and onwasm32linear memory never shrinks. Constrained runtimes must bound payload size at the caller.src/byte_storage.rs; the fuzz and Kani jobs are smoke checks.Scope
No behaviour or constants changed. The only code edit is
ByteStorage::validate()'s doc comment, which said it validates "without extracting data"; it now says it runsextractand allocates up to the bound.Closes LAB-2504 for this repo.
Summary
Documentation-only change that condenses the
ByteStoragedecompression-bound section ofSECURITY.mdand corrects the misleading doc comment onByteStorage::validate(). No runtime behavior is changed.Changes
src/byte_storage.rs— public API documentationByteStorage::validate(&self, envelope_bytes: &[u8]) -> bool: The doc comment previously read "Validate envelope without extracting data", which was inaccurate. It now states that the method fully runsStorageEnvelope::extract, decompresses the payload, allocates up to the decompression bound, and discards the result. It also states that the method is not a cheap structural pre-screen for untrusted envelopes. The signature and behavior are unchanged.SECURITY.md— decompression bound sectionCondensed:
u64viachecked_mul, overflow is treated as a bomb, and zero-length compressed data is rejected unconditionally, including whenoriginal_size == 0.min(512 MiB, 1000 × compressed_data.len())becauselz4_flexreturnsOutputTooSmall.extractre-checks the produced length afterwards.lz4_flex's defaultsafe-decodefeature must stay enabled.original_sizenote no longer references the LAB-2505 ticket. It describes allocation amplification as occurring within the bound, not as a bypass of it.Added:
ByteStorage::retrieveholds three buffers at once: the serialized envelope, the deserializedcompressed_datacopy, and the decompressed output. Peak memory per call can therefore reach roughly 1.5 GiB at the limits, not 512 MiB.Removed / summarized:
compression_bombfuzz target assertions, CIquick-fuzz/deep-fuzz corpus behavior, and Kani harness tautologies is replaced by a short "Test coverage" paragraph. It states that:src/byte_storage.rsare the only merge-time enforcement of the bound.StorageEnvelope::extract, and cannot detect a wrong predicate.