open-standards/wai/SECURITY.md

Security

This file says where WAI's reference implementation reads untrusted input, what it bounds, and what it does not yet do. The rules a sink must follow are in SPEC.md and extensions/; this file points at them and adds nothing normative.

Reporting a vulnerability

Email [email protected]. Do not open a public issue or pull request for a vulnerability that is not yet fixed.

Say which component is affected (a crate and file, a verifier, or a section of the specification), the commit you tested, the input or steps that trigger it, what happened and what you expected. An input file is the most useful thing to send.

In scope:

• the specification and its extensions, where a rule lets a conforming sink or verifier be made to misbehave;
• the reference implementation: wai-rs, wai-quantum, wai-web, wai-pack, wai-relay, wai-verify and the other crates under this directory;
• the conformance verifiers (*-conformance/*.py) and their corpora.

Out of scope: services anyone deploys with this code, which their operators answer for.

WAI is pre-1.0. Fixes go to main of the canonical repository, https://git.transaction.science/Transaction-Science/open-standards; no earlier release line is maintained.

Manifest parsing

Every envelope a sink receives is untrusted. Its container and manifest are read by wai-rs/src/container.rs (Wai::from_bytes, WaiMulti::from_bytes, manifest_bytes) and, over HTTP, by wai-rs/src/http_binding/head.rs (parse_head).

What the readers bound:

what	bound	where it is specified
every length field (man_len, payload_len, each rendition's offset and length)	checked against the bytes in hand before anything is sliced; sums formed so they cannot wrap (saturating_add; the WAI2 payload total in u64)	SPEC §2, §7.1
manifest length	at most 134 217 728 bytes (2²⁷), sized to the largest manifest a conforming staged layer has. A longer one is refused from man_len alone, before it is read or, over HTTP, requested (manifest_too_large; head_malformed from parse_head)	SPEC §2, Reader limits
manifest values	at most 4 194 304 JSON values (2²²), the manifest object and every value in it at any depth; a member's name is not one. One more is refused while the values are counted, before anything is built from the manifest (manifest_too_many_values; head_malformed from parse_head)	SPEC §2, Reader limits
manifest nesting	at most 127 nested arrays and objects, the manifest object counting as one; 128 are refused	SPEC §2, Reader limits
member names	a member named twice in one object, at any depth, is refused	SPEC §2
WAI2 rendition table	at most 65 535 rows (u16), as many as the manifest's renditions, packed contiguously from offset 0	SPEC §7.1
an HTTP object	exactly one envelope; any other length is refused (object_length_mismatch)	http-delivery §5.1
a head proof	at most 64 chaining values; covered_len is the object's length, or shorter and ending on a chunk boundary of the hashed input	http-delivery §6

A reader copies a payload only after its bounds are checked, so a payload never makes it allocate more than it was given. A WAI1 reader ignores bytes after the payload; an HTTP object that carries any is refused (http-delivery §5.1).

What parsing a manifest takes. A parsed value takes more memory than its text, and many small values can take many times their length, so the length limit bounds what a reader must read and not what a parse costs. The value limit bounds the rest. The reference readers count a manifest's values before they build anything from it (check_manifest_text) and refuse it as soon as the count passes the limit; they then build each array and object at the size counted, and move the manifest's members out of that one tree (read_manifest, read_manifest_v2). On a 64-bit target a read allocates at most 128 bytes for each value and one for each byte of the manifest, and for a WAI2 one rendition entry of 768 bytes for each it lists: at most 640 MiB for a WAI1 manifest at both limits, whatever its shape, and 48 MiB more for a WAI2's entries.

tests/manifest_parse_memory.rs measures this with an allocator that counts every byte asked for: a manifest of small values at the length limit is refused for its values before anything is built; one at the length limit within the value limit is read; a manifest of each shape at the value limit is read and one value past it refused; and so are the largest manifest a conforming staged layer has and a WAI2 listing as many renditions as its table holds. Each stays within the bound above. tests/manifest_reader_equivalence.rs holds these readers to the result of the derived reader they replace, for every envelope committed under wai/ and for each with every member removed or given a value of each JSON type.

The library tests in container.rs and http_binding/tests.rs check the manifest limits at their edges: from its first eight bytes, every reader refuses a manifest one byte past the length limit as too large and one at the limit only as truncated; a 2 MiB manifest is read and written back byte for byte; and 127 levels are read where 128 are refused. The writers (Wai::to_bytes, WaiMulti::to_bytes, and pack and packMulti in wai-web) refuse a manifest past the length limit, and a payload whose length does not fit its u32 field. The Rust writers stop serialising as soon as the manifest passes the limit; the test runs both writers at limits equal to an envelope's manifest and payload lengths and one byte below them, since serialising 128 MiB takes a debug build many seconds. verify_pack.mjs checks the browser writers at the real limit.

The stdlib Python verifiers read envelopes too. staged_verify.py, claim_verify.py, pack_verify.py and http_verify.py refuse a manifest past the length limit, or past the value limit, as the reference does. http_verify.py and wai_http_vectors agree on parse_head at each limit in the HTTP corpus; the other three each build an envelope in memory with a manifest one byte past the length limit, and one value past the value limit, and require each refused. Those that read manifests through staged_verify.strict_json refuse nesting past 127; pack_verify.py reads up to the interpreter's recursion limit and refuses past it as unparseable.

Payload formats that WAI defines carry their own caps, checked before the first symbol is decoded: a tensor of at most 2²⁶ elements, a WIV1 clip of at most 2³⁰ decoded bytes and 2²⁰ frames, and a sink cost limit (int_payload::check_cost, cost_over_limit) (integer-payloads §7, §8). wai.neural.int_mlicpp refuses a header whose shapes, or whose buffers, pass its cap before it allocates them (tests/int_mlicpp_preflight.rs).

wai.text.zstd and wai.text.xz carry streams whose formats bound neither what a payload decodes to nor the history its decoder must keep, so the specification sets both (SPEC §2): at most 2²⁷ decoded bytes, and at most 2²⁷ bytes of window or dictionary. The reference decoders (codecs/text.rs) write their output into a buffer that is never sized past the length limit and refuse the payload when its output would pass it (text_decoded_too_long); a Zstandard frame that declares a longer content size is refused before anything is decoded. They give the decoding library the window limit, so a stream that declares more is refused from its header, before the library allocates it (text_window_too_large). The encoders refuse input past the length limit. tests/text_decode_bounds.rs checks each limit at its edge, on streams made in the test that are at it and one step past it; tests/text_decode_memory.rs measures, with a counting allocator, that refusing a payload of a few kilobytes that would expand to four times a limit takes no more output buffer than the limit. The C entry points wai_text_zstd_decode and wai_text_xz_decode are these decoders.

Native codec libraries and sandboxed decode

The default codecs feature binds C and C++ libraries for the floor and the recommended set: libzstd and liblzma for wai.text.zstd and wai.text.xz, libjxl for JPEG XL, libavif for AVIF decode, libopus, dav1d for AV1 decode, and libFLAC for FLAC encode. PNG, JPEG and FLAC decode run in Rust (the image and claxon crates). The neural feature binds ONNX Runtime for the float neural decoders. These libraries parse payload bytes directly, and they are the usual place a memory-safety defect in a media stack lives.

They run in the caller's process, with its privileges: the reference implementation has no process or system-call sandbox. A sink that decodes payloads from origins it does not trust through these libraries should run the decode in a separate process with no file-system or network access, or use a decoder compiled to WebAssembly and run it in a WebAssembly runtime.

What does not touch them:

• Most CI builds. One job builds codecs: codec-wrappers in wai-conformance.yml, on the macOS runner, which runs the wrappers' fuzz smoke, the text decode limits and the C entry points through the libraries. wai-neural-gates.yml and wai-neural-e2e.yml build the neural feature, which links ONNX Runtime and libzstd, and run the float neural decoders through them. No workflow enables semantic, and no other workflow's build links any of these libraries.
• WAI's own decoders are Rust: the integer codecs, worlds, scores, films and the generator. Outside the C ABI (ffi.rs, ffi_codecs.rs, ffi_neural.rs), the only unsafe in wai-rs's library code is in the AArch64 SIMD kernels of int_kernels.rs, each block with the bounds argument beside it.
• wai-web, the browser crate, links none of the C libraries and runs inside the browser's WebAssembly sandbox.

Model checking covers part of the integer core: Kani harnesses beside the code in rans.rs, int_entropy.rs, int_transform.rs, wai_video_policy.rs, staged/predict.rs and staged/wir1_arith.rs, hosted by wai-verify and run by .github/workflows/wai-formal-verification.yml. wai-verify also compiles int_kernels.rs, on its scalar path, because int_transform.rs calls it, but no harness reaches it: its AArch64 SIMD kernels, the only unsafe named above, are not model-checked. Equality tests hold them to the scalar reference kernels, element by element beside each kernel and over every committed integer-decoder vector (tests/int_kernels_vectors.rs), on every platform leg of wai-conformance.yml.

Known gaps:

• The image, audio and video decoders set no output limit of their own. They allocate what the bitstream declares, within the libraries' own limits.
• Fuzzing is shallow. tests/fuzz_smoke.rs feeds seeded random bytes to each codec wrapper and to Wai::from_bytes and requires that nothing panics. It runs in CI on one platform (codec-wrappers in wai-conformance.yml). There is no coverage-guided fuzzing of the manifest parser, the integer payload parsers or the codec wrappers.

Module authorisation

A module is code a sink obtains apart from an envelope and runs to decode a capability or to rebuild a reconstruction (SPEC §4, Module authorisation). A sink runs one only if it holds it under jwp-receipts §7.9: a grant, signed under a module origin's key log, naming the SHA-256 of the module's bytes exactly as the sink instantiates them, its form, the capabilities it may decode and a validity window, with revocation through the same key log. A module the sink does not hold leaves its capability unadvertised for that envelope, and the envelope's fallback still runs. Parameter sets pinned by digest are checked before use and revoked the same way (SPEC §3.1, checks 5–8).

The reference dispatcher runs no module obtained apart from an envelope. With a key log (key_log::SinkKeyLog, feature key_discovery) it refuses a revoked parameter set (prior_revoked).

Authorisation decides which code runs, not what that code can reach. The environment a module runs in (its imports, a memory limit, a time limit, and the determinism rules an exact replay needs) is not specified yet. Until it is, a sink that runs modules confines them itself, for example in a WebAssembly runtime that gives the module no file-system, network, clock or random-number imports.

Fingerprinting: advertised parameter sets

The Sink Capability Descriptor (http-delivery §12.1) can list held_sets: for each capability, the digests of the parameter sets the sink holds. Which sets a sink holds depends on what it has fetched before, so the list can identify a sink across the sites it is sent to, much as a cookie does. A shared compression dictionary named by its hash raises the same concern (RFC 9842).

Privacy rules for the descriptor and for parameter-set fetches are not written yet. They are to be added to http-delivery: the list kept per top-level site, clearable by the user, disclosed only as far as a choice of rendition needs it, never sent across sites, and off by default. Until they land, send held_sets only to the origin that needs it to choose a rendition, and never across sites. No code in this tree builds or sends the descriptor.

Protected content

End-to-end encryption at the application layer (mls-sframe, SPEC §6) keeps content from relays and from anyone outside the group, not from a member's device: the decrypted payload and the decoded output are in the application's memory. It is not a protected-path robustness level (SPEC §6.5).