poc-netcdf-external-file-read

PoC — arbitrary local file read when loading a NetCDF (.nc) file

A .nc file can make netCDF4/xarray read an arbitrary file off the machine that opens it. NetCDF-4 is HDF5 underneath, and HDF5 lets a variable's raw data live in an external file (H5Pset_external). libnetcdf follows that reference silently, so when you open the file and read the variable, you get back the bytes of whatever file the author pointed at — including absolute paths well outside the working directory.

Nothing here is malicious to run; the "secret" is a marker string I put next to the file.

Files

• poc_secret.nc — variable leak is backed by the relative path secret.txt (shipped here). Works on any OS.
• secret.txt — the stand-in secret so PoC 1 is self-contained.
• poc_passwd.nc — variable leak is backed by the absolute path /etc/passwd. On a Linux box this reads the real passwd file.
• verify.py — opens a .nc and prints the leak variable's bytes.
• build_poc_hf.py — how the two .nc files were built (with h5py).

Reproduce

pip install netCDF4          # or: pip install xarray netCDF4
python verify.py poc_secret.nc     # prints the contents of secret.txt
python verify.py poc_passwd.nc     # on Linux, prints /etc/passwd

Or with xarray:

import xarray as xr
print(bytes(xr.open_dataset("poc_passwd.nc")["leak"].values.tobytes()))

Why it matters

Anywhere a service ingests a user-supplied .nc (model weights, scientific datasets, data-conversion pipelines) and later reads the variable — or re-saves it — the targeted file's bytes are pulled in and can end up in the output. It's a pure default-path read; there's no safe-mode flag in netCDF4/xarray that turns external references off.