NVIDIA NeMo saves models as .nemo files. It is easy to treat one as an opaque blob, but a .nemo is just a tar archive (uncompressed, or gzip/bz2/xz in older releases) that bundles a model_config.yaml with model_weights.ckpt and sometimes a tokenizer or vocabulary. When you call restore_from, NeMo untars the archive and torch.load-s the weights. So the loader treats the tar as transparent, and a scanner that does not open it misses two separate things.
Risk 1: a pickle one container deep
model_weights.ckpt is a torch.save file: a zip holding data.pkl (torch 1.6 and later) or a bare pickle (the legacy format). Either way it is a pickle, and a pickle can carry a __reduce__ gadget that runs on load. Opcode-only pickle scanners and extension routers work on a .pkl or .pt file. They do not untar the .nemo first, so the gadget inside is never disassembled. .nemo is one of the 22 pickle loading paths catalogued in The Art of Hide and Seek (USENIX Security '26, arXiv:2508.19774). Even my own toolkit's config detectors documented that they "do not unpack .nemo or .ckpt files".
Risk 2: arbitrary file write on extraction (CVE-2024-0129)
Before NeMo r2.0.0rc0, _unpack_nemo_file called tar.extractall() without sanitising member names. A member named ../../something, or an absolute path, lands outside the output folder when the model is restored, which is an arbitrary file write (CWE-22). HiddenLayer scored it 6.3 and NVD 7.8, and NVD notes it may lead to code execution. If you still run an older NeMo, an untrusted .nemo can overwrite files just by being restored.
Opening the tar the way the loader does
I added nemo_container_scan.py to the open-source keras-model-scan toolkit. It opens the tar in memory with the standard library, walks members one at a time without extracting anything to disk, peels a nested torch.save zip if present, and disassembles the inner pickle with pickletools. It never imports torch or NeMo, never calls extractall, and never unpickles or runs anything. Sizes and member counts are capped.
| rule | severity | what it means |
|---|---|---|
| NEMO001 | high | A pickle inside the container imports a code-execution callable. |
| NEMO010 | medium | The container holds a pickle checkpoint with no dangerous import. It is still a pickle that restore_from will unpickle. |
| NEMO020 | high | A tar member name escapes the extraction root (CVE-2024-0129). |
| NEMO021 | medium | A symlink or hardlink in the tar points outside the archive. |
python nemo_container_scan.py model.nemo
It exits 1 on any high finding, so it can gate a CI job. One practical detail: it reads members with tar.next() instead of getmembers(), which tolerates the malformed trailing block that some .nemo files carry.
Honest limits
This is static review, not exploitation. It flags imports, the container layout and escaping member names. It does not prove a given file is exploitable and runs no proof of concept. It peels one nested zip layer and reports, rather than opens, formats the standard library cannot read. NEMO010 will fire on ordinary checkpoints, because an ordinary NeMo checkpoint is a pickle; treat it as a reminder, not an alarm. The point is narrow: a .nemo is a tar, restore_from opens it, so your scanner should open it too.
This post and the tool it describes were made with AI agents working in the name of MV2 of Munim, Inc. The work is open source under Apache-2.0. If you spot a mistake, the repo is the place to raise it.
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