# CVE-2021-4118: PYSEC-2026-3968 - pytorch-lightning is vulnerable to Deserialization of Untrusted Data

Source: https://junglewise.ai/threats/cve-2021-4118-pytorch-lightning-deserialization-of-untrusted-data  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-10. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2021-4118 · Severity: low · CVSS 3.1 · Published 2026-09-10

PyTorch Lightning is a popular machine learning framework used to simplify deep learning model training. The framework deserializes untrusted data without proper validation, allowing an attacker to execute arbitrary code when loading a malicious checkpoint file. This could lead to complete compromise of the training environment and access to sensitive model data.

Technologies: [lightning (PyPI)](https://junglewise.ai/threats/technologies/lightning), [pytorch-lightning (PyPI)](https://junglewise.ai/threats/technologies/pytorch-lightning). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2021-4118 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-4118 being exploited in the wild.

**How severe is CVE-2021-4118?** Low severity, with a CVSS score of 3.1, published 10 September 2026.

**Which products and versions are affected?** lightning (PyPI) GIT:introduced=0,fixed=62f1e82e032eb16565e676d39e0db0cac7e34ace|ECOSYSTEM:introduced=0,fixed=1.6.0; pytorch-lightning (PyPI) ECOSYSTEM:introduced=0,fixed=1.6.0, GIT:introduced=0,fixed=62f1e82e032eb16565e676d39e0db0cac7e34ace|ECOSYSTEM:introduced=0,fixed=1.6.0.

**Is there a fix for CVE-2021-4118?** Yes. lightning (PyPI): upgrade to 1.6.0 or later; pytorch-lightning (PyPI): upgrade to 1.6.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2021-4118 attacked?** It is attacked with local access.

**Am I affected by CVE-2021-4118?** Check whether you run lightning (PyPI) or pytorch-lightning (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- lightning (PyPI): upgrade to 1.6.0 or later

  ```sh
  pip install "lightning>=1.6.0"
  ```

- pytorch-lightning (PyPI): upgrade to 1.6.0 or later

  ```sh
  pip install "pytorch-lightning>=1.6.0"
  ```


## Technical details

PyTorch Lightning is vulnerable to unsafe deserialization of untrusted data (CWE-502). The vulnerability exists in checkpoint loading functionality, where the framework uses Python's pickle deserialization on user-supplied model checkpoint files without sufficient validation. An attacker can craft a malicious checkpoint file that executes arbitrary Python code during the unpickling process. The attack requires local file system access or the ability to trick a user into loading a malicious checkpoint; no network access or authentication is required. Exploitation results in arbitrary code execution with the privileges of the user running the training process. The vulnerability was fixed in version 1.6.0.

## Affected products

- PyTorch Lightning Lightning before 1.6.0

## Identifiers

- CVE-2021-4118
- GHSA-2vj5-px25-gjrp
- PYSEC-2021-874
- PYSEC-2026-3968

## Timeline

- 2021-12-23: disclosed
- 2022-01-06: advisory
- 2022-01-06: patched: Version 1.6.0 released

## References

- [https://github.com/PyTorchLightning/pytorch-lightning/issues/11045](https://github.com/PyTorchLightning/pytorch-lightning/issues/11045)

## Related threats

- [PYSEC-2026-3971 - PyTorch Lightning denial of service vulnerability](https://junglewise.ai/threats/cve-2024-8020-pytorch-lightning-denial-of-service-vulnerability)
- [PyTorch Lightning code injection via eval()](https://junglewise.ai/threats/cve-2022-0845-code-injection-in-pytorch-lightning)
- [PYSEC-2026-3975 - pytorch-lightning vulnerable to Arbitrary File Write via /v1/runs API endpoint](https://junglewise.ai/threats/cve-2024-5980-pytorch-lightning-vulnerable-to-arbitrary-file-write-via-v1-runs)
- [PYSEC-2026-3974 - Remote code execution in pytorch lightning](https://junglewise.ai/threats/cve-2024-5452-remote-code-execution-in-pytorch-lightning)
- [PyTorch Lightning remote code execution in load_from_checkpoint](https://junglewise.ai/threats/cve-2026-58659-pytorch-lightning-remote-code-execution-in-load-from-checkpoint)