# CVE-2026-31221: PyTorch Lightning load_from_checkpoint has an insecure checkpoint deserialization

> Attackers can execute arbitrary code by tricking users into loading malicious checkpoint files in PyTorch Lightning due to insecure deserialization.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 12 May 2026
- **Updated:** 15 May 2026
- **Also known as:** GHSA-75m9-98v2-hjpm, PYSEC-2026-3043, PYSEC-2026-3972, UBUNTU-CVE-2026-31221

## What is CVE-2026-31221?

CVE-2026-31221 is a high-severity unsafe deserialization vulnerability in n/a, published on 12 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

PyTorch-Lightning versions 2.6.0 and earlier contain an insecure deserialization vulnerability (CWE-502) in the checkpoint loading mechanism. The LightningModule.load_from_checkpoint() method, which is commonly used to load saved model states, internally calls torch.load() without setting the security-restrictive weights_only=True parameter. This default behavior allows the deserialization of arbitrary Python objects via the Pickle module. A remote attacker can exploit this by providing a maliciously crafted checkpoint file, leading to arbitrary code execution on the victim's system when the file is loaded.

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2026-31221?

4 open source packages ship an affected version, including pytorch-lightning and lightning. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `pytorch-lightning` | PyPI | All versions to 2.6.0 | None |
| `pytorch-lightning` | PyPI | All versions to 2.6.0 | None |
| `pytorch-lightning` | PyPI | All versions to 2.6.0 | None |
| `lightning` | PyPI | All versions to 2.6.0 | None |

Products: Lightningai Pytorch Lightning.

## How do I fix CVE-2026-31221?

No fix has been published for CVE-2026-31221 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-31221 being exploited?

There is no known exploitation of CVE-2026-31221 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2024-5452](https://insights.sbomify.com/CVE-2024-5452/): RCE via Property/Class Pollution in lightning-ai/pytorch-lightning (critical 9.8)
- [CVE-2026-44484](https://insights.sbomify.com/CVE-2026-44484/): Compromise of PyTorch Lightning PyPi Package Versions (critical 9.3)
- [CVE-2024-8019](https://insights.sbomify.com/CVE-2024-8019/): Arbitrary File Write/Overwrite in lightning-ai/pytorch-lightning (critical 9.1)
- [CVE-2024-5980](https://insights.sbomify.com/CVE-2024-5980/): Arbitrary File Write via /v1/runs API endpoint in lightning-ai/pytorch-lightning (critical 9.1)
- [CVE-2026-58659](https://insights.sbomify.com/CVE-2026-58659/): PyTorch Lightning Arbitrary Code Execution via _instantiator Hyperparameter (high 8.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31221 and fix it if it is affected.

What it is: CVE-2026-31221 is a high-severity unsafe deserialization vulnerability in n/a, published on 12 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-31221 in the wild and no public exploit code.

Affected packages and fixed versions:
- pytorch-lightning (PyPI): affected all versions up to 2.6.0; no fix listed
- pytorch-lightning (PyPI): affected all versions up to 2.6.0; no fix listed
- pytorch-lightning (PyPI): affected all versions up to 2.6.0; no fix listed
- lightning (PyPI): affected all versions up to 2.6.0; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-31221/ (Markdown: https://insights.sbomify.com/CVE-2026-31221/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31221
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31221
- OSV: https://osv.dev/vulnerability/CVE-2026-31221

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31221/
Summaries are generated and may contain mistakes; the official description above is the record.