# CVE-2026-9856: Path Traversal in huggingface/transformers

> Attackers can write arbitrary files via path traversal in save_pretrained methods when processing malicious tokenizer configs.

- **Severity:** High 7.1 (`CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:L`)
- **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:** Yes
- **Published:** 2 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** CLSA-2026-1788512070, ECHO-230d-a6b3-d275, GHSA-xrqw-3rrv-vx5w, MINI-c5x3-wmfp-xrp6, MINI-h559-7p2f-hm8g, MINI-hr3g-w8w2-xrp2, PYSEC-2026-3929, ROOT-APP-PYPI-CVE-2026-9856

## What is CVE-2026-9856?

CVE-2026-9856 is a high-severity path traversal vulnerability in huggingface/transformers, published on 2 Aug 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

A vulnerability in huggingface/transformers versions <=5.8.0.dev0 allows an attacker to perform arbitrary file writes via path traversal. The issue resides in the `save_pretrained()` methods of `PreTrainedTokenizerBase` and `ProcessorMixin`, where keys from the `chat_template` dictionary are used directly as filenames without proper validation. An attacker can exploit this by publishing a malicious Hugging Face Hub repository with a crafted `tokenizer_config.json` file. When a victim downloads and saves the tokenizer or processor, the attacker-controlled keys can escape the intended save directory, enabling arbitrary file writes with attacker-controlled content. This vulnerability affects multiple processors inheriting from `ProcessorMixin`, including Idefics, Florence, Gemma, Phi, and Qwen-VL.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-9856?

3 open source packages ship an affected version, including transformers.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `transformers` | PyPI | All versions | 5.10.0 |
| `transformers` | PyPI | All versions | 5.10.0 |
| `transformers` | PyPI | All versions | 5.10.0 |

## How do I fix CVE-2026-9856?

Upgrade transformers to 5.10.0 or later.

## Is CVE-2026-9856 being exploited?

There is no known exploitation of CVE-2026-9856 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9856 and fix it if it is affected.

What it is: CVE-2026-9856 is a high-severity path traversal vulnerability in huggingface/transformers, published on 2 Aug 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-2026-9856 in the wild and no public exploit code.

Affected packages and fixed versions:
- transformers (PyPI): affected all versions; fixed in 5.10.0
- transformers (PyPI): affected all versions; fixed in 5.10.0
- transformers (PyPI): affected all versions; fixed in 5.10.0

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-2026-9856/ (Markdown: https://insights.sbomify.com/CVE-2026-9856/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9856
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9856
- OSV: https://osv.dev/vulnerability/CVE-2026-9856

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9856/
Summaries are generated and may contain mistakes; the official description above is the record.