CVE-2026-9856: Transformers Path Traversal Vulnerability

HIGH
Advisory Identifier: GHSA-xrqw-3rrv-vx5w
Share
CVSS 7.1
CWE-22
Published
Advisory Summary

CVE-2026-9856 is a HIGH (CVSS 7.1) path traversal vulnerability (CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')) affecting Transformers libraries in the PyPI ecosystem. The issue involves transformers save_pretrained path traversal allows arbitrary file writes through chat template names. Remediation is available by upgrading affected packages to version 5.10.0.

CVE-2026-9856 at a Glance

CVE Identifier CVE-2026-9856
GHSA Identifier GHSA-xrqw-3rrv-vx5w
Severity HIGH
CVSS v3.1 Base Score 7.1
CWE Classification CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Affected Product Transformers
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability Low (A:L)

How to Fix CVE-2026-9856

To remediate this vulnerability, upgrade all affected Transformers dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

transformers fixed: 5.10.0 
pip install transformers>=5.10.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show transformers

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-9856, inspect your installed packages using pip:

pip show transformers

Analyze the terminal output to locate any version of Transformers in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 5.10.0), your application is affected.

Note: The pip show transformers command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'), where the product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. In Transformers, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

A vulnerability in huggingface/transformers versions < 5.10.0 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.

CVE-2026-9856 Affected Transformers Versions

1 component(s)
Fixed Versions:
5.10.0
Product / Package Ecosystem Affected Versions Fixed Version
transformers
pypi/transformers
PyPI
< 5.10.0
5.10.0

References

NVD — CVE-2026-9856 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-9856
Transformers Source Repository REPOSITORY
https://github.com/huggingface/transformers/pull/46191
Security Fix Commit FIX COMMIT
https://github.com/huggingface/transformers/commit/eaaaf8494dd5386634ae37d1d122212fdc315be5
Transformers Source Repository REPOSITORY
https://github.com/huggingface/transformers
huntr.com/bounties/362824d5-f... WEB
https://huntr.com/bounties/362824d5-fe18-40e8-a6cf-62277f97a170

Frequently Asked Questions

What is CVE-2026-9856?
CVE-2026-9856 is a security vulnerability in Transformers involving Transformers save_pretrained path traversal allows arbitrary file writes through chat template names under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
Which Transformers versions are affected?
Affected versions include releases prior to 5.10.0 across affected packages: transformers.
What version fixes CVE-2026-9856?
Upgrading to 5.10.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is transformers affected?
Yes, transformers is affected in releases prior to 5.10.0. Upgrade your dependency to 5.10.0 or later.
What is the CVSS score of CVE-2026-9856?
CVE-2026-9856 has a CVSS v3.1 base score of 7.1 (HIGH) with vector string CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:L.
What CWE is associated with CVE-2026-9856?
CVE-2026-9856 is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') (The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.).
How can I check PyPI dependencies for CVE-2026-9856?
Run the dependency tree command: 'pip show transformers'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xrqw-3rrv-vx5w",
  "modified" : "2026-09-01T18:58:08Z",
  "published" : "2026-08-02T18:30:20Z",
  "aliases" : [ "CVE-2026-9856" ],
  "summary" : "Transformers save_pretrained path traversal allows arbitrary file writes through chat template names",
  "details" : "A vulnerability in huggingface/transformers versions < 5.10.0 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "transformers"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.10.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-9856"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/huggingface/transformers/pull/46191"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/huggingface/transformers/commit/eaaaf8494dd5386634ae37d1d122212fdc315be5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/huggingface/transformers"
  }, {
    "type" : "WEB",
    "url" : "https://huntr.com/bounties/362824d5-fe18-40e8-a6cf-62277f97a170"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-01T18:58:08Z",
    "nvd_published_at" : "2026-08-02T16:16:25Z"
  }
}