CVE-2025-6921: Transformers Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-4w7r-h757-3r74
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CVSS 5.3
CWE-400
Published
Advisory Summary

CVE-2025-6921 is a MODERATE (CVSS 5.3) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Transformers libraries in the PyPI ecosystem. The issue involves hugging face transformers vulnerable to regular expression denial of service (redos) in the adamweightdecay optimizer. Remediation is available by upgrading affected packages to version 4.53.0.

CVE-2025-6921 at a Glance

CVE Identifier CVE-2025-6921
GHSA Identifier GHSA-4w7r-h757-3r74
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Transformers
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.3

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:N/S:U/C:N/I:N/A:L).

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

How to Fix CVE-2025-6921

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: 4.53.0 
pip install transformers>=4.53.0
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-2025-6921, 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 4.53.0), 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-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. 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

The huggingface/transformers library, versions prior to 4.53.0, is vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer. The vulnerability arises from the _do_use_weight_decay method, which processes user-controlled regular expressions in the include_in_weight_decay and exclude_from_weight_decay lists. Malicious regular expressions can cause catastrophic backtracking during the re.search call, leading to 100% CPU utilization and a denial of service. This issue can be exploited by attackers who can control the patterns in these lists, potentially causing the machine learning task to hang and rendering services unresponsive.

CVE-2025-6921 Affected Transformers Versions

1 component(s)
Fixed Versions:
4.53.0
Product / Package Ecosystem Affected Versions Fixed Version
transformers
pypi/transformers
PyPI
< 4.53.0
4.53.0

References

NVD — CVE-2025-6921 NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-6921
Security Fix Commit FIX COMMIT
https://github.com/huggingface/transformers/commit/47c34fba5c303576560cb29767efb452ff12b8be
Security Fix Commit FIX COMMIT
https://github.com/huggingface/transformers/commit/d37f7517972f67e3f2194c000ed0f87f064e5099
Transformers Source Repository REPOSITORY
https://github.com/huggingface/transformers
huntr.com/bounties/287d15a7-6... WEB
https://huntr.com/bounties/287d15a7-6e7c-45d2-8c05-11e305776f1f

Frequently Asked Questions

What is CVE-2025-6921?
CVE-2025-6921 is a security vulnerability in Transformers involving Hugging Face Transformers vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer under CWE-400: Uncontrolled Resource Consumption.
Which Transformers versions are affected?
Affected versions include releases prior to 4.53.0 across affected packages: transformers.
What version fixes CVE-2025-6921?
Upgrading to 4.53.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is transformers affected?
Yes, transformers is affected in releases prior to 4.53.0. Upgrade your dependency to 4.53.0 or later.
What is the CVSS score of CVE-2025-6921?
CVE-2025-6921 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2025-6921?
CVE-2025-6921 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2025-6921?
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 
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  "published" : "2025-09-23T15:31:09Z",
  "aliases" : [ "CVE-2025-6921" ],
  "summary" : "Hugging Face Transformers vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer",
  "details" : "The huggingface/transformers library, versions prior to 4.53.0, is vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer. The vulnerability arises from the _do_use_weight_decay method, which processes user-controlled regular expressions in the include_in_weight_decay and exclude_from_weight_decay lists. Malicious regular expressions can cause catastrophic backtracking during the re.search call, leading to 100% CPU utilization and a denial of service. This issue can be exploited by attackers who can control the patterns in these lists, potentially causing the machine learning task to hang and rendering services unresponsive.",
  "severity" : [ {
    "type" : "CVSS_V3",
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  "database_specific" : {
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