---
id: CVE-2025-6921
aliases:
  - GHSA-4w7r-h757-3r74
  - PYSEC-2026-1980
title: >-
  Hugging Face Transformers vulnerable to Regular Expression Denial of Service
  (ReDoS) in the AdamWeightDecay optimizer
summary: >-
  Hugging Face Transformers vulnerable to Regular Expression Denial of Service
  (ReDoS) in the AdamWeightDecay optimizer
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
vendor: transformers
product: transformers
ecosystem: pip
affected:
  - transformers < 4.53.0
patched:
  - transformers 4.53.0
published: '2025-09-23'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:27.867584538Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-4w7r-h757-3r74'
references:
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2025-6921'
  - url: >-
      https://github.com/huggingface/transformers/commit/47c34fba5c303576560cb29767efb452ff12b8be
  - url: >-
      https://github.com/huggingface/transformers/commit/d37f7517972f67e3f2194c000ed0f87f064e5099
  - url: 'https://github.com/huggingface/transformers'
  - url: 'https://huntr.com/bounties/287d15a7-6e7c-45d2-8c05-11e305776f1f'
tags:
  - osv
  - pip
epss: 0.00511
epssPercentile: 0.41073
ingestedAt: '2026-07-08T18:25:45.712Z'
---

## Overview

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.

## Affected packages

- `transformers < 4.53.0`

## Remediation

Upgrade to a patched release:

- `transformers 4.53.0`