Hugging Face Transformers vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2025-6921: 0.51% estimated exploitation activity in the next 30 days; percentile 43.63 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 douseweightdecay method, which processes user-controlled regular expressions in the includeinweightdecay and excludefromweightdecay 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.

Published by the source

23 Sep 2025, 15:31 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:35 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4w7r-h757-3r74

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4w7r-h757-3r74

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# Patch Next advisory brief: CVE-2025-6921

Advisory: Hugging Face Transformers vulnerable to Regular Expression Denial of Service (ReDoS) in the AdamWeightDecay optimizer
IDs: CVE-2025-6921, GHSA-4w7r-h757-3r74
Record: https://patchnext.com/advisories/cve-2025-6921 (revision 2, source checked 26 Oct 2026, 13:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: transformers 4.53.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI transformers: affected < 4.53.0; fixed in 4.53.0

## Suggested inspection (read-only)
- `python -m pip show transformers`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on transformers.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4w7r-h757-3r74 (OSV record GHSA-4w7r-h757-3r74 lists affected ranges: PyPI transformers >=0 <4.53.0 (fixed))
- Verified: https://github.com/advisories/GHSA-4w7r-h757-3r74 (GitHub-reviewed advisory GHSA-4w7r-h757-3r74 lists affected ranges: PyPI transformers >=0 <4.53.0)

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