---
id: CVE-2026-12261
title: >-
  A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for
  cross-package resource and model poisoning
summary: >-
  A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for
  cross-package resource and model poisoning. The downloader extracts package
  archives into shared namespaces such as `corpora/` and `taggers/` instead of
  packa…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N'
cwe:
  - CWE-284
vendor: nltk
product: nltk
affected:
  - nltk <= 3.9.4
published: '2026-08-07'
updated: '2026-09-04'
sourceUpdated: '2026-09-04T14:40:11.537'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-12261'
references:
  - url: 'https://huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f'
    label: security@huntr.dev
  - url: 'https://huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - cve.org
  - exploit-available
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-08-07T15:25:29.448356Z'
scores:
  nvd: 6.5
  cna: 5.3
ingestedAt: '2026-09-13T12:16:14.875Z'
epss: 0.00205
epssPercentile: 0.10828
---

## Overview

A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces such as `corpora/` and `taggers/` instead of package-isolated roots, and validates package integrity only after the archive has been written and extracted. This design flaw enables one package to overwrite another package's trusted resources within the same namespace, making the changes immediately active through ordinary NLTK APIs. This issue persists across fresh interpreter restarts and can affect downstream workflows, including machine learning pipelines and reproducibility-sensitive environments.

## Affected

- `nltk <= 3.9.4`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.