---
id: CVE-2026-12259
title: >-
  In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()`
  function writes downloaded package bytes to disk and may extract them before
  enforcing SHA-256 or MD5 checksum validation
summary: >-
  In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()`
  function writes downloaded package bytes to disk and may extract them before
  enforcing SHA-256 or MD5 checksum validation. This allows an attacker to
  tamper with…
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-494
  - CWE-354
vendor: nltk
product: nltk
affected:
  - nltk = 3.9.4
patched:
  - nltk 3.9.3
published: '2026-08-03'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T19:21:42.683'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-12259'
references:
  - url: 'https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d'
    label: security@huntr.dev
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-12259.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-12259'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2510340'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-12259'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-12259'
  - url: 'https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q'
  - url: 'https://github.com/nltk/nltk/pull/3449'
  - url: >-
      https://github.com/nltk/nltk/commit/0e26734a61094b628d93e26dc18dd7302567ac46
  - url: 'https://github.com/nltk/nltk'
  - url: 'https://github.com/nltk/nltk/releases/tag/3.9.3'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3729.yaml
  - url: >-
      https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity-verification
  - url: 'https://github.com/advisories/GHSA-5wp5-5229-5g6q'
tags:
  - nvd
  - csaf
  - vex
  - red-hat
  - osv
  - pip
  - ghsa
epss: 0.00101
epssPercentile: 0.00891
aliases:
  - GHSA-5wp5-5229-5g6q
  - CVE-2026-63310
  - GHSA-gf32-cmjh-8m9v
  - PYSEC-2026-3729
  - PYSEC-2026-4318
ecosystem: pip
scores:
  nvd: 6.5
  vendor: 5.3
  osv: 5.3
  ghsa: 5.3
ingestedAt: '2026-09-08T15:33:26.986Z'
---

## Overview

In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for `info.url` through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.

## Affected

- `nltk = 3.9.4`

## Remediation

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

## Package advisory (CVE-2026-12259)

Affected packages:

- `nltk < 3.9.3`

Patched in:

- `nltk 3.9.3`

Source: https://osv.dev/vulnerability/GHSA-5wp5-5229-5g6q

## Vendor advisories

- **Red Hat VEX** · Moderate · affected: Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed, Red Hat Ansible Automation Platform 2, Red Hat OpenShift AI (RHOAI) · no fix planned: Exploit Intelligence, Red Hat Ansible Automation Platform 2, Lightspeed Core, OpenShift Lightspeed, … · updated 2026-10-15 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-12259.json)