---
id: CVE-2026-12259
aliases:
  - GHSA-5wp5-5229-5g6q
  - CVE-2026-63310
  - GHSA-gf32-cmjh-8m9v
  - PYSEC-2026-3729
title: >-
  NLTK: Missing Post-Download Integrity Verification Allows Malicious Package
  Injection
summary: >-
  NLTK: Missing Post-Download Integrity Verification Allows Malicious Package
  Injection
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N'
vendor: nltk
product: nltk
ecosystem: pip
affected:
  - nltk < 3.9.3
patched:
  - nltk 3.9.3
published: '2026-09-08'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T11:25:57.355843699Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-5wp5-5229-5g6q'
references:
  - url: 'https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-12259'
  - 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://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d'
  - url: >-
      https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity-verification
  - url: 'https://github.com/advisories/GHSA-5wp5-5229-5g6q'
tags:
  - osv
  - pip
  - ghsa
epss: 0.0014
epssPercentile: 0.02753
cwe:
  - CWE-494
ingestedAt: '2026-09-08T15:33:26.986Z'
---

## Overview

NLTK's package downloader in nltk/downloader.py does not verify file integrity after download and before extraction.

The download flow at lines 789-825:
1. File is downloaded to a temp path via HTTP
2. os.replace(tmp_filepath, filepath) moves it to the final location (line 799)
3. Extraction begins via _unzip_iter() (line 825)

Between steps 2 and 3, there is no SHA-256 verification. The checksum logic exists in _pkg_status() (lines 982-1015) but it is only used BEFORE download as a status check ("is this package already installed and up-to-date?"). It is never called after download to verify the file that was actually received.

Attack vectors:
1. MITM during HTTP download (NLTK downloads from http:// by default on some mirrors)
2. Race condition on shared filesystems (attacker replaces file between os.replace and _unzip_iter)
3. DNS poisoning redirecting to attacker-controlled server

PoC:
```python
import nltk
import unittest.mock
import zipfile
import io
import os

# Create a malicious zip that will be "downloaded"
malicious_zip = io.BytesIO()
with zipfile.ZipFile(malicious_zip, 'w') as zf:
    zf.writestr('punkt_tab/tokenizers/punkt_tab/english.pickle', 
                b'MALICIOUS PAYLOAD - attacker controlled content')

# Patch urllib to return our malicious zip
with unittest.mock.patch('urllib.request.urlopen') as mock_urlopen:
    mock_response = unittest.mock.MagicMock()
    mock_response.read.return_value = malicious_zip.getvalue()
    mock_response.headers = {'Content-Length': str(len(malicious_zip.getvalue()))}
    mock_urlopen.return_value = mock_response
    
    # Download proceeds, no integrity check catches the swap
    # nltk.download('punkt_tab')  # Would install attacker payload
```

This is distinct from CVE-2024-39705 (pickle deserialization via download) and CVE-2025-14009 (zip-slip path traversal). Those address what happens AFTER extraction. This finding addresses the gap BEFORE extraction where integrity is never verified.

Suggested fix: After os.replace() and before _unzip_iter(), compute SHA-256 of the final file and compare against the expected checksum from the package index. Reject and delete the file if the hash does not match.

## Affected packages

- `nltk < 3.9.3`

## Remediation

Upgrade to a patched release:

- `nltk 3.9.3`