NLTKGHSA-5wp5-5229-5g6q

NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection

NLTK

CVE-2026-12259 · Published Sep 8, 2026 · updated Oct 16, 2026

Medium5.3

Fix: upgrade to 3.9.3 or later

GitHub advisory

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 =...

Affected versions

Package	Affected	Fixed in
nltk PyPI	< 3.9.3	3.9.3

Details and references

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.

CVSS 3.1: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N
Severity from: GitHub (reviewed advisory)
Weakness: CWE-494
Also known as: CVE-2026-12259, CVE-2026-63310, GHSA-gf32-cmjh-8m9v, PYSEC-2026-3729, PYSEC-2026-4318

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