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

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:

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.