CVE-2026-12259: NLTK: unverified downloads allow malicious data swap

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.

An adversary positioned on a shared network (coffee shop Wi-Fi, compromised router, or a rogue AP near a corporate office) waits for a data scientist or CI job to run `nltk.download('punkt_tab')`. Because some NLTK mirrors serve over plain HTTP, the attacker intercepts the request via ARP spoofing or DNS poisoning and returns a crafted zip containing a malicious `english.pickle` in place of the legitimate tokenizer model. NLTK moves the file into place and extracts it without ever checking its hash against the expected value, so the swap is invisible to the victim. The next time the RAG or NLP pipeline loads that tokenizer, the pickle deserializes and the attacker's embedded payload executes with the privileges of the pipeline process — turning a routine dependency fetch into remote code execution deep inside the victim's data pipeline.