CVE-2026-12259: 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