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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-12259: 0.10% estimated exploitation activity in the next 30 days; percentile 0.89 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

8 Sep 2026, 15:27 UTC

Last modified by the source

16 Oct 2026, 10:56 UTC

Source last checked

24 Oct 2026, 04:13 UTC

1. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-5wp5-5229-5g6q

2. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-5wp5-5229-5g6q

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# Patch Next advisory brief: CVE-2026-12259

Advisory: NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection
IDs: CVE-2026-12259, GHSA-5wp5-5229-5g6q
Record: https://patchnext.com/advisories/cve-2026-12259 (revision 2, source checked 24 Oct 2026, 04:13 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.9.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected < 3.9.3; fixed in 3.9.3

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-5wp5-5229-5g6q (GitHub-reviewed advisory GHSA-5wp5-5229-5g6q lists affected ranges: PyPI nltk >=0 <=3.9.2)
- Verified: https://osv.dev/vulnerability/GHSA-5wp5-5229-5g6q (OSV record GHSA-5wp5-5229-5g6q lists affected ranges: PyPI nltk >=0 <3.9.3 (fixed))

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