# CVE-2026-12259

## Summary

- **CVE ID:** CVE-2026-12259
- **Severity:** MEDIUM
- **CVSS Score:** 5.3 (CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N)
- **CWE:** CWE-494
- **Published:** Aug 3, 2026
- **Last Modified:** Aug 3, 2026

## 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.

## Affected Products

- nltk — nltk/nltk (unspecified)

## References

- [CNA](https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.10%
- **EPSS Percentile:** 1.0

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._