CVE-2026-12259: Nltk Nltk/nltk

Medium severity, CVSS 5.3. EPSS: 0.1% chance of exploitation in the next 30 days.

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: affected versions not specified

Published 2026-08-03. Last modified 2026-08-31.