CVE-2026-63310

NLTK versions before 3.9.3 contain a download-integrity flaw (CWE-494) in its downloader module: files fetched via nltk.download() are extracted without any integrity or authenticity verification. An attacker positioned on the network path — through a man-in-the-middle attack or DNS poisoning — can substitute malicious package contents that NLTK then extracts onto the host unvalidated. Successful injection could place attacker-controlled files into the environment, with CVSS v4.0 scoring high impact to confidentiality, integrity, and availability of the target. Any developer, CI pipeline, or data-processing environment that installs NLTK via pip and uses its downloader to fetch packages over a network an attacker can influence is affected. No exploitation has been reported in the wild (EPSS ~0.1%, not in CISA KEV); the issue is documented in a published GitHub security advisory (GHSA-5wp5-5229-5g6q).

What to do: Upgrade NLTK to 3.9.3 or later (pip install --upgrade nltk) and ensure CI images, requirements files, and containers are rebuilt against the fixed version. As interim mitigation, avoid running nltk.download() over untrusted or attacker-influenceable networks; instead pre-fetch packages from a trusted source and verify their integrity before extraction. Audit build and data pipelines for runtime calls to the NLTK downloader, since static installs that never download packages are not practically exposed.