NLTK before 3.9.3 fails to verify file integrity after downloading packages and before extraction in the downloader module. Attackers can perform man-in-the-middle attacks or DNS poisoning to inject malicious package contents that are extracted without validation.

NLTK versions before 3.9.3 contain a vulnerability in the downloader module that fails to verify file integrity after downloading packages and before extraction. This flaw allows attackers to perform man-in-the-middle (MITM) attacks or DNS poisoning to inject malicious package contents. Because no validation occurs prior to extraction, the malicious content is processed without detection. The vulnerability poses a significant supply chain risk for any application or workflow relying on NLTK's package downloading functionality. Users are advised to upgrade to NLTK 3.9.3 or later to mitigate the risk. The issue is tracked as CVE-2026-63310 and has been acknowledged in a GitHub security advisory.