NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.

NLTK versions before 3.10.3 contain a server-side request forgery (SSRF) vulnerability in nltk.pathsec.urlopen and its callers (nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. The vulnerability arises because pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers, causing the actual fetch to be performed by the proxy without re-validation of the destination. An attacker can supply a publicly validated URL that the proxy then forwards to an internal loopback-only service. This allows disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content. The flaw represents a significant supply chain risk for Python NLP environments using NLTK with proxy configurations. Users are advised to upgrade to NLTK 3.10.3 or later to mitigate the vulnerability.