NLTK versions prior to 3.10.3 have a server‑side request forgery vulnerability in nltk.pathsec.urlopen and its callers. The function validates the hostname locally, but when an HTTP proxy is configured, proxy‑handler inheritance disables the safe HTTP/HTTPS handlers so the proxy performs the fetch without re‑validation. An attacker can trick the proxy into requesting an internal loopback‑only service by providing a validated public URL, resulting in disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker‑chosen package content.

The NLTK library from the nltk vendor, in all versions earlier than 3.10.3. No other products are mentioned.

The CVSS score of 8.7 indicates high severity and the vulnerability could be exploited when the application is configured to use an HTTP proxy. The EPSS score is not available, but the lack of a KEV listing does not diminish the risk for environments that rely on the vulnerable library. An attacker with access to supply parameters to the vulnerable function can leverage the bypass to reach internal services and introduce malicious content.