GHSA-qvv7-cg9c-w4x3: Natural Language Toolkit (NLTK): DNS-rebinding SSRF filter bypass in nltk.pathsec.urlopen (nltk.download / nltk.data.load) defeats ENFORCE mode

Summary nltk.pathsec provides an SSRF filter that NLTK documents as a security control, blocking loopback, private, link-local, and multicast ranges (including obfuscated forms) and recommending strict ENFORCE mode for security-sensitive environments. The filter is bypassable by DNS rebinding: validate_network_url() resolves the hostname and checks the resulting IP, but the actual HTTP connection re-resolves the hostname independently at connect time and connects to that second result. The validated IP is never the one connected to. An attacker controlling DNS for a hostname (a TTL-0 rebinding record) returns a public IP for the validation lookup and an internal/loopback IP for the connection lookup, defeating the filter even under nltk.pathsec.ENFORCE = True. Details urlopen() validates, then hands the raw hostname to urllib, which performs a second name resolution deep in the connection layer (http.client.HTTPConnection.connect → socket.create_connection → socket.getaddrinfo). The validation-side and connection-side resolutions are fully independent code paths with independent caches: 1. validate_network_url() calls _resolve_hostname(parsed.hostname) and checks each returned IP against loopback/link-local/multicast/private, blocking under ENFORCE. (Resolution #1.) 2. urlopen() then calls build_opener(...).open(url) with the original URL (raw hostname), so urllib resolves the hostname again at connect time. (Resolution #2 — the address actually connected to.) _resolve_hostname is decorated with lru_cache and its docstring claims to mitigate DNS rebinding, but the cache only memoizes the validation-side lookup. The connection layer's getaddrinfo does not consult that cache, so it provides no protection. The annotation is a false assurance: an operator reading it may believe rebinding is handled when it is not. PoC import socket import threading import warnings from collections import defaultdict from http.server import BaseHTTPRequestHandler, HTTPServer warnin