fix(security): pin webhook delivery to the SSRF-validated IP (DNS rebinding) (#5147)

validate_webhook_url resolves the host to accept/reject, but the delivery
connect (httpx.AsyncClient.post) re-resolved independently — a DNS record
flipping between the two lookups (rebinding) could slip an internal IP
(127.0.0.1 / 169.254.169.254 / LAN) past the check and receive the signed
payload. The module docstring already flagged this as only a "partial
defense".

Resolve + validate once via _validated_public_ips, then pin the delivery
TCP connect to that approved IP with an async _PinnedAsyncTransport built
on the public httpcore/httpx APIs (mirrors the sync search-fetch pin from
#704). The URL, Host header, and TLS SNI are unchanged, so certificate
validation and vhost routing still target the original hostname; only the
socket destination is pinned.

Delivery now uses a per-request pinned client instead of one shared client,
so close() is a no-op kept for API compatibility. Adds end-to-end tests that
drive the real transport against loopback servers, proving the connect
follows the pin rather than re-resolving the URL host.

Fixes #5146

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/test_webhook_dns_rebinding_pin.py

@@ -0,0 +1,156 @@
+"""Regression: webhook delivery must pin the TCP connect to the SSRF-approved IP.
+
+validate_webhook_url resolves the host to accept/reject, but the delivery
+connect previously re-resolved independently — a DNS record flipping between
+the two lookups (rebinding) could slip an internal IP past the check. _deliver
+now resolves+validates once via _validated_public_ips and pins the connect to
+that IP through _PinnedAsyncTransport. These tests drive the real transport
+against local servers so the pin is exercised end-to-end, not mocked away.
+"""
+import asyncio
+import http.server
+import ipaddress
+import socketserver
+import threading
+
+import pytest
+
+from tests.helpers.import_state import clear_module, preserve_import_state
+
+import os
+import sys
+from unittest.mock import patch
+
+with patch.dict(os.environ, {"DATABASE_URL": "sqlite:///:memory:"}), \
+        preserve_import_state("src.database", "core.database"):
+    clear_module("src.database")
+    _core_database = sys.modules.get("core.database")
+    if _core_database is not None and not getattr(_core_database, "__file__", None):
+        del sys.modules["core.database"]
+    import src.webhook_manager as wm
+
+
+# ---------------------------------------------------------------------------
+# _validated_public_ips
+# ---------------------------------------------------------------------------
+
+def test_validated_public_ips_rejects_metadata_literal():
+    with pytest.raises(ValueError):
+        wm._validated_public_ips("http://169.254.169.254/")
+
+
+def test_validated_public_ips_rejects_loopback_literal():
+    with pytest.raises(ValueError):
+        wm._validated_public_ips("http://127.0.0.1/")
+
+
+def test_validated_public_ips_returns_public_literal():
+    ips = wm._validated_public_ips("http://93.184.216.34/")
+    assert ips == [ipaddress.ip_address("93.184.216.34")]
+
+
+def test_validated_public_ips_rejects_hostname_resolving_private(monkeypatch):
+    # Rebinding shape: a hostname that (now) resolves into loopback space.
+    monkeypatch.setattr(wm, "_resolve_hostname_ips",
+                        lambda h: [ipaddress.ip_address("127.0.0.1")])
+    with pytest.raises(ValueError):
+        wm._validated_public_ips("http://evil.rebind.example/")
+
+
+# ---------------------------------------------------------------------------
+# End-to-end: the pinned transport actually routes to the pinned IP
+# ---------------------------------------------------------------------------
+
+def _serve(handler):
+    srv = socketserver.TCPServer(("127.0.0.1", 0), handler)
+    port = srv.server_address[1]
+    threading.Thread(target=srv.serve_forever, daemon=True).start()
+    return srv, port
+
+
+def test_pinned_transport_connects_to_pinned_ip():
+    """A request whose URL host is a throwaway hostname is still delivered to
+    the pinned loopback IP — proving the socket destination comes from the pin,
+    not from resolving the URL host."""
+    hits = []
+
+    class _Handler(http.server.BaseHTTPRequestHandler):
+        def do_POST(self):  # noqa: N802
+            length = int(self.headers.get("Content-Length", 0))
+            self.rfile.read(length)
+            hits.append(self.path)
+            self.send_response(204)
+            self.end_headers()
+
+        def log_message(self, *a):
+            pass
+
+    srv, port = _serve(_Handler)
+    try:
+        ip = ipaddress.ip_address("127.0.0.1")
+        transport = wm._PinnedAsyncTransport(ip)
+
+        async def go():
+            async with __import__("httpx").AsyncClient(
+                transport=transport, timeout=5, follow_redirects=False,
+            ) as client:
+                # Host "unresolvable.invalid" would never resolve; the pin is
+                # what makes this reach the loopback server on `port`.
+                return await client.post(
+                    f"http://unresolvable.invalid:{port}/hook", content=b"{}",
+                )
+
+        resp = asyncio.run(go())
+        assert resp.status_code == 204
+        assert hits == ["/hook"]
+    finally:
+        srv.shutdown()
+
+
+def test_deliver_pins_to_validated_ip_end_to_end(monkeypatch):
+    """Full _deliver path: a hostname that validation resolves to loopback is
+    pinned to loopback and the local server receives the signed POST."""
+    received = {}
+
+    class _Handler(http.server.BaseHTTPRequestHandler):
+        def do_POST(self):  # noqa: N802
+            length = int(self.headers.get("Content-Length", 0))
+            received["body"] = self.rfile.read(length)
+            received["event"] = self.headers.get("X-Odysseus-Event")
+            self.send_response(200)
+            self.end_headers()
+
+        def log_message(self, *a):
+            pass
+
+    srv, port = _serve(_Handler)
+
+    class _Query:
+        def filter(self, *a, **k): return self
+        def update(self, values): return None
+
+    class _Db:
+        def query(self, _m): return _Query()
+        def commit(self): pass
+        def rollback(self): pass
+        def close(self): pass
+
+    # Make both the validation resolve and the pin target loopback, and treat
+    # loopback as allowed for this test (production blocks it — here we only
+    # want to prove the pin routes to the validated IP).
+    monkeypatch.setattr(wm, "SessionLocal", lambda: _Db())
+    monkeypatch.setattr(wm, "_is_private_url", lambda url: False)
+    monkeypatch.setattr(wm, "_resolve_hostname_ips",
+                        lambda h: [ipaddress.ip_address("127.0.0.1")])
+    monkeypatch.setattr(wm, "_ip_is_private", lambda a: False)
+
+    manager = wm.WebhookManager()
+    try:
+        asyncio.run(manager._deliver(
+            "hook-1", f"http://webhook.test:{port}/cb", "s3cret",
+            "webhook.test", {"ok": True},
+        ))
+        assert received.get("event") == "webhook.test"
+        assert b'"ok": true' in received["body"]
+    finally:
+        srv.shutdown()