fix(companion): honor configured pairing address (#6060)

* fix(companion): honor configured pairing origin

* fix(companion): keep configured pairing on v1 LAN contract

* fix(companion): reject numeric pairing hosts

---------

Co-authored-by: RaresKeY <158580472+RaresKeY@users.noreply.github.com>

companion/pairing.py

@@ -6,11 +6,14 @@ units so the route layer stays thin and the logic is directly testable.
 
 from __future__ import annotations
 
+import ipaddress
 import json
 import os
+import re
 import secrets
 import socket
 import uuid
+from urllib.parse import urlsplit
 
 import bcrypt
 
@@ -20,6 +23,102 @@ PAIRING_VERSION = 1
 COMPANION_SCOPE = "chat"
 
 
+_COMPANION_IPV4_NETWORKS = tuple(
+    ipaddress.ip_network(cidr)
+    for cidr in (
+        "10.0.0.0/8",
+        "100.64.0.0/10",
+        "127.0.0.0/8",
+        "169.254.0.0/16",
+        "172.16.0.0/12",
+        "192.168.0.0/16",
+    )
+)
+_DNS_LABEL_RE = re.compile(r"[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?\Z")
+
+
+def _valid_companion_client_host(host: str) -> bool:
+    """Match the host forms supported by the current v1 Expo client."""
+    if not host or len(host) > 253 or not host.isascii() or "%" in host:
+        return False
+
+    try:
+        address = ipaddress.ip_address(host)
+    except ValueError:
+        labels = host.split(".")
+        if any(not _DNS_LABEL_RE.fullmatch(label) for label in labels):
+            return False
+        if any(label.startswith("xn--") for label in labels):
+            return False
+        # WHATWG URL parsers treat a decimal or ``0x`` single-label hostname
+        # as an IPv4 number even though Python's strict ``ipaddress`` parser
+        # rejects that spelling.  The v1 client interpolates this host back
+        # into a URL, so accepting e.g. ``134744072`` would make the phone send
+        # its bearer token to public 8.8.8.8.  Keep DNS labels unambiguous.
+        if len(labels) == 1 and (
+            labels[0].isdigit()
+            or re.fullmatch(r"0x[0-9a-f]*", labels[0]) is not None
+        ):
+            return False
+        return len(labels) == 1 or (len(labels) >= 2 and labels[-1] == "local")
+
+    return isinstance(address, ipaddress.IPv4Address) and any(
+        address in network for network in _COMPANION_IPV4_NETWORKS
+    )
+
+
+def parse_companion_base_url(value: str) -> tuple[str, int]:
+    """Validate a v1 companion address and return its legacy (host, port).
+
+    The deployed client understands only HTTP plus a LAN-style host and port.
+    Reject anything outside that exact contract instead of advertising a URL
+    the client would reject, downgrade, or interpret differently.
+    """
+    if not isinstance(value, str) or not value:
+        raise ValueError("COMPANION_BASE_URL must be a canonical HTTP LAN origin")
+    if not value.isascii():
+        raise ValueError("COMPANION_BASE_URL must contain only ASCII characters")
+    if any(
+        ord(char) <= 32 or ord(char) == 127 or char in {"\\", "%"}
+        for char in value
+    ):
+        raise ValueError(
+            "COMPANION_BASE_URL contains a forbidden character"
+        )
+
+    try:
+        parsed = urlsplit(value)
+        port = parsed.port
+    except ValueError as exc:
+        raise ValueError("COMPANION_BASE_URL must be a valid HTTP LAN origin") from exc
+
+    host = parsed.hostname
+    if parsed.scheme.lower() != "http" or not parsed.netloc or not host:
+        raise ValueError("COMPANION_BASE_URL must be a canonical HTTP LAN origin")
+    if parsed.username is not None or parsed.password is not None:
+        raise ValueError("COMPANION_BASE_URL must not contain credentials")
+    if parsed.path or parsed.query or parsed.fragment:
+        raise ValueError("COMPANION_BASE_URL must not contain a path, query, or fragment")
+    if port is not None and not 1 <= port <= 65535:
+        raise ValueError("COMPANION_BASE_URL port must be between 1 and 65535")
+    if not _valid_companion_client_host(host):
+        raise ValueError("COMPANION_BASE_URL host is not supported by companion v1")
+
+    netloc = f"{host}:{port}" if port is not None else host
+    origin = f"http://{netloc}"
+    if value != origin:
+        raise ValueError("COMPANION_BASE_URL must be a canonical HTTP LAN origin")
+    return host, port or 80
+
+
+def configured_companion_origin() -> tuple[str, int] | None:
+    """Return the validated operator-configured v1 address, if any."""
+    value = os.environ.get("COMPANION_BASE_URL")
+    if value is None or value == "":
+        return None
+    return parse_companion_base_url(value)
+
+
 def default_port() -> int:
     """Best guess at the port the server is reachable on. Callers that know the
     real request port should pass it explicitly."""

companion/routes.py

@@ -200,19 +200,27 @@ def setup_companion_routes() -> APIRouter:
         the code works immediately, no restart. `?format=json` returns the
         payload for an in-app pairing screen."""
         require_admin(request)
+        try:
+            configured_origin = _pairing.configured_companion_origin()
+        except ValueError as exc:
+            raise HTTPException(500, str(exc)) from None
         owner = get_current_user(request)
         invalidate = getattr(request.app.state, "invalidate_token_cache", None)
         token_id, raw_token = mint_pairing_token(owner, invalidate)
 
-        hosts = _pairing.lan_ip_candidates()
-        host = hosts[0] if hosts else "127.0.0.1"
-        port = request.url.port or _pairing.default_port()
+        if configured_origin:
+            host, port = configured_origin
+            hosts = [host]
+        else:
+            hosts = _pairing.lan_ip_candidates()
+            host = hosts[0] if hosts else "127.0.0.1"
+            port = request.url.port or _pairing.default_port()
         payload = _pairing.pairing_payload(host, port, raw_token)
         qr = _pairing.pairing_qr_png_data_uri(payload)
         qr_ok = bool(qr and qr.startswith("data:image/png;base64,"))
 
         if (request.query_params.get("format") or "").lower() == "json":
-            return {
+            response = {
                 "host": host,
                 "port": port,
                 "token": raw_token,
@@ -221,6 +229,7 @@ def setup_companion_routes() -> APIRouter:
                 "payload": payload,
                 "qr": qr if qr_ok else None,
             }
+            return response
 
         import json as _json
         payload_json = _json.dumps(payload, separators=(",", ":"))