Harden loopback HTTP and document AMO unlisted signing.

host/firefox_agent_bridge_host.py

@@ -2,6 +2,7 @@
 """Native messaging host: 127.0.0.1 HTTP <-> Firefox extension stdio."""
 from __future__ import annotations
 
+import hmac
 import json
 import os
 import struct
@@ -21,6 +22,8 @@ if sys.platform == "win32":
 
 HOST = "127.0.0.1"
 PORT = int(os.environ.get("FAB_PORT", "17634"))
+MAX_BODY = 256 * 1024
+ALLOWED_HOSTS = {f"127.0.0.1:{PORT}", f"localhost:{PORT}"}
 STATE_DIR = Path(os.environ.get("LOCALAPPDATA", str(Path.home()))) / "firefox-agent-bridge"
 TOKEN_PATH = STATE_DIR / "token"
 STDIN_LOCK = threading.Lock()
@@ -91,20 +94,36 @@ class Handler(BaseHTTPRequestHandler):
         self.end_headers()
         self.wfile.write(body)
 
+    def _gate_ok(self) -> bool:
+        """Block browser-origin calls and non-loopback Host headers."""
+        origin = self.headers.get("Origin")
+        if origin:
+            self._send(403, {"ok": False, "error": "browser origin not allowed"})
+            return False
+        host = (self.headers.get("Host") or "").split("%")[0].lower()
+        if host not in ALLOWED_HOSTS:
+            self._send(403, {"ok": False, "error": "host not allowed"})
+            return False
+        return True
+
     def _auth_ok(self) -> bool:
+        if not self._gate_ok():
+            return False
         token = read_token()
         if not token:
             self._send(500, {"ok": False, "error": "bridge token missing; run install-native-host.ps1"})
             return False
         header = self.headers.get("Authorization", "")
         got = header[7:].strip() if header.lower().startswith("bearer ") else ""
-        if got != token:
+        if not hmac.compare_digest(got, token):
             self._send(401, {"ok": False, "error": "missing or invalid bearer token"})
             return False
         return True
 
     def _json_body(self) -> dict[str, Any]:
         length = int(self.headers.get("Content-Length") or "0")
+        if length > MAX_BODY:
+            raise ValueError("request body too large")
         if length <= 0:
             return {}
         raw = self.rfile.read(length)
@@ -118,6 +137,8 @@ class Handler(BaseHTTPRequestHandler):
     def do_GET(self) -> None:
         parsed = urlparse(self.path)
         if parsed.path == "/health":
+            if not self._gate_ok():
+                return
             self._send(200, {"ok": True, "service": "firefox-agent-bridge", "port": PORT})
             return
         if not self._auth_ok():