add basic twilio signature checking and tests

gateway/platforms/sms.py

@@ -10,6 +10,8 @@ Shares credentials with the optional telephony skill — same env vars:
 
 Gateway-specific env vars:
   - SMS_WEBHOOK_PORT     (default 8080)
+  - SMS_WEBHOOK_HOST     (default 0.0.0.0)
+  - SMS_WEBHOOK_URL      (public URL for Twilio signature validation)
   - SMS_ALLOWED_USERS    (comma-separated E.164 phone numbers)
   - SMS_ALLOW_ALL_USERS  (true/false)
   - SMS_HOME_CHANNEL     (phone number for cron delivery)
@@ -17,6 +19,8 @@ Gateway-specific env vars:
 
 import asyncio
 import base64
+import hashlib
+import hmac
 import logging
 import os
 import re
@@ -29,6 +33,7 @@ from gateway.platforms.base import (
     MessageEvent,
     MessageType,
     SendResult,
+    is_network_accessible,
 )
 
 logger = logging.getLogger(__name__)
@@ -36,6 +41,7 @@ logger = logging.getLogger(__name__)
 TWILIO_API_BASE = "https://api.twilio.com/2010-04-01/Accounts"
 MAX_SMS_LENGTH = 1600  # ~10 SMS segments
 DEFAULT_WEBHOOK_PORT = 8080
+DEFAULT_WEBHOOK_HOST = "0.0.0.0"
 
 # E.164 phone number pattern for redaction
 _PHONE_RE = re.compile(r"\+[1-9]\d{6,14}")
@@ -77,6 +83,8 @@ class SmsAdapter(BasePlatformAdapter):
         self._webhook_port: int = int(
             os.getenv("SMS_WEBHOOK_PORT", str(DEFAULT_WEBHOOK_PORT))
         )
+        self._webhook_host: str = os.getenv("SMS_WEBHOOK_HOST", DEFAULT_WEBHOOK_HOST)
+        self._webhook_url: str = os.getenv("SMS_WEBHOOK_URL", "").strip()
         self._runner = None
         self._http_session: Optional["aiohttp.ClientSession"] = None
 
@@ -98,13 +106,21 @@ class SmsAdapter(BasePlatformAdapter):
             logger.error("[sms] TWILIO_PHONE_NUMBER not set — cannot send replies")
             return False
 
+        if not self._webhook_url:
+            logger.warning(
+                "[sms] SMS_WEBHOOK_URL not set — Twilio signature validation is "
+                "DISABLED. Any client that can reach port %d can inject messages. "
+                "Set SMS_WEBHOOK_URL to enable signature validation.",
+                self._webhook_port,
+            )
+
         app = web.Application()
         app.router.add_post("/webhooks/twilio", self._handle_webhook)
         app.router.add_get("/health", lambda _: web.Response(text="ok"))
 
         self._runner = web.AppRunner(app)
         await self._runner.setup()
-        site = web.TCPSite(self._runner, "0.0.0.0", self._webhook_port)
+        site = web.TCPSite(self._runner, self._webhook_host, self._webhook_port)
         await site.start()
         self._http_session = aiohttp.ClientSession(
             timeout=aiohttp.ClientTimeout(total=30),
@@ -112,7 +128,8 @@ class SmsAdapter(BasePlatformAdapter):
         self._running = True
 
         logger.info(
-            "[sms] Twilio webhook server listening on port %d, from: %s",
+            "[sms] Twilio webhook server listening on %s:%d, from: %s",
+            self._webhook_host,
             self._webhook_port,
             _redact_phone(self._from_number),
         )
@@ -203,6 +220,28 @@ class SmsAdapter(BasePlatformAdapter):
         content = re.sub(r"\n{3,}", "\n\n", content)
         return content.strip()
 
+    # ------------------------------------------------------------------
+    # Twilio signature validation
+    # ------------------------------------------------------------------
+
+    def _validate_twilio_signature(
+        self, url: str, post_params: dict, signature: str,
+    ) -> bool:
+        """Validate ``X-Twilio-Signature`` header (HMAC-SHA1, base64).
+
+        Algorithm: https://www.twilio.com/docs/usage/security#validating-requests
+        """
+        data_to_sign = url
+        for key in sorted(post_params.keys()):
+            data_to_sign += key + post_params[key]
+        mac = hmac.new(
+            self._auth_token.encode("utf-8"),
+            data_to_sign.encode("utf-8"),
+            hashlib.sha1,
+        )
+        computed = base64.b64encode(mac.digest()).decode("utf-8")
+        return hmac.compare_digest(computed, signature)
+
     # ------------------------------------------------------------------
     # Twilio webhook handler
     # ------------------------------------------------------------------
@@ -213,7 +252,7 @@ class SmsAdapter(BasePlatformAdapter):
         try:
             raw = await request.read()
             # Twilio sends form-encoded data, not JSON
-            form = urllib.parse.parse_qs(raw.decode("utf-8"))
+            form = urllib.parse.parse_qs(raw.decode("utf-8"), keep_blank_values=True)
         except Exception as e:
             logger.error("[sms] webhook parse error: %s", e)
             return web.Response(
@@ -222,6 +261,27 @@ class SmsAdapter(BasePlatformAdapter):
                 status=400,
             )
 
+        # Validate Twilio request signature when SMS_WEBHOOK_URL is configured
+        if self._webhook_url:
+            twilio_sig = request.headers.get("X-Twilio-Signature", "")
+            if not twilio_sig:
+                logger.warning("[sms] Rejected: missing X-Twilio-Signature header")
+                return web.Response(
+                    text='<?xml version="1.0" encoding="UTF-8"?><Response></Response>',
+                    content_type="application/xml",
+                    status=403,
+                )
+            flat_params = {k: v[0] for k, v in form.items() if v}
+            if not self._validate_twilio_signature(
+                self._webhook_url, flat_params, twilio_sig
+            ):
+                logger.warning("[sms] Rejected: invalid Twilio signature")
+                return web.Response(
+                    text='<?xml version="1.0" encoding="UTF-8"?><Response></Response>',
+                    content_type="application/xml",
+                    status=403,
+                )
+
         # Extract fields (parse_qs returns lists)
         from_number = (form.get("From", [""]))[0].strip()
         to_number = (form.get("To", [""]))[0].strip()