diff --git a/src/openjarvis/server/webhook_routes.py b/src/openjarvis/server/webhook_routes.py
index 0c3db1da..ecd9983d 100644
--- a/src/openjarvis/server/webhook_routes.py
+++ b/src/openjarvis/server/webhook_routes.py
@@ -34,15 +34,25 @@ def _validate_twilio_signature(
     params: dict,
     signature: str,
 ) -> bool:
-    """Validate Twilio webhook signature using the SDK."""
+    """Validate Twilio webhook signature using the SDK.
+
+    Fails closed: returns ``False`` if the SDK is not installed or
+    if no auth_token is configured.
+    """
+    if not auth_token:
+        logger.error("Twilio auth token not configured — rejecting webhook")
+        return False
     try:
         from twilio.request_validator import RequestValidator
 
         validator = RequestValidator(auth_token)
         return validator.validate(url, params, signature)
     except ImportError:
-        logger.warning("twilio SDK not installed — skipping signature validation")
-        return True
+        logger.error(
+            "twilio SDK not installed — rejecting webhook. "
+            "Install it: pip install twilio"
+        )
+        return False
 
 
 def _format_for_sms(text: str) -> str:
@@ -236,6 +246,11 @@ def create_webhook_router(
             header_secret = request.headers.get("x-sendblue-secret", "")
             if header_secret != sb.webhook_secret:
                 return Response("Invalid secret", status_code=403)
+        elif sb:
+            logger.warning(
+                "SendBlue webhook received without secret verification. "
+                "Set webhook_secret for HMAC validation."
+            )
 
         # Ignore outbound status callbacks
         if payload.get("is_outbound", False):
diff --git a/tests/security/test_webhook_validation.py b/tests/security/test_webhook_validation.py
new file mode 100644
index 00000000..25b35fec
--- /dev/null
+++ b/tests/security/test_webhook_validation.py
@@ -0,0 +1,32 @@
+"""Tests for webhook fail-closed validation (Section 3)."""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+
+class TestTwilioValidationFailClosed:
+    """Twilio validation must reject when SDK is unavailable."""
+
+    def test_missing_sdk_returns_false(self) -> None:
+        from openjarvis.server.webhook_routes import _validate_twilio_signature
+
+        with patch.dict("sys.modules", {"twilio": None, "twilio.request_validator": None}):
+            result = _validate_twilio_signature(
+                auth_token="test_token",
+                url="https://example.com/webhooks/twilio",
+                params={"Body": "hello"},
+                signature="invalid",
+            )
+            assert result is False
+
+    def test_empty_auth_token_returns_false(self) -> None:
+        from openjarvis.server.webhook_routes import _validate_twilio_signature
+
+        result = _validate_twilio_signature(
+            auth_token="",
+            url="https://example.com/webhooks/twilio",
+            params={},
+            signature="",
+        )
+        assert result is False