diff --git a/README.md b/README.md
index 7640a72b..60233f06 100644
--- a/README.md
+++ b/README.md
@@ -910,7 +910,7 @@ Create or reuse a Microsoft Teams / Azure bot app registration. Set the bot mess
 
 > - `replyInThread: true` replies to the triggering Teams activity when a stored `activity_id` is available.
 > - `mentionOnlyResponse` controls what Nanobot receives when a user sends only a bot mention (`<at>Nanobot</at>`). Set to `""` to ignore mention-only messages.
-> - `validateInboundAuth: true` (recommended for production) enables inbound Bot Framework bearer-token validation (signature, issuer, audience, lifetime, `serviceUrl`). **Default is `false`** — set explicitly to `true` for production deployments.
+> - `validateInboundAuth: true` enables inbound Bot Framework bearer-token validation (signature, issuer, audience, lifetime, `serviceUrl`). This is the safe default for public deployments. Only set it to `false` for local development or tightly controlled testing.
 
 **4. Run**
 
diff --git a/nanobot/channels/msteams.py b/nanobot/channels/msteams.py
index 06b707f8..427b35f8 100644
--- a/nanobot/channels/msteams.py
+++ b/nanobot/channels/msteams.py
@@ -57,7 +57,7 @@ class MSTeamsConfig(Base):
     allow_from: list[str] = Field(default_factory=list)
     reply_in_thread: bool = True
     mention_only_response: str = "Hi — what can I help with?"
-    validate_inbound_auth: bool = False
+    validate_inbound_auth: bool = True
 
 
 @dataclass
@@ -116,9 +116,9 @@ class MSTeamsChannel(BaseChannel):
 
         if not self.config.validate_inbound_auth:
             logger.warning(
-                "MSTeams inbound auth validation is DISABLED. "
+                "MSTeams inbound auth validation was explicitly DISABLED in config. "
                 "Anyone who knows the webhook URL can send messages as any user. "
-                "Set validateInboundAuth: true in config for production use."
+                "Only disable this for local development or controlled testing."
             )
 
         self._loop = asyncio.get_running_loop()
@@ -274,6 +274,14 @@ class MSTeamsChannel(BaseChannel):
                 logger.debug("MSTeams ignoring empty message after Teams text sanitization")
                 return
 
+        if not self.is_allowed(sender_id):
+            logger.warning(
+                "Access denied for sender {} on channel {}. "
+                "Add them to allowFrom list in config to grant access.",
+                sender_id, self.name,
+            )
+            return
+
         self._conversation_refs[conversation_id] = ConversationRef(
             service_url=service_url,
             conversation_id=conversation_id,
diff --git a/tests/test_msteams.py b/tests/test_msteams.py
index 67ffb853..f5597c38 100644
--- a/tests/test_msteams.py
+++ b/tests/test_msteams.py
@@ -147,6 +147,40 @@ async def test_handle_activity_ignores_group_messages(make_channel):
     assert ch._conversation_refs == {}
 
 
+@pytest.mark.asyncio
+async def test_handle_activity_denied_sender_does_not_store_ref(make_channel, tmp_path):
+    ch = make_channel(allowFrom=["allowed-user"])
+
+    activity = {
+        "type": "message",
+        "id": "activity-denied",
+        "text": "Hello from denied user",
+        "serviceUrl": "https://smba.trafficmanager.net/amer/",
+        "conversation": {
+            "id": "conv-denied",
+            "conversationType": "personal",
+        },
+        "from": {
+            "id": "29:user-id",
+            "aadObjectId": "aad-user-1",
+            "name": "Bob",
+        },
+        "recipient": {
+            "id": "28:bot-id",
+            "name": "nanobot",
+        },
+        "channelData": {
+            "tenant": {"id": "tenant-id"},
+        },
+    }
+
+    await ch._handle_activity(activity)
+
+    assert ch.bus.inbound == []
+    assert ch._conversation_refs == {}
+    assert not (tmp_path / "state" / "msteams_conversations.json").exists()
+
+
 @pytest.mark.asyncio
 async def test_handle_activity_mention_only_uses_default_response(make_channel):
     ch = make_channel()
@@ -520,6 +554,7 @@ async def test_start_logs_install_hint_when_pyjwt_missing(make_channel, monkeypa
 def test_msteams_default_config_includes_restart_notify_fields():
     cfg = MSTeamsChannel.default_config()
 
+    assert cfg["validateInboundAuth"] is True
     assert "restartNotifyEnabled" not in cfg
     assert "restartNotifyPreMessage" not in cfg
     assert "restartNotifyPostMessage" not in cfg