diff --git a/gateway/platforms/msgraph_webhook.py b/gateway/platforms/msgraph_webhook.py
index 88781d1f547..04058a41a01 100644
--- a/gateway/platforms/msgraph_webhook.py
+++ b/gateway/platforms/msgraph_webhook.py
@@ -34,6 +34,7 @@ DEFAULT_HOST = "0.0.0.0"
 DEFAULT_PORT = 8646
 DEFAULT_WEBHOOK_PATH = "/msgraph/webhook"
 DEFAULT_MAX_SEEN_RECEIPTS = 5000
+DEFAULT_MAX_BODY_BYTES = 1_048_576
 NotificationScheduler = Callable[[Dict[str, Any], MessageEvent], Awaitable[None] | None]
 
 
@@ -63,6 +64,9 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
         self._max_seen_receipts = max(
             1, int(extra.get("max_seen_receipts", DEFAULT_MAX_SEEN_RECEIPTS))
         )
+        self._max_body_bytes = max(
+            1, int(extra.get("max_body_bytes", DEFAULT_MAX_BODY_BYTES))
+        )
         self._allowed_source_networks: list[ipaddress._BaseNetwork] = (
             self._parse_allowed_source_cidrs(extra.get("allowed_source_cidrs"))
         )
@@ -152,7 +156,7 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
             )
             return False
 
-        app = web.Application()
+        app = web.Application(client_max_size=self._max_body_bytes)
         app.router.add_get(self._health_path, self._handle_health)
         app.router.add_get(self._webhook_path, self._handle_validation)
         app.router.add_post(self._webhook_path, self._handle_notification)
@@ -229,8 +233,24 @@ class MSGraphWebhookAdapter(BasePlatformAdapter):
             return web.Response(text=validation_token, content_type="text/plain")
 
         try:
-            body = await request.json()
+            content_length = request.content_length
         except Exception:
+            content_length = None
+        if content_length is not None and content_length > self._max_body_bytes:
+            return web.Response(status=413)
+
+        try:
+            raw_body = await request.read()
+        except Exception:
+            return web.Response(status=400)
+        if len(raw_body) > self._max_body_bytes:
+            return web.Response(status=413)
+
+        try:
+            body = json.loads(raw_body.decode("utf-8"))
+        except (json.JSONDecodeError, UnicodeDecodeError):
+            return web.Response(status=400)
+        if not isinstance(body, dict):
             return web.Response(status=400)
 
         notifications = body.get("value")
diff --git a/tests/gateway/test_msgraph_webhook.py b/tests/gateway/test_msgraph_webhook.py
index caa141c6a44..a731e9287da 100644
--- a/tests/gateway/test_msgraph_webhook.py
+++ b/tests/gateway/test_msgraph_webhook.py
@@ -19,9 +19,19 @@ def _make_adapter(**extra_overrides) -> MSGraphWebhookAdapter:
 
 
 class _FakeRequest:
-    def __init__(self, *, query=None, json_payload=None, remote="127.0.0.1"):
+    def __init__(
+        self,
+        *,
+        query=None,
+        json_payload=None,
+        raw_body: bytes | None = None,
+        content_length: int | None = None,
+        remote="127.0.0.1",
+    ):
         self.query = query or {}
         self._json_payload = json_payload
+        self._raw_body = raw_body
+        self.content_length = content_length
         self.remote = remote
 
     async def json(self):
@@ -29,6 +39,11 @@ class _FakeRequest:
             raise self._json_payload
         return self._json_payload
 
+    async def read(self):
+        if self._raw_body is not None:
+            return self._raw_body
+        return json.dumps(self._json_payload or {}).encode("utf-8")
+
 
 class TestMSGraphWebhookConfig:
     def test_gateway_config_accepts_msgraph_webhook_platform(self):
@@ -183,6 +198,62 @@ class TestMSGraphNotifications:
         assert event.source.chat_type == "webhook"
         assert event.message_id == "id:notif-1"
 
+    @pytest.mark.anyio
+    async def test_oversized_notification_rejected_by_content_length(self):
+        adapter = _make_adapter(max_body_bytes=100)
+        payload = {
+            "value": [
+                {
+                    "id": "notif-oversized",
+                    "subscriptionId": "sub-1",
+                    "changeType": "updated",
+                    "resource": "communications/onlineMeetings/meeting-1",
+                    "clientState": "expected-client-state",
+                }
+            ]
+        }
+
+        resp = await adapter._handle_notification(
+            _FakeRequest(json_payload=payload, content_length=101)
+        )
+
+        assert resp.status == 413
+
+    @pytest.mark.anyio
+    async def test_chunked_oversized_notification_rejected_after_read(self):
+        adapter = _make_adapter(max_body_bytes=100)
+        payload = {
+            "value": [
+                {
+                    "id": "notif-chunked-oversized",
+                    "subscriptionId": "sub-1",
+                    "changeType": "updated",
+                    "resource": "communications/onlineMeetings/meeting-1",
+                    "clientState": "expected-client-state",
+                }
+            ]
+        }
+
+        resp = await adapter._handle_notification(
+            _FakeRequest(
+                json_payload=payload,
+                raw_body=b"x" * 101,
+                content_length=None,
+            )
+        )
+
+        assert resp.status == 413
+
+    @pytest.mark.anyio
+    async def test_non_object_notification_body_rejected(self):
+        adapter = _make_adapter()
+
+        resp = await adapter._handle_notification(
+            _FakeRequest(json_payload=[], raw_body=b"[]")
+        )
+
+        assert resp.status == 400
+
     @pytest.mark.anyio
     async def test_bad_client_state_rejected_as_auth_failure(self):
         """Every-item-bad-clientState batches return 403 so forged POSTs stop retrying."""