🔒️(backend) enhance API input validation to strengthen security

During the bug bounty, attempts were made to pass unexpected hidden
fields to manipulate room behavior and join as a ghost.

Treat these parameters as suspicious. They are not sent by the
frontend, so their presence likely indicates tampering.

Explicitly allow the parameters but emit warning logs to help detect
and investigate suspicious activity.

src/backend/core/api/serializers.py

@@ -2,9 +2,11 @@
 
 # pylint: disable=abstract-method,no-name-in-module
 
+from django.core.exceptions import SuspiciousOperation
 from django.utils.translation import gettext_lazy as _
 
-from livekit.api import ParticipantPermission
+from django_pydantic_field.rest_framework import SchemaField
+from pydantic import BaseModel, Field
 from rest_framework import serializers
 from rest_framework.exceptions import PermissionDenied
 from timezone_field.rest_framework import TimeZoneSerializerField
@@ -261,6 +263,28 @@ class MuteParticipantSerializer(BaseParticipantsManagementSerializer):
     )
 
 
+class ParticipantPermission(BaseModel):
+    """Mirror the LiveKit ParticipantPermission protobuf.
+
+    Control what a participant is allowed to publish, subscribe, and do within a room.
+    Unknown fields are rejected.
+    """
+
+    can_subscribe: bool | None = None
+    can_publish: bool | None = None
+    can_publish_data: bool | None = None
+    can_publish_sources: list[int] = Field(
+        default_factory=list
+    )  # TrackSource enum values
+    hidden: bool | None = None
+    recorder: bool | None = None
+    can_update_metadata: bool | None = None
+    agent: bool | None = None
+    can_subscribe_metrics: bool | None = None
+
+    model_config = {"extra": "forbid"}
+
+
 class UpdateParticipantSerializer(BaseParticipantsManagementSerializer):
     """Validate participant update data."""
 
@@ -272,10 +296,11 @@ class UpdateParticipantSerializer(BaseParticipantsManagementSerializer):
         allow_null=True,
         help_text="Participant attributes as JSON object",
     )
-    permission = serializers.DictField(
+    permission = SchemaField(
+        schema=ParticipantPermission | None,
         required=False,
         allow_null=True,
-        help_text="Participant permission as JSON object",
+        help_text="Participant permissions",
     )
     name = serializers.CharField(
         max_length=255,
@@ -285,6 +310,33 @@ class UpdateParticipantSerializer(BaseParticipantsManagementSerializer):
         help_text="Display name for the participant",
     )
 
+    def validate_permission(self, permission):
+        """Validate that the given permission does not include forbidden or unimplemented fields."""
+
+        if permission is None:
+            return None
+
+        suspicious_fields = [
+            field
+            for field in ("hidden", "recorder", "agent")
+            if getattr(permission, field) is not None
+        ]
+        if suspicious_fields:
+            raise SuspiciousOperation(
+                f"Setting the following participant permissions is not allowed: "
+                f"{', '.join(suspicious_fields)}."
+            )
+        if permission.can_subscribe_metrics is not None:
+            raise serializers.ValidationError(
+                {
+                    "permission": {
+                        "can_subscribe_metrics": "This permission is not implemented."
+                    }
+                }
+            )
+
+        return permission
+
     def validate(self, attrs):
         """Ensure at least one update field is provided."""
         update_fields = ["metadata", "attributes", "permission", "name"]
@@ -300,12 +352,4 @@ class UpdateParticipantSerializer(BaseParticipantsManagementSerializer):
                 f"{', '.join(update_fields)}."
             )
 
-        if "permission" in attrs:
-            try:
-                ParticipantPermission(**attrs["permission"])
-            except ValueError as e:
-                raise serializers.ValidationError(
-                    {"permission": f"Invalid permission: {str(e)}"}
-                ) from e
-
         return attrs

src/backend/core/api/viewsets.py

@@ -607,13 +607,15 @@ class RoomViewSet(
         serializer = serializers.UpdateParticipantSerializer(data=request.data)
         serializer.is_valid(raise_exception=True)
 
+        permission = serializer.validated_data.get("permission")
+
         try:
             ParticipantsManagement().update(
                 room_name=str(room.pk),
                 identity=str(serializer.validated_data["participant_identity"]),
                 metadata=serializer.validated_data.get("metadata"),
                 attributes=serializer.validated_data.get("attributes"),
-                permission=serializer.validated_data.get("permission"),
+                permission=permission.model_dump() if permission else None,
                 name=serializer.validated_data.get("name"),
             )
         except ParticipantsManagementException: