diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3b1f47488c91..1510a5cf23db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Memory leak related to unclosed av container (<https://github.com/opencv/cvat/pull/6501>)
 - Using initial frame from query parameter to open specific frame in a job
  (<https://github.com/opencv/cvat/pull/6506>)
+- Server-side validation for attribute specifications
+  (<https://github.com/opencv/cvat/pull/6447>)
 
 ### Security
 - TDB
diff --git a/cvat/apps/dataset_manager/tests/test_formats.py b/cvat/apps/dataset_manager/tests/test_formats.py
index e1fa84d874d3..c710013d547d 100644
--- a/cvat/apps/dataset_manager/tests/test_formats.py
+++ b/cvat/apps/dataset_manager/tests/test_formats.py
@@ -259,7 +259,8 @@ def _generate_task(self, images, **overrides):
                             "name": "parked",
                             "mutable": True,
                             "input_type": "checkbox",
-                            "default_value": False
+                            "default_value": "false",
+                            "values": [],
                         },
                     ]
                 },
@@ -561,7 +562,8 @@ def _generate_task(self, images):
                             "name": "parked",
                             "mutable": True,
                             "input_type": "checkbox",
-                            "default_value": False
+                            "default_value": "false",
+                            "values": [],
                         },
                     ]
                 },
@@ -723,7 +725,8 @@ def _generate_task(self, images, annotation_format, **overrides):
                             "name": "parked",
                             "mutable": True,
                             "input_type": "checkbox",
-                            "default_value": False
+                            "default_value": "false",
+                            "values": [],
                         }
                     ]
                 },
diff --git a/cvat/apps/engine/serializers.py b/cvat/apps/engine/serializers.py
index f3303ea0ab03..52987755f25d 100644
--- a/cvat/apps/engine/serializers.py
+++ b/cvat/apps/engine/serializers.py
@@ -212,8 +212,15 @@ class Meta:
             'last_login': { 'allow_null': True }
         }
 
+class DelimitedStringListField(serializers.ListField):
+    def to_representation(self, value):
+        return super().to_representation(value.split('\n'))
+
+    def to_internal_value(self, data):
+        return '\n'.join(super().to_internal_value(data))
+
 class AttributeSerializer(serializers.ModelSerializer):
-    values = serializers.ListField(allow_empty=True,
+    values = DelimitedStringListField(allow_empty=True,
         child=serializers.CharField(allow_blank=True, max_length=200),
     )
 
@@ -221,21 +228,6 @@ class Meta:
         model = models.AttributeSpec
         fields = ('id', 'name', 'mutable', 'input_type', 'default_value', 'values')
 
-    # pylint: disable=no-self-use
-    def to_internal_value(self, data):
-        attribute = data.copy()
-        attribute['values'] = '\n'.join(data.get('values', []))
-        return attribute
-
-    def to_representation(self, instance):
-        if instance:
-            rep = super().to_representation(instance)
-            rep['values'] = instance.values.split('\n')
-        else:
-            rep = instance
-
-        return rep
-
 class SublabelSerializer(serializers.ModelSerializer):
     id = serializers.IntegerField(required=False)
     attributes = AttributeSerializer(many=True, source='attributespec_set', default=[],
diff --git a/cvat/apps/engine/tests/test_rest_api.py b/cvat/apps/engine/tests/test_rest_api.py
index 5663e47e4a71..4cb37551c555 100644
--- a/cvat/apps/engine/tests/test_rest_api.py
+++ b/cvat/apps/engine/tests/test_rest_api.py
@@ -1811,7 +1811,8 @@ def _create_project(project_data):
                             "name": "bool_attribute",
                             "mutable": True,
                             "input_type": AttributeType.CHECKBOX,
-                            "default_value": "true"
+                            "default_value": "true",
+                            "values": [],
                         }],
                     }, {
                         "name": "person",
@@ -2572,7 +2573,8 @@ def test_api_v2_tasks_admin(self):
                     "name": "my_attribute",
                     "mutable": True,
                     "input_type": AttributeType.CHECKBOX,
-                    "default_value": "true"
+                    "default_value": "true",
+                    "values": [],
                 }]
             }]
         }
@@ -2895,7 +2897,8 @@ def _create_task(task_data, media_data):
                         "name": "bool_attribute",
                         "mutable": True,
                         "input_type": AttributeType.CHECKBOX,
-                        "default_value": "true"
+                        "default_value": "true",
+                        "values": [],
                     }],
                     }, {
                         "name": "person",
@@ -2915,7 +2918,8 @@ def _create_task(task_data, media_data):
                         "name": "bool_attribute",
                         "mutable": True,
                         "input_type": AttributeType.CHECKBOX,
-                        "default_value": "true"
+                        "default_value": "true",
+                        "values": [],
                     }],
                     }, {
                         "name": "person",
@@ -4649,7 +4653,8 @@ def _create_task(self, owner, assignee, annotation_format=""):
                             "name": "parked",
                             "mutable": True,
                             "input_type": "checkbox",
-                            "default_value": "false"
+                            "default_value": "false",
+                            "values": [],
                         },
                     ]
                 },
diff --git a/cvat/apps/lambda_manager/tests/assets/tasks.json b/cvat/apps/lambda_manager/tests/assets/tasks.json
index 019716f38961..7ae07a1c0d0a 100644
--- a/cvat/apps/lambda_manager/tests/assets/tasks.json
+++ b/cvat/apps/lambda_manager/tests/assets/tasks.json
@@ -18,7 +18,8 @@
             "name": "parked",
             "mutable": true,
             "input_type": "checkbox",
-            "default_value": false
+            "default_value": "false",
+            "values": []
           }
         ]
       },
@@ -47,7 +48,8 @@
             "name": "parked",
             "mutable": true,
             "input_type": "checkbox",
-            "default_value": false
+            "default_value": "false",
+            "values": []
           }
         ]
       },