Fixed #28431 -- Added a system check for BinaryField to prevent strings defaults.

Thanks Claude Paroz for the initial patch.

django/db/models/fields/__init__.py

@@ -2252,6 +2252,21 @@ class BinaryField(Field):
         if self.max_length is not None:
             self.validators.append(validators.MaxLengthValidator(self.max_length))
 
+    def check(self, **kwargs):
+        return [*super().check(**kwargs), *self._check_str_default_value()]
+
+    def _check_str_default_value(self):
+        if self.has_default() and isinstance(self.default, str):
+            return [
+                checks.Error(
+                    "BinaryField's default cannot be a string. Use bytes "
+                    "content instead.",
+                    obj=self,
+                    id='fields.E170',
+                )
+            ]
+        return []
+
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
         if self.editable:

docs/ref/checks.txt

@@ -159,6 +159,8 @@ Model fields
 * **fields.W161**: Fixed default value provided.
 * **fields.W162**: ``<database>`` does not support a database index on
   ``<field data type>`` columns.
+* **fields.E170**: ``BinaryField``’s ``default`` cannot be a string. Use bytes
+  content instead.
 * **fields.E900**: ``IPAddressField`` has been removed except for support in
   historical migrations.
 * **fields.W900**: ``IPAddressField`` has been deprecated. Support for it

tests/invalid_models_tests/test_ordinary_fields.py

@@ -39,6 +39,33 @@ class AutoFieldTests(SimpleTestCase):
         ])
 
 
+@isolate_apps('invalid_models_tests')
+class BinaryFieldTests(SimpleTestCase):
+
+    def test_valid_default_value(self):
+        class Model(models.Model):
+            field1 = models.BinaryField(default=b'test')
+            field2 = models.BinaryField(default=None)
+
+        for field_name in ('field1', 'field2'):
+            field = Model._meta.get_field(field_name)
+            self.assertEqual(field.check(), [])
+
+    def test_str_default_value(self):
+        class Model(models.Model):
+            field = models.BinaryField(default='test')
+
+        field = Model._meta.get_field('field')
+        self.assertEqual(field.check(), [
+            Error(
+                "BinaryField's default cannot be a string. Use bytes content "
+                "instead.",
+                obj=field,
+                id='fields.E170',
+            ),
+        ])
+
+
 @isolate_apps('invalid_models_tests')
 class CharFieldTests(SimpleTestCase):