diff --git a/ironic_python_agent/disk_utils.py b/ironic_python_agent/disk_utils.py
index 55fa44869..e9b2a6115 100644
--- a/ironic_python_agent/disk_utils.py
+++ b/ironic_python_agent/disk_utils.py
@@ -444,7 +444,9 @@ def get_and_validate_image_format(filename, ironic_disk_format):
                 raise errors.InvalidImage(
                     details=msg % (img_format, fmts)
                 )
-            elif ironic_disk_format and ironic_disk_format != img_format:
+            elif (ironic_disk_format
+                  and ironic_disk_format != img_format
+                  and ironic_disk_format != 'unknown'):
                 msg = ("Security: Expected format was %s, but image was "
                        "actually %s" % (ironic_disk_format, img_format))
                 LOG.error(msg)
diff --git a/ironic_python_agent/tests/unit/test_disk_utils.py b/ironic_python_agent/tests/unit/test_disk_utils.py
index 1d2a2a88b..6dae2c260 100644
--- a/ironic_python_agent/tests/unit/test_disk_utils.py
+++ b/ironic_python_agent/tests/unit/test_disk_utils.py
@@ -1013,6 +1013,18 @@ class GetAndValidateImageFormat(base.IronicAgentTest):
                           disk_utils.get_and_validate_image_format,
                           '/fake/path', fmt)
 
+    @mock.patch.object(disk_utils, '_image_inspection', autospec=True)
+    def test_format_unknown_happy(self, mock_ii):
+        """ironic_disk_format=unknown, but we detect it as a qcow3"""
+        CONF.set_override('disable_deep_image_inspection', False)
+        fmt = 'unknown'
+        mock_ii.return_value = MockFormatInspectorCls('qcow2', 0, True)
+        self.assertEqual(
+            ('qcow2', 0),
+            disk_utils.get_and_validate_image_format('/fake/path', fmt)
+        )
+        mock_ii.assert_called_once_with('/fake/path')
+
     @mock.patch.object(disk_utils, '_image_inspection', autospec=True)
     @mock.patch.object(qemu_img, 'image_info', autospec=True)
     def test_format_mismatch_but_disabled(self, mock_info, mock_ii):