diff --git a/stix2/v21/vocab.py b/stix2/v21/vocab.py
index dc790d7..9443d0e 100644
--- a/stix2/v21/vocab.py
+++ b/stix2/v21/vocab.py
@@ -87,6 +87,22 @@ ENCRYPTION_ALGORITHM = [
 ]
 
 
+EXTENSION_TYPE_NEW_SDO = "new-sdo"
+EXTENSION_TYPE_NEW_SCO = "new-sco"
+EXTENSION_TYPE_NEW_SRO = "new-sro"
+EXTENSION_TYPE_PROPERTY_EXTENSION = "property-extension"
+EXTENSION_TYPE_TOPLEVEL_PROPERTY_EXTENSION = "toplevel-property-extension"
+
+
+EXTENSION_TYPE = [
+    EXTENSION_TYPE_NEW_SDO,
+    EXTENSION_TYPE_NEW_SCO,
+    EXTENSION_TYPE_NEW_SRO,
+    EXTENSION_TYPE_PROPERTY_EXTENSION,
+    EXTENSION_TYPE_TOPLEVEL_PROPERTY_EXTENSION
+]
+
+
 GROUPING_CONTEXT_SUSPICIOUS_ACTIVITY = "suspicious-activity"
 GROUPING_CONTEXT_MALWARE_ANALYSIS = "malware-analysis"
 GROUPING_CONTEXT_UNSPECIFIED = "unspecified"