diff --git a/stix2/v21/__init__.py b/stix2/v21/__init__.py
index 77a28ee..70f2a79 100644
--- a/stix2/v21/__init__.py
+++ b/stix2/v21/__init__.py
@@ -21,7 +21,7 @@ from .bundle import Bundle
 from .common import (
     TLP_AMBER, TLP_GREEN, TLP_RED, TLP_WHITE, CustomMarking, ExternalReference,
     GranularMarking, KillChainPhase, LanguageContent, MarkingDefinition,
-    StatementMarking, TLPMarking,
+    StatementMarking, STIXExtension, TLPMarking,
 )
 from .observables import (
     URL, AlternateDataStream, ArchiveExt, Artifact, AutonomousSystem,
@@ -65,6 +65,7 @@ OBJ_MAP = {
     'threat-actor': ThreatActor,
     'tool': Tool,
     'sighting': Sighting,
+    'stix-extension': STIXExtension,
     'vulnerability': Vulnerability,
 }
 
@@ -121,7 +122,7 @@ __all__ = """
 
     TLP_AMBER, TLP_GREEN, TLP_RED, TLP_WHITE, CustomMarking, ExternalReference,
     GranularMarking, KillChainPhase, LanguageContent, MarkingDefinition,
-    StatementMarking, TLPMarking,
+    StatementMarking, STIXExtension, TLPMarking,
 
     URL, AlternateDataStream, ArchiveExt, Artifact, AutonomousSystem,
     CustomExtension, CustomObservable, Directory, DomainName, EmailAddress,
diff --git a/stix2/v21/base.py b/stix2/v21/base.py
index 8b5a5f1..830a209 100644
--- a/stix2/v21/base.py
+++ b/stix2/v21/base.py
@@ -6,7 +6,11 @@ from ..base import (
 
 
 class _STIXBase21(_STIXBase):
-    pass
+
+    def __init__(self, **kwargs):
+        if 'extensions' in self._properties:
+            self._properties['extensions'].allow_custom = kwargs.get('allow_custom', False)
+        super(_STIXBase21, self).__init__(**kwargs)
 
 
 class _Observable(_Observable, _STIXBase21):
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index eae0865..60c4068 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -197,12 +197,13 @@ class MarkingDefinition(_STIXBase21, _MarkingsMixin):
         ('id', IDProperty(_type, spec_version='2.1')),
         ('created_by_ref', ReferenceProperty(valid_types='identity', spec_version='2.1')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond', precision_constraint='min')),
-        ('definition_type', StringProperty(required=True)),
+        ('definition_type', StringProperty()),
         ('name', StringProperty()),
-        ('definition', MarkingProperty(required=True)),
+        ('definition', MarkingProperty()),
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def __init__(self, **kwargs):
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index a359aa6..d538852 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -12,9 +12,10 @@ from ..exceptions import (
     InvalidValueError, PropertyPresenceError, STIXDeprecationWarning,
 )
 from ..properties import (
-    BooleanProperty, EnumProperty, FloatProperty, IDProperty, IntegerProperty,
-    ListProperty, ObservableProperty, PatternProperty, ReferenceProperty,
-    StringProperty, TimestampProperty, TypeProperty,
+    BooleanProperty, EnumProperty, ExtensionsProperty, FloatProperty,
+    IDProperty, IntegerProperty, ListProperty, ObservableProperty,
+    PatternProperty, ReferenceProperty, StringProperty, TimestampProperty,
+    TypeProperty,
 )
 from ..utils import NOW
 from .base import _DomainObject
@@ -45,6 +46,7 @@ class AttackPattern(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -74,6 +76,7 @@ class Campaign(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -109,6 +112,7 @@ class CourseOfAction(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -136,6 +140,7 @@ class Grouping(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -165,6 +170,7 @@ class Identity(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -197,6 +203,7 @@ class Indicator(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def __init__(self, *args, **kwargs):
@@ -254,6 +261,7 @@ class Infrastructure(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -296,6 +304,7 @@ class IntrusionSet(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -340,6 +349,7 @@ class Location(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -449,6 +459,7 @@ class Malware(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -504,6 +515,7 @@ class MalwareAnalysis(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -536,6 +548,7 @@ class Note(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -564,6 +577,7 @@ class ObservedData(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def __init__(self, *args, **kwargs):
@@ -628,6 +642,7 @@ class Opinion(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -656,6 +671,7 @@ class Report(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -692,6 +708,7 @@ class ThreatActor(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     def _check_object_constraints(self):
@@ -731,6 +748,7 @@ class Tool(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -756,6 +774,7 @@ class Vulnerability(_DomainObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
 
@@ -807,6 +826,7 @@ def CustomObject(type='x-custom-type', properties=None):
                 ('external_references', ListProperty(ExternalReference)),
                 ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
                 ('granular_markings', ListProperty(GranularMarking)),
+                ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=type)),
             ],
             sorted([x for x in properties if x[0].startswith('x_')], key=lambda x: x[0]),
         ]))
diff --git a/stix2/v21/sro.py b/stix2/v21/sro.py
index d287373..d0e07f8 100644
--- a/stix2/v21/sro.py
+++ b/stix2/v21/sro.py
@@ -3,8 +3,9 @@
 from collections import OrderedDict
 
 from ..properties import (
-    BooleanProperty, IDProperty, IntegerProperty, ListProperty,
-    ReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
+    BooleanProperty, ExtensionsProperty, IDProperty, IntegerProperty,
+    ListProperty, ReferenceProperty, StringProperty, TimestampProperty,
+    TypeProperty,
 )
 from ..utils import NOW
 from .base import _RelationshipObject
@@ -39,6 +40,7 @@ class Relationship(_RelationshipObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     # Explicitly define the first three kwargs to make readable Relationship declarations.
@@ -95,6 +97,7 @@ class Sighting(_RelationshipObject):
         ('external_references', ListProperty(ExternalReference)),
         ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
         ('granular_markings', ListProperty(GranularMarking)),
+        ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
     ])
 
     # Explicitly define the first kwargs to make readable Sighting declarations.