diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index 1d97261..0f4a5a7 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -173,7 +173,7 @@ class Identity(STIXDomainObject):
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
         ('roles', ListProperty(StringProperty)),
-        ('identity_class', StringProperty(required=True)),
+        ('identity_class', StringProperty()),
         ('sectors', ListProperty(StringProperty)),
         ('contact_information', StringProperty()),
         ('revoked', BooleanProperty(default=lambda: False)),
@@ -202,7 +202,7 @@ class Indicator(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty()),
         ('description', StringProperty()),
-        ('indicator_types', ListProperty(StringProperty, required=True)),
+        ('indicator_types', ListProperty(StringProperty)),
         ('pattern', PatternProperty(required=True)),
         ('pattern_type', StringProperty(required=True)),
         ('pattern_version', StringProperty()),
@@ -269,7 +269,7 @@ class Infrastructure(STIXDomainObject):
         ('granular_markings', ListProperty(GranularMarking)),
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
-        ('infrastructure_types', ListProperty(StringProperty, required=True)),
+        ('infrastructure_types', ListProperty(StringProperty)),
         ('aliases', ListProperty(StringProperty)),
         ('kill_chain_phases', ListProperty(KillChainPhase)),
         ('first_seen', TimestampProperty()),
@@ -454,7 +454,7 @@ class Malware(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty()),
         ('description', StringProperty()),
-        ('malware_types', ListProperty(StringProperty, required=True)),
+        ('malware_types', ListProperty(StringProperty)),
         ('is_family', BooleanProperty(required=True)),
         ('aliases', ListProperty(StringProperty)),
         ('kill_chain_phases', ListProperty(KillChainPhase)),
@@ -672,7 +672,7 @@ class Report(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
-        ('report_types', ListProperty(StringProperty, required=True)),
+        ('report_types', ListProperty(StringProperty)),
         ('published', TimestampProperty(required=True)),
         ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1'), required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
@@ -701,7 +701,7 @@ class ThreatActor(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
-        ('threat_actor_types', ListProperty(StringProperty, required=True)),
+        ('threat_actor_types', ListProperty(StringProperty)),
         ('aliases', ListProperty(StringProperty)),
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
@@ -748,7 +748,7 @@ class Tool(STIXDomainObject):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
-        ('tool_types', ListProperty(StringProperty, required=True)),
+        ('tool_types', ListProperty(StringProperty)),
         ('aliases', ListProperty(StringProperty)),
         ('kill_chain_phases', ListProperty(KillChainPhase)),
         ('tool_version', StringProperty()),

Because of "unknown", several properties have been changed to optional
See oasis-tcs/cti-stix2#211
diff --git a/stix2/test/v21/test_indicator.py b/stix2/test/v21/test_indicator.py
index 152f253..6965989 100644
--- a/stix2/test/v21/test_indicator.py
+++ b/stix2/test/v21/test_indicator.py
@@ -14,9 +14,6 @@ EXPECTED_INDICATOR = """{
     "id": "indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7",
     "created": "2017-01-01T00:00:01.000Z",
     "modified": "2017-01-01T00:00:01.000Z",
-    "indicator_types": [
-        "malicious-activity"
-    ],
     "pattern": "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
     "pattern_type": "stix",
     "pattern_version": "2.1",
@@ -29,7 +26,6 @@ EXPECTED_INDICATOR_REPR = "Indicator(" + " ".join("""
     id='indicator--a740531e-63ff-4e49-a9e1-a0a3eed0e3e7',
     created='2017-01-01T00:00:01.000Z',
     modified='2017-01-01T00:00:01.000Z',
-    indicator_types=['malicious-activity'],
     pattern="[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
     pattern_type='stix',
     pattern_version='2.1',
@@ -49,7 +45,6 @@ def test_indicator_with_all_required_properties():
         pattern="[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
         pattern_type="stix",
         valid_from=epoch,
-        indicator_types=['malicious-activity'],
     )
 
     assert ind.revoked is False
@@ -103,8 +98,8 @@ def test_indicator_required_properties():
         stix2.v21.Indicator()
 
     assert excinfo.value.cls == stix2.v21.Indicator
-    assert excinfo.value.properties == ["indicator_types", "pattern", "pattern_type", "valid_from"]
-    assert str(excinfo.value) == "No values for required properties for Indicator: (indicator_types, pattern, pattern_type, valid_from)."
+    assert excinfo.value.properties == ["pattern", "pattern_type", "valid_from"]
+    assert str(excinfo.value) == "No values for required properties for Indicator: (pattern, pattern_type, valid_from)."
 
 
 def test_indicator_required_property_pattern():
@@ -163,9 +158,6 @@ def test_created_modified_time_are_identical_by_default():
             "id": INDICATOR_ID,
             "created": "2017-01-01T00:00:01Z",
             "modified": "2017-01-01T00:00:01Z",
-            "indicator_types": [
-                "malicious-activity",
-            ],
             "pattern": "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']",
             "pattern_type": "stix",
             "valid_from": "1970-01-01T00:00:01Z",
@@ -181,7 +173,6 @@ def test_parse_indicator(data):
     assert idctr.created == dt.datetime(2017, 1, 1, 0, 0, 1, tzinfo=pytz.utc)
     assert idctr.modified == dt.datetime(2017, 1, 1, 0, 0, 1, tzinfo=pytz.utc)
     assert idctr.valid_from == dt.datetime(1970, 1, 1, 0, 0, 1, tzinfo=pytz.utc)
-    assert idctr.indicator_types[0] == "malicious-activity"
     assert idctr.pattern == "[file:hashes.MD5 = 'd41d8cd98f00b204e9800998ecf8427e']"
 
 
diff --git a/stix2/test/v21/test_infrastructure.py b/stix2/test/v21/test_infrastructure.py
index 30632bb..3e9feb7 100644
--- a/stix2/test/v21/test_infrastructure.py
+++ b/stix2/test/v21/test_infrastructure.py
@@ -13,10 +13,7 @@ EXPECTED_INFRASTRUCTURE = """{
     "id": "infrastructure--3000ae1b-784c-f03d-8abc-0a625b2ff018",
     "created": "2017-01-01T12:34:56.000Z",
     "modified": "2017-01-01T12:34:56.000Z",
-    "name": "Poison Ivy C2",
-    "infrastructure_types": [
-        "command-and-control"
-    ]
+    "name": "Poison Ivy C2"
 }"""
 
 
@@ -29,7 +26,6 @@ def test_infrastructure_with_all_required_properties():
         created=now,
         modified=now,
         name="Poison Ivy C2",
-        infrastructure_types=["command-and-control"],
     )
 
     assert str(infra) == EXPECTED_INFRASTRUCTURE
@@ -76,7 +72,7 @@ def test_infrastructure_required_properties():
         stix2.v21.Infrastructure()
 
     assert excinfo.value.cls == stix2.v21.Infrastructure
-    assert excinfo.value.properties == ["infrastructure_types", "name"]
+    assert excinfo.value.properties == ["name"]
 
 
 def test_infrastructure_required_property_name():
@@ -105,7 +101,6 @@ def test_invalid_kwarg_to_infrastructure():
             "id": INFRASTRUCTURE_ID,
             "created": "2017-01-01T12:34:56.000Z",
             "modified": "2017-01-01T12:34:56.000Z",
-            "infrastructure_types": ["command-and-control"],
             "name": "Poison Ivy C2",
         },
     ],
@@ -118,7 +113,6 @@ def test_parse_infrastructure(data):
     assert infra.id == INFRASTRUCTURE_ID
     assert infra.created == dt.datetime(2017, 1, 1, 12, 34, 56, tzinfo=pytz.utc)
     assert infra.modified == dt.datetime(2017, 1, 1, 12, 34, 56, tzinfo=pytz.utc)
-    assert infra.infrastructure_types == ['command-and-control']
     assert infra.name == 'Poison Ivy C2'
 
 
diff --git a/stix2/test/v21/test_malware.py b/stix2/test/v21/test_malware.py
index 53838c9..a9158cf 100644
--- a/stix2/test/v21/test_malware.py
+++ b/stix2/test/v21/test_malware.py
@@ -1,5 +1,5 @@
 import datetime as dt
-import re
+import json
 
 import pytest
 import pytz
@@ -16,9 +16,6 @@ EXPECTED_MALWARE = """{
     "created": "2016-05-12T08:17:27.000Z",
     "modified": "2016-05-12T08:17:27.000Z",
     "name": "Cryptolocker",
-    "malware_types": [
-        "ransomware"
-    ],
     "is_family": false
 }"""
 
@@ -31,7 +28,6 @@ def test_malware_with_all_required_properties():
         id=MALWARE_ID,
         created=now,
         modified=now,
-        malware_types=["ransomware"],
         name="Cryptolocker",
         is_family=False,
     )
@@ -80,7 +76,7 @@ def test_malware_required_properties():
         stix2.v21.Malware()
 
     assert excinfo.value.cls == stix2.v21.Malware
-    assert excinfo.value.properties == ["is_family", "malware_types"]
+    assert excinfo.value.properties == ["is_family"]
 
 
 def test_malware_required_property_name():
@@ -116,7 +112,6 @@ def test_invalid_kwarg_to_malware():
             "id": MALWARE_ID,
             "created": "2016-05-12T08:17:27.000Z",
             "modified": "2016-05-12T08:17:27.000Z",
-            "malware_types": ["ransomware"],
             "name": "Cryptolocker",
             "is_family": False,
         },
@@ -130,13 +125,14 @@ def test_parse_malware(data):
     assert mal.id == MALWARE_ID
     assert mal.created == dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
     assert mal.modified == dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
-    assert mal.malware_types == ['ransomware']
     assert mal.name == 'Cryptolocker'
     assert not mal.is_family
 
 
-def test_parse_malware_invalid_labels():
-    data = re.compile('\\[.+\\]', re.DOTALL).sub('1', EXPECTED_MALWARE)
+def test_parse_malware_invalid_types():
+    data = json.loads(EXPECTED_MALWARE)
+    data["malware_types"] = 1  # Oops, not a list
+    data = json.dumps(data)
     with pytest.raises(InvalidValueError) as excinfo:
         stix2.parse(data)
     assert "Invalid value for Malware 'malware_types'" in str(excinfo.value)

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest
apt-get update apt-get install -y gcc
3.7
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
antlr4-python3-runtime==4.9.3
attrs==24.2.0
Babel==2.14.0
backcall==0.2.0
beautifulsoup4==4.13.3
bleach==6.0.0
bump2version==1.0.1
bumpversion==0.6.0
cachetools==5.5.2
certifi @ file:///croot/certifi_1671487769961/work/certifi
cfgv==3.3.1
chardet==5.2.0
charset-normalizer==3.4.1
colorama==0.4.6
coverage==7.2.7
decorator==5.1.1
defusedxml==0.7.1
distlib==0.3.9
docutils==0.17.1
entrypoints==0.4
exceptiongroup==1.2.2
fastjsonschema==2.21.1
filelock==3.12.2
identify==2.5.24
idna==3.10
imagesize==1.4.1
importlib-metadata==6.7.0
importlib-resources==5.12.0
iniconfig==2.0.0
ipython==7.34.0
jedi==0.19.2
Jinja2==3.1.6
jsonschema==4.17.3
jupyter_client==7.4.9
jupyter_core==4.12.0
jupyterlab-pygments==0.2.2
MarkupSafe==2.1.5
matplotlib-inline==0.1.6
mistune==3.0.2
nbclient==0.7.4
nbconvert==7.6.0
nbformat==5.8.0
nbsphinx==0.4.3
nest-asyncio==1.6.0
nodeenv==1.9.1
packaging==24.0
pandocfilters==1.5.1
parso==0.8.4
pexpect==4.9.0
pickleshare==0.7.5
pkgutil_resolve_name==1.3.10
platformdirs==4.0.0
pluggy==1.2.0
pre-commit==2.21.0
prompt_toolkit==3.0.48
ptyprocess==0.7.0
Pygments==2.17.2
pyproject-api==1.5.3
pyrsistent==0.19.3
pytest==7.4.4
pytest-cov==4.1.0
python-dateutil==2.9.0.post0
pytz==2025.2
PyYAML==6.0.1
pyzmq==26.2.1
requests==2.31.0
simplejson==3.20.1
six==1.17.0
snowballstemmer==2.2.0
soupsieve==2.4.1
Sphinx==1.8.6
sphinx-prompt==1.5.0
sphinxcontrib-serializinghtml==1.1.5
sphinxcontrib-websupport==1.2.4
-e git+https://github.com/oasis-open/cti-python-stix2.git@c2b71672f5504b975b113e3671e3279ceb503a65#egg=stix2
stix2-patterns==2.0.0
tinycss2==1.2.1
tomli==2.0.1
tornado==6.2
tox==4.8.0
traitlets==5.9.0
typing_extensions==4.7.1
urllib3==2.0.7
virtualenv==20.26.6
wcwidth==0.2.13
webencodings==0.5.1
zipp==3.15.0

name: cti-python-stix2
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2022.12.7=py37h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=22.3.1=py37h06a4308_0
  - python=3.7.16=h7a1cb2a_0
  - readline=8.2=h5eee18b_0
  - setuptools=65.6.3=py37h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.38.4=py37h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - antlr4-python3-runtime==4.9.3
      - attrs==24.2.0
      - babel==2.14.0
      - backcall==0.2.0
      - beautifulsoup4==4.13.3
      - bleach==6.0.0
      - bump2version==1.0.1
      - bumpversion==0.6.0
      - cachetools==5.5.2
      - cfgv==3.3.1
      - chardet==5.2.0
      - charset-normalizer==3.4.1
      - colorama==0.4.6
      - coverage==7.2.7
      - decorator==5.1.1
      - defusedxml==0.7.1
      - distlib==0.3.9
      - docutils==0.17.1
      - entrypoints==0.4
      - exceptiongroup==1.2.2
      - fastjsonschema==2.21.1
      - filelock==3.12.2
      - identify==2.5.24
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==6.7.0
      - importlib-resources==5.12.0
      - iniconfig==2.0.0
      - ipython==7.34.0
      - jedi==0.19.2
      - jinja2==3.1.6
      - jsonschema==4.17.3
      - jupyter-client==7.4.9
      - jupyter-core==4.12.0
      - jupyterlab-pygments==0.2.2
      - markupsafe==2.1.5
      - matplotlib-inline==0.1.6
      - mistune==3.0.2
      - nbclient==0.7.4
      - nbconvert==7.6.0
      - nbformat==5.8.0
      - nbsphinx==0.4.3
      - nest-asyncio==1.6.0
      - nodeenv==1.9.1
      - packaging==24.0
      - pandocfilters==1.5.1
      - parso==0.8.4
      - pexpect==4.9.0
      - pickleshare==0.7.5
      - pkgutil-resolve-name==1.3.10
      - platformdirs==4.0.0
      - pluggy==1.2.0
      - pre-commit==2.21.0
      - prompt-toolkit==3.0.48
      - ptyprocess==0.7.0
      - pygments==2.17.2
      - pyproject-api==1.5.3
      - pyrsistent==0.19.3
      - pytest==7.4.4
      - pytest-cov==4.1.0
      - python-dateutil==2.9.0.post0
      - pytz==2025.2
      - pyyaml==6.0.1
      - pyzmq==26.2.1
      - requests==2.31.0
      - simplejson==3.20.1
      - six==1.17.0
      - snowballstemmer==2.2.0
      - soupsieve==2.4.1
      - sphinx==1.8.6
      - sphinx-prompt==1.5.0
      - sphinxcontrib-serializinghtml==1.1.5
      - sphinxcontrib-websupport==1.2.4
      - stix2-patterns==2.0.0
      - tinycss2==1.2.1
      - tomli==2.0.1
      - tornado==6.2
      - tox==4.8.0
      - traitlets==5.9.0
      - typing-extensions==4.7.1
      - urllib3==2.0.7
      - virtualenv==20.26.6
      - wcwidth==0.2.13
      - webencodings==0.5.1
      - zipp==3.15.0
prefix: /opt/conda/envs/cti-python-stix2

stix2/test/v21/test_indicator.py::test_indicator_with_all_required_properties stix2/test/v21/test_indicator.py::test_indicator_required_properties stix2/test/v21/test_indicator.py::test_parse_indicator[{\n stix2/test/v21/test_indicator.py::test_parse_indicator[data1] stix2/test/v21/test_infrastructure.py::test_infrastructure_with_all_required_properties stix2/test/v21/test_infrastructure.py::test_infrastructure_required_properties stix2/test/v21/test_infrastructure.py::test_parse_infrastructure[{\n stix2/test/v21/test_infrastructure.py::test_parse_infrastructure[data1] stix2/test/v21/test_infrastructure.py::test_parse_infrastructure_kill_chain_phases stix2/test/v21/test_infrastructure.py::test_parse_infrastructure_clean_kill_chain_phases stix2/test/v21/test_malware.py::test_malware_with_all_required_properties stix2/test/v21/test_malware.py::test_malware_required_properties stix2/test/v21/test_malware.py::test_parse_malware[{\n stix2/test/v21/test_malware.py::test_parse_malware[data1] stix2/test/v21/test_malware.py::test_parse_malware_kill_chain_phases stix2/test/v21/test_malware.py::test_parse_malware_clean_kill_chain_phases
stix2/test/v21/test_indicator.py::test_indicator_stix20_invalid_pattern
stix2/test/v21/test_indicator.py::test_indicator_autogenerated_properties stix2/test/v21/test_indicator.py::test_indicator_type_must_be_indicator stix2/test/v21/test_indicator.py::test_indicator_id_must_start_with_indicator stix2/test/v21/test_indicator.py::test_indicator_required_property_pattern stix2/test/v21/test_indicator.py::test_indicator_created_ref_invalid_format stix2/test/v21/test_indicator.py::test_indicator_revoked_invalid stix2/test/v21/test_indicator.py::test_cannot_assign_to_indicator_attributes stix2/test/v21/test_indicator.py::test_invalid_kwarg_to_indicator stix2/test/v21/test_indicator.py::test_created_modified_time_are_identical_by_default stix2/test/v21/test_indicator.py::test_invalid_indicator_pattern stix2/test/v21/test_indicator.py::test_indicator_with_custom_embedded_objs stix2/test/v21/test_indicator.py::test_indicator_with_custom_embed_objs_extra_props_error stix2/test/v21/test_infrastructure.py::test_infrastructure_autogenerated_properties stix2/test/v21/test_infrastructure.py::test_infrastructure_type_must_be_infrastructure stix2/test/v21/test_infrastructure.py::test_infrastructure_id_must_start_with_infrastructure stix2/test/v21/test_infrastructure.py::test_infrastructure_required_property_name stix2/test/v21/test_infrastructure.py::test_invalid_kwarg_to_infrastructure stix2/test/v21/test_infrastructure.py::test_infrastructure_invalid_last_before_first stix2/test/v21/test_malware.py::test_malware_autogenerated_properties stix2/test/v21/test_malware.py::test_malware_type_must_be_malware stix2/test/v21/test_malware.py::test_malware_id_must_start_with_malware stix2/test/v21/test_malware.py::test_malware_required_property_name stix2/test/v21/test_malware.py::test_cannot_assign_to_malware_attributes stix2/test/v21/test_malware.py::test_invalid_kwarg_to_malware stix2/test/v21/test_malware.py::test_parse_malware_invalid_types stix2/test/v21/test_malware.py::test_malware_invalid_last_before_first stix2/test/v21/test_malware.py::test_malware_family_no_name stix2/test/v21/test_malware.py::test_malware_non_family_no_name
stix2/v21/sdo.py
stix2/v21/sdo.py