diff --git a/stix2/test/v21/test_external_reference.py b/stix2/test/v21/test_external_reference.py
index d192a11..f5a3e23 100644
--- a/stix2/test/v21/test_external_reference.py
+++ b/stix2/test/v21/test_external_reference.py
@@ -120,3 +120,13 @@ def test_external_reference_source_required():
 
     assert excinfo.value.cls == stix2.v21.ExternalReference
     assert excinfo.value.properties == ["source_name"]
+
+
+def test_external_reference_bad_hash():
+    with pytest.raises(stix2.exceptions.InvalidValueError):
+        stix2.v21.ExternalReference(
+            source_name="ACME Threat Intel",
+            hashes={
+                "SHA-123": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+            }
+        )
\ No newline at end of file
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index cf3a3b3..ac8daf1 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -4,6 +4,7 @@ from collections import OrderedDict
 
 from ..base import _STIXBase
 from ..custom import _custom_marking_builder
+from ..exceptions import InvalidValueError
 from ..markings import _MarkingsMixin
 from ..markings.utils import check_tlp_marking
 from ..properties import (
@@ -28,10 +29,26 @@ class ExternalReference(_STIXBase):
         ('external_id', StringProperty()),
     ])
 
+    # This is hash-algorithm-ov
+    _LEGAL_HASHES = {
+        "MD5", "SHA-1", "SHA-256", "SHA-512", "SHA3-256", "SHA3-512", "SSDEEP",
+        "TLSH",
+    }
+
     def _check_object_constraints(self):
         super(ExternalReference, self)._check_object_constraints()
         self._check_at_least_one_property(['description', 'external_id', 'url'])
 
+        if "hashes" in self:
+            if any(
+                hash_ not in self._LEGAL_HASHES
+                for hash_ in self["hashes"]
+            ):
+                raise InvalidValueError(
+                    ExternalReference, "hashes",
+                    "Hash algorithm names must be members of hash-algorithm-ov",
+                )
+
 
 class KillChainPhase(_STIXBase):
     # TODO: Add link