diff --git a/src/sentry/workflow_engine/endpoints/organization_open_periods.py b/src/sentry/workflow_engine/endpoints/organization_open_periods.py
index 3cf0efbc769f56..cc122a88c2ffe2 100644
--- a/src/sentry/workflow_engine/endpoints/organization_open_periods.py
+++ b/src/sentry/workflow_engine/endpoints/organization_open_periods.py
@@ -41,7 +41,7 @@ class OrganizationOpenPeriodsEndpoint(OrganizationEndpoint):
     permission_classes = (OrganizationDetectorPermission,)
 
     def get_group_from_detector_id(
-        self, detector_id: str, organization: Organization
+        self, request: Request, detector_id: str, organization: Organization
     ) -> Group | None:
         validated_detector_id = to_valid_int_id("detectorId", detector_id)
         try:
@@ -56,13 +56,18 @@ def get_group_from_detector_id(
         if detector.project.organization_id != organization.id:
             raise ValidationError({"detectorId": "Detector not found"})
 
+        if not request.access.has_project_access(detector.project):
+            raise ValidationError({"detectorId": "Detector not found"})
+
         detector_group = (
             DetectorGroup.objects.filter(detector=detector).order_by("-date_added").first()
         )
 
         return detector_group.group if detector_group else None
 
-    def get_group_from_group_id(self, group_id: str, organization: Organization) -> Group | None:
+    def get_group_from_group_id(
+        self, request: Request, group_id: str, organization: Organization
+    ) -> Group | None:
         validated_group_id = to_valid_int_id("groupId", group_id)
         try:
             group = Group.objects.select_related("project").get(id=validated_group_id)
@@ -72,6 +77,9 @@ def get_group_from_group_id(self, group_id: str, organization: Organization) ->
         if group.project.organization_id != organization.id:
             raise ValidationError({"groupId": "Group not found"})
 
+        if not request.access.has_project_access(group.project):
+            raise ValidationError({"groupId": "Group not found"})
+
         return group
 
     @extend_schema(
@@ -134,10 +142,10 @@ def get(self, request: Request, organization: Organization) -> Response:
             raise ValidationError({"detail": "Must provide only one of detectorId or groupId"})
 
         target_group: Group | None = (
-            self.get_group_from_detector_id(detector_id_param, organization)
+            self.get_group_from_detector_id(request, detector_id_param, organization)
             if detector_id_param
             else (
-                self.get_group_from_group_id(group_id_param, organization)
+                self.get_group_from_group_id(request, group_id_param, organization)
                 if group_id_param
                 else None
             )
diff --git a/tests/sentry/workflow_engine/endpoints/test_organization_open_periods.py b/tests/sentry/workflow_engine/endpoints/test_organization_open_periods.py
index 0ce126959d0fc1..4d5f68cc0f5314 100644
--- a/tests/sentry/workflow_engine/endpoints/test_organization_open_periods.py
+++ b/tests/sentry/workflow_engine/endpoints/test_organization_open_periods.py
@@ -84,6 +84,68 @@ def test_open_periods_group_id(self) -> None:
     def test_validation_error_when_missing_params(self) -> None:
         self.get_error_response(*self.get_url_args(), status_code=400)
 
+    def test_group_id_returns_400_when_user_lacks_project_access(self) -> None:
+        self.organization.flags.allow_joinleave = False
+        self.organization.save()
+
+        restricted_team = self.create_team(organization=self.organization)
+        restricted_project = self.create_project(
+            organization=self.organization, teams=[restricted_team]
+        )
+        restricted_group = self.create_group(
+            project=restricted_project,
+            type=MetricIssue.type_id,
+            priority=PriorityLevel.LOW,
+        )
+
+        other_team = self.create_team(organization=self.organization)
+        outsider = self.create_user()
+        self.create_member(
+            user=outsider,
+            organization=self.organization,
+            role="member",
+            teams=[other_team],
+        )
+        self.login_as(user=outsider)
+
+        self.get_error_response(
+            *self.get_url_args(),
+            qs_params={"groupId": restricted_group.id},
+            status_code=400,
+        )
+
+    def test_detector_id_returns_400_when_user_lacks_project_access(self) -> None:
+        self.organization.flags.allow_joinleave = False
+        self.organization.save()
+
+        restricted_team = self.create_team(organization=self.organization)
+        restricted_project = self.create_project(
+            organization=self.organization, teams=[restricted_team]
+        )
+        restricted_detector = self.create_detector(project=restricted_project)
+        restricted_group = self.create_group(
+            project=restricted_project,
+            type=MetricIssue.type_id,
+            priority=PriorityLevel.LOW,
+        )
+        DetectorGroup.objects.create(detector=restricted_detector, group=restricted_group)
+
+        other_team = self.create_team(organization=self.organization)
+        outsider = self.create_user()
+        self.create_member(
+            user=outsider,
+            organization=self.organization,
+            role="member",
+            teams=[other_team],
+        )
+        self.login_as(user=outsider)
+
+        self.get_error_response(
+            *self.get_url_args(),
+            qs_params={"detectorId": restricted_detector.id},
+            status_code=400,
+        )
+
     def test_open_periods_resolved_group(self) -> None:
         self.group.status = GroupStatus.RESOLVED
         self.group.save()