diff --git a/src/appengine/handlers/crash_query.py b/src/appengine/handlers/crash_query.py
index 7427ab644d..49a452dab6 100644
--- a/src/appengine/handlers/crash_query.py
+++ b/src/appengine/handlers/crash_query.py
@@ -56,10 +56,13 @@ def post(self):
 
     duplicate_testcase = data_handler.find_testcase(
         project, state.crash_type, state.crash_state, security_flag)
-    if (duplicate_testcase and duplicate_testcase.security_flag and
-        not access.can_user_access_testcase(duplicate_testcase)):
-      # Do not disclose the existence or identifiers of a security-confidential
-      # testcase to a user who is not allowed to access it.
+    if duplicate_testcase and not access.can_user_access_testcase(
+        duplicate_testcase):
+      # find_testcase only filters by project/crash signature, not by
+      # whether the caller has any relationship to that project. Without
+      # this check, any authenticated user could pass an arbitrary project
+      # name and learn whether a matching testcase exists there, plus its
+      # duplicate_id/bug_id, regardless of project ownership.
       duplicate_testcase = None
     if duplicate_testcase:
       result['result'] = 'duplicate'
diff --git a/src/clusterfuzz/_internal/tests/appengine/handlers/crash_query_test.py b/src/clusterfuzz/_internal/tests/appengine/handlers/crash_query_test.py
index e78a2f4195..b2e3c6c9e5 100644
--- a/src/clusterfuzz/_internal/tests/appengine/handlers/crash_query_test.py
+++ b/src/clusterfuzz/_internal/tests/appengine/handlers/crash_query_test.py
@@ -161,6 +161,37 @@ def test_duplicate_security_flag_no_access(self):
         'security': True,
     }, response.json)
 
+  def test_duplicate_not_disclosed_without_project_access(self):
+    """A non-security-flagged duplicate belonging to a project the caller
+    has no relationship to is not disclosed either. find_testcase only
+    filters by project name and crash signature, so without this check any
+    authenticated user could learn about (and get the bug_id for) another
+    project's testcases just by guessing/knowing its project name and crash
+    signature."""
+    self.mock.can_user_access_testcase.return_value = False
+    t = data_types.Testcase(
+        open=True,
+        status='Processed',
+        crash_state='Foo\nBar\nMain\n',
+        crash_type='Heap-buffer-overflow\nWRITE 4',
+        project_name='someone-elses-project',
+        security_flag=False,
+        bug_information='1337')
+    t.put()
+
+    response = self.app.post_json(
+        '/', {
+            'project': 'someone-elses-project',
+            'fuzz_target': 'target',
+            'stacktrace': TEST_STACKTRACE_OVERFLOW,
+        })
+    self.assertEqual({
+        'result': 'new',
+        'type': 'Heap-buffer-overflow\nWRITE 4',
+        'state': 'Foo\nBar\nMain\n',
+        'security': True,
+    }, response.json)
+
   def test_oom(self):
     """Test OOM parsing."""
     response = self.app.post_json(