Escape special characters in search by recipient

SQLAlchemy handles escaping anything that could allow a SQL injection
attack. But it doesn’t escape the characters used for wildcard
searching. This is the reason we’re able to do `.like('%example%')`
at all.

But we shouldn’t be letting our users search with wildcard characters,
so we need to escape them. Which is what this commit does.

app/dao/notifications_dao.py

@@ -455,6 +455,12 @@ def dao_get_notifications_by_to_field(service_id, search_term, notification_type
     else:
         raise InvalidRequest("Only email and SMS can use search by recipient", 400)
 
+    for special_character in {'_', '%', '/'}:
+        normalised = normalised.replace(
+            special_character,
+            '\\{}'.format(special_character)
+        )
+
     filters = [
         Notification.service_id == service_id,
         Notification.normalised_to.like("%{}%".format(normalised)),

tests/app/dao/notification_dao/test_notification_dao.py

@@ -1769,6 +1769,45 @@ def test_dao_get_notifications_by_to_field_matches_partial_emails(sample_email_t
     assert notification_2.id not in notification_ids
 
 
+@pytest.mark.parametrize('search_term, expected_result_count', [
+    ('foobar', 1),
+    ('foo', 2),
+    ('bar', 2),
+    ('foo%', 1),
+    ('%%bar', 1),
+    ('%_', 1),
+    ('%', 2),
+    ('_', 1),
+    ('/', 1),
+    ('%foo', 0),
+    ('%_%', 0),
+    ('example.com', 4),
+])
+def test_dao_get_notifications_by_to_field_escapes(
+    sample_email_template,
+    search_term,
+    expected_result_count,
+):
+
+    for email_address in {
+        'foo%_@example.com',
+        '%%bar@example.com',
+        'foobar@example.com',
+        '/@example.com',
+    }:
+        create_notification(
+            template=sample_email_template,
+            to_field=email_address,
+            normalised_to=email_address,
+        )
+
+    assert len(dao_get_notifications_by_to_field(
+        sample_email_template.service_id,
+        search_term,
+        notification_type='email',
+    )) == expected_result_count
+
+
 @pytest.mark.parametrize('search_term', [
     '001',
     '100',