diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 2be4c83..8575851 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -683,6 +683,11 @@ class _singlefileMailbox(Mailbox):
 _sync_close(new_file)
 # self._file is about to get replaced, so no need to sync.
 self._file.close()
+
+ # Ensure the new file's mode is the same as the old file's
+ mode = os.stat(self._path).st_mode
+ os.chmod(new_file.name, mode)
+
 try:
 os.rename(new_file.name, self._path)
 except OSError as e:
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 91c8983..85126e5 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -942,7 +942,26 @@ class TestMaildir(TestMailbox, unittest.TestCase):
 self._box._refresh()
 self.assertTrue(refreshed())
 
-class _TestMboxMMDF(TestMailbox):
+
+class _TestSingleFile(TestMailbox):
+ '''Common tests for single-file mailboxes'''
+
+ def test_permissions_after_flush(self):
+ # See issue #5346
+
+ # Make the mailbox world writable. It's unlikely that the
+ # mailbox would have these permissions after flush, because
+ # umask prevents it.
+ mode = os.stat(self._path).st_mode | 0o666
+ os.chmod(self._path, mode)
+
+ self._box.add(self._template % 0)
+ self._box.flush()
+
+ self.assertEqual(os.stat(self._path).st_mode, mode)
+
+
+class _TestMboxMMDF(_TestSingleFile):
 
 def tearDown(self):
 super().tearDown()
@@ -1217,7 +1236,7 @@ class TestMH(TestMailbox, unittest.TestCase):
 return os.path.join(self._path, '.mh_sequences.lock')
 
 
-class TestBabyl(TestMailbox, unittest.TestCase):
+class TestBabyl(_TestSingleFile, unittest.TestCase):
 
 _factory = lambda self, path, factory=None: mailbox.Babyl(path, factory)
 
</div><div class="naked_ctrl">
<form action="/index.cgi/contrast" method="get" name="gate">
<p><a href="http://altstyle.alfasado.net">AltStyle</a> によって変換されたページ <a href="https://bugs.python.org/file26166/issue5346.patch">(-&gt;オリジナル)</a>
/ <label>アドレス: <input type="text" name="naked_post_url" value="https://bugs.python.org/file26166/issue5346.patch" size="22" /></label> <label>モード: <select name="naked_post_mode">
<option value="default">デフォルト</option>
<option value="speech">音声ブラウザ</option>
<option value="ruby">ルビ付き</option>
<option value="contrast" selected="selected">配色反転</option>
<option value="larger-text">文字拡大</option>
<option value="mobile">モバイル</option>
</select>
<input type="submit" value="表示" />
</p>
</form>
</div>