Found way to change directory in rbash!

[boris . valkovski]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/local/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -g -O2 
-Wno-parentheses -Wno-format-security
uname output: Linux bobi-HP-xw4300-Workstation 4.15.0-34-generic 
#37~16.04.1-Ubuntu SMP Tue Aug 28 10:44:06 UTC 2018 x86_64 x86_64 x86_64 
GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.4
Patch Level: 12
Release Status: release

Description:
        As you know, the cd command is restricted when running rbash.Well, 
        the PWD variable can be modified, so directory can be changed.
Repeat-By:
        I ran rbash, so cd was restricted.Then I tried to modify the PWD
        variable, and I changed the current working directory successfully.
Fix:
        Restrict modifying the PWD variable.