Background
HomeMatic is a home automation system consisting of various components for automating several parts of a building, including different sensors and actuators. The HomeMatic CCU2 is a central control unit, which is responsible for integrating these components with each other.
From the vendor’s website (eQ-3):
“The Homematic Central Control Unit CCU2 is responsible for numerous control, signalling and supervision functions across all areas of the Homematic system. The device includes a browser based user interface to the up the system. It can be controlled in the local network as well as via Internet. The Homematic software offers various configuration and application possibilities, so that there are no limits to creativity for installing your individual smart home.”
Issue Description
While analyzing the implementation of the CCU2’s web interface, one Directory Traversal vulnerabilities has been identified, which can be exploited in order to read files from the CCU2’s filesystem. This vulnerability can be exploited by unauthenticated attackers with access to the web interface.
The system provides a JSON API, which exposes various methods. While some methods require the connecting user to be authenticated, a number of methods can also be used anonymously.
A code review of the file api/methods/user/getlanguage.tcl revealed that the User.getLanguage method allows unauthenticated attackers to read the first line of an arbitrary file on the CCU2’s filesystem.
Please consider the following excerpt from the file api/methods/user/getlanguage.tcl:
if {[catch {set fp [open "/etc/config/userprofiles/$args(userName).lang" r]}] == 0} {
set data [read $fp]
set lang [split $data "\n"]
close $fp
} else {
set lang "0"
}
jsonrpc_response [lindex $lang 0]
Please note that the web server is running as root, which means that this vulnerability can be exploited in order to disclose the first line of any file on the target’s filesystem.
One particular way to exploit this issue is to read the file /tmp/event/subscriber.list, which contains existing session IDs. These session IDs can subsequently be used to impersonate other users on the system.
CVE
CVSSv3 Base Score
CVSS Base Score: 7.5
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Credit
Gregor Kopf of Secfault Security GmbH
Patrick Muench of SVA System Vertrieb Alexander GmbH
Disclaimer
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The contents of this advisory are copyright (c) 2017 Secfault Security GmbH & SVA System Vertrieb Alexander GmbH and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.