  Siemens produce a range of industrial human machine interface (HMI) screens
which allow operators to both view information about and control physical
processes. For scenarios where an operator cannot physically access the screen,
Siemens provide the SM@rtServer features on HMIs, which when activated provides
remote access either through their own Sm@rtClient application, or through
third party VNC client software. Through analysing this server, we discovered a
lack of protection against brute-force password attacks on basic devices. On
advanced devices which include a brute-force protection mechanism, we
discovered an attacker strategy that is able to evade the mechanism allowing
for unlimited password guess attempts with minimal effect on the guess rate.
This vulnerability has been assigned two CVEs - CVE-2020-15786 and
CVE-2020-157867. In this report, we provide an overview of this vulnerability,
discuss the impact of a successful exploitation and propose mitigations to
provide protection against this vulnerability. This report accompanies a demo
presented at CPSIoTSec 2020.
