  Pipeline bursting, production lines shut down, frenzy traffic, trains
confrontation, nuclear reactor shut down, disrupted electric supply,
interrupted oxygen supply in ICU - these catastrophic events could result
because of an erroneous SCADA system/ Industrial Control System(ICS). SCADA
systems have become an essential part of automated control and monitoring of
many of the Critical Infrastructures (CI). Modern SCADA systems have evolved
from standalone systems into sophisticated complex, open systems, connected to
the Internet. This geographically distributed modern SCADA system is vulnerable
to threats and cyber attacks. In this paper, we first review the SCADA system
architectures that have been proposed/implemented followed by attacks on such
systems to understand and highlight the evolving security needs for SCADA
systems. A short investigation of the current state of intrusion detection
techniques in SCADA systems is done , followed by a brief study of testbeds for
SCADA systems. The cloud and Internet of things (IoT) based SCADA systems are
studied by analysing the architecture of modern SCADA systems. This review
paper ends by highlighting the critical research problems that need to be
resolved to close the gaps in the security of SCADA systems.
