66
Security Challenges in Embedded Systems
DIMITRIOS N. SERPANOS, University of Patras, Greece and Industrial Systems Institute/RC
‘Athena’, Greece
ARTEMIOS G. VOYIATZIS, Industrial Systems Institute/RC ‘Athena’, Greece
Embedded systems security is a significant requirement in emerging environments, considering the
increasing deployment of embedded systems in several application domains. The large number of deployed
embedded systems, their limited resources and their increasing complexity render systems vulnerable to an
increasing number of threats. Additionally, the involvement of sensitive, often private, information and the
expectation for safe and dependable embedded platforms lead to strong security requirements, even legal
ones, which require new technologies for their provision. In this article, we provide an overview of embedded
security issues, used methods and technologies, identifying important challenges in this emerging field.
Categories and Subject Descriptors: C.3 [Computer Systems Organization]: Special-Purpose and
Application-based Systems—Real-time and embedded systems
General Terms: Security, Design, Reliability
Additional Key Words and Phrases: Communication system security, computer security, embedded systems,
security
ACM Reference Format:
Serpanos, D. N. and Voyiatzis, A. G. 2013. Security challenges in embedded systems. ACM Trans. Embedd.
Comput. Syst. 12, 1s, Article 66 (March 2013), 10 pages.
DOI: http://dx.doi.org/10.1145/2435227.2435262
1. INTRODUCTION
Embedded systems constitute the majority of computational and communications base
deployed worldwide. Their adoption in a wide range of products and processes in the
consumer, enterprise, and industrial domains have led to embedded system presence in
almost all technology products, from toys and telephones to cars and industrial process
control.
Advances in embedded systems hardware and software technology enable the development of increasingly complex embedded systems. Additionally, the penetration of
embedded systems to products and processes increases continuously, since it enables
control of complex processes and leads to cost effective solutions at a wide front of uses.
As embedded systems are being used and are continuously being employed in emerging applications in everyday life, they are increasingly involved in services that require
security. However, security is a term that is being used differently, depending on the
This work is supported in part by a grant (PENED 2003), which is funded by the European Commission
by 75%, from the Hellenic State-Greek Ministry of Development-General Secretariat of Research and Technology (GSRT) by 25% and by private industry through the Operational Programme “Competitiveness,”
Measure 8.3, 2000-2006.
Author’s addresses: D. N. Serpanos, Department of Electrical and Computer Engineering, University of
Patras, Patras, GR-26504, Greece; email: [email protected]; A. G. Voyiatzis, Industrial Systems
Institute/RC Athena, Patras Science Park building, Stadiou Str., Plarani GR-26504, Greece.
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DOI: http://dx.doi.org/10.1145/2435227.2435262
ACM Transactions on Embedded Computing Systems, Vol. 12, No. 1s, Article 66, Publication date: March 2013.
66:2 D. N. Serpanos and A. G. Voyiatzis
user, system, and process involved. For example, security in an e-commerce transaction
implies protection of financial data, while in a car it implies safety and dependability.
Importantly, all these interpretations of security require development and adoption of
new technologies, in order to provide the required properties.
Although security has received attention in the past in the context of conventional
computational and communication environments, the dramatic evolution of embedded
systems and related applications leads to need for new technologies to be developed,
since embedded systems have brought significant differentiations to the models
considered in the past. The large numbers of users, the limited resources of embedded
systems and the complexity of developed embedded platforms leads to the need
to reconsider the models for the capabilities of attackers, the necessary levels of
protection of devices and systems, including physical protection, the performance
limitations of systems for computationally intensive operations, etc.
The goal of this article is to provide a comprehensive overview of the directions and
challenges of embedded systems security. Since security has different interpretations,
as mentioned earlier, we specify the security issues addressed in the article and we
distinguish security from privacy, safety and dependability. As these properties are
important and considered as parts of security in many contexts, we also address their
relationship and dependence on security.
The article is organized as follows. Section 2 presents the security requirements
of applications and services on embedded systems, the differentiators of embedded
systems from non-embedded ones and distinguishes security from privacy, safety and
dependability. Section 3 describes the security issues and challenges in autonomous
embedded systems, while Section 4 addresses issues and challenges in networked
embedded systems and their applications and services. Finally, Section 5 addresses
privacy protection, safety and dependability, presenting challenges in these technical
areas, which demonstrate the need to differentiate them from security, as they require
advances in different direction.
2. SECURITY AND EMBEDDED SYSTEMS
Security is considered a significant requirement in emerging embedded systems, especially considering the connectivity that most of them provide to system networks,
private networks, or the Internet. However, the term security is being used indicating
different properties or services, depending on the context of system use and considering
the attacks that have been and are being developed for systems overall.
In order to specify the requirements placed on systems for security, one needs to
identify the areas of applications and services where embedded systems are used,
since applications designate the security requirements, which, in turn, need to be met
through appropriate security mechanisms and policies. A useful classification of the
driving applications for embedded systems appears in the ARTEMIS Strategic Research Agenda [ARTEMIS 2006], where applications and services are classified in 4
major categories: (i) industrial systems, including automotive and aerospace transport, (ii) nomadic environments, targeting on mobile communications and information
services, (iii) private spaces, focusing on home services, and (iv) public infrastructure,
aiming to secure and dependable infrastructures. Although this classification is not
unique for embedded systems applications and services, it is definitely a comprehensive
one, indicating the areas of industrial interest driving the development of embedded
systems technology.
Analyzing the application/service areas and considering their requirements for security from all perspectives, vendors, providers and customers, it becomes clear that
the range of applications places several security requirements.
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Security Challenges in Embedded Systems 66:3
(1) Confidentiality: protection of stored or transmitted data from disclosure
(2) Integrity: verification of the correctness of stored or transmitted data
(3) Authentication: identification of the producer or sender of data
(4) Access control: availability of services only to authorized users
(5) Non-repudiation: inability of transaction participants to deny actions
(6) Dependability: provision of functionality in a dependable fashion, i.e. continuous
service, expected/promised quality of service (such as real-time requirements), etc.
(7) Safety: provision of services without possibility of hazard to the users
(8) Privacy: protection of personal information and resources from unauthorized
parties
In order to provide security in systems, one does not need to consider only the
functional requirements of applications and services, but other factors as well, such as
the profile of attackers and the resources that are available for the provision of secure
applications. The last two factors, attacker profile and available resources, are the main
differentiators of security provision in embedded systems and services relatively to nonembedded ones. Specifically, the main differentiators that need to be considered are:
(1) deployment in significantly larger numbers than non-embedded systems (servers,
desktops, etc.);
(2) simpler and limited resources, including power, computational and communications;
(3) low cost requirements, considering their adoption in consumer devices of various
levels;
(4) deployment in hostile environments of various types and ability of malicious users
to capture systems;
(5) strict safety requirements, especially in some application domains, such as industrial, automotive and aeronautics.
These differentiators of embedded systems actually make security requirements on
embedded systems significantly stricter than in traditional computational systems.
Specifically, embedded systems are deployed in more hostile environments, in much
larger numbers than traditional computational systems, and they are available to
a wider population, which includes larger numbers of potential attackers; thus, one
needs to provide hard—and thus demanding—security mechanisms in order to protect
against a significantly larger number of potential attacks. Furthermore, the safety
requirements in a wide range of applications, such as transport systems, surveillance,
etc., as well as dependability and protection of privacy, lead to additional requirements,
considering the criticality of safety and continuous operation in several application
domains of embedded systems. Importantly, all these stricter requirements have to be
provided in systems with limited resources and at low cost, in order to meet consumer
and client expectations.
Safety, privacy and dependability are considered security requirements in many application domains. However, they differ from the typical security considerations in
several ways. Privacy protection and safety, for example, differ in that they are usually requirements for processes, applications and services, rather than system-level
requirements, which are met with the typical security mechanisms (encryption, authentication, etc.). Dependability is typically considered as an application and/or system requirement, providing reliability, availability, etc., in the context of accidental and
nonmalicious failures, in contrast to security that is associated to malicious interventions. Importantly, in our view, privacy and safety are dependent on security, since they
need to employ security mechanisms for their implementation; furthermore, safety and
privacy protection can be considered to overlap, since privacy protection is considered a
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66:4 D. N. Serpanos and A. G. Voyiatzis
Fig. 1. Security property layers.
safety issue in several contexts, as in the case of financial transactions. In contrast, dependability is mainly a system issue, which is complementary to security, as indicated
in Figure 1. The layering shown in the figure indicates that security and dependability
are a requirement for privacy and safety. One can easily derive this, considering that,
if security mechanisms are not provided, an attacker can (i) easily collect any data, attacking privacy, and (ii) alter processes, leading to unsafe conditions. Dependability is
complementary, since an attacker can insert faults and failures that the dependability
mechanisms cannot recover from, in analogy to attacks on security mechanisms.
3. AUTONOMOUS EMBEDDED SYSTEM SECURITY
Embedded computing systems employ structures similar to the ones for generalpurpose computing systems. The typical structure of an embedded computing system
is shown in Figure 2, where the system contains 4 main subsystems: (i) processing,
(ii) memory, (iii) communication, and (iv) power. In a secure system, all components
need to be protected individually as well as the complete system, depending on the
specific requirements and the operational environment. For example, in a sensor system, where sensors are distributed in a potentially hostile environment, protection
of the complete system may be infeasible, and thus, subsystems have to be protected
individually and the complete system needs to be adaptive to operating with a dynamic
sensor population.
There are several levels of protection used for security of stand-alone embedded
systems, ranging from physical and hardware security to trusted computing platforms.
Physical security of complete systems or subsystems is addressed through antitampering techniques, and specifically techniques for tamperevidence, tamperresponse
and tamper-resistance. Tamper-evidence techniques enable the identification of tampering of devices, while tamper-response techniques include tamper-detection and reaction with actions that protect sensitive information appropriately. Tamperresistance
techniques, finally, prevent tampering attacks and protect sensitive information from
non-intrusive attacks that have been developed. Anti-tamper technologies are especially important in sensor systems, which are often deployed for critical applications,
like surveillance, in highly hostile environments.
Anti-tamper technologies are increasingly important to deployed embedded systems,
because system protection requires physical as well as algorithmic mechanisms. For
example, encryption had been long considered a sufficient solution for many applications and services, but the advances of attacks for more than a decade have demonstrated that it is not sufficient, especially in low-cost embedded systems with limited
resources, where encryption mechanisms can be broken in simple ways. Special attention has to be paid to side-channel attacks [Zhou and Feng 2005], which have changed
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Security Challenges in Embedded Systems 66:5
Fig. 2. Structure of a typical embedded computing system.
the model of attacks, breaking cryptosystems using methods that rather than attacking the cryptographic algorithms themselves exploit physical characteristics (timing,
power, etc.) [Kocher 1996; Kocher et al. 1999; Quisquater and Samyde 2001] or introduce faults during the cryptographic computations [Bar-El et al. 2006; Joye 2009].
Anti-tampering techniques protect against attacks after system deployment. New
business environments can drive embedded systems insecure by planting hardware
Trojans during the design and manufacture phase [Jin and Makris 2010].
Importantly, physical and hardware attacks have not been proven successful only
against specialized circuits, but against more complex systems as well, such as microcontrollers and processors [Anderson and Kuhn 1996; Blythe et al. 1993]. Several hardware and architectural techniques have been developed for protection against physical
and hardware attacks. Dedicated hardware is necessary, for example, to protect sensitive parts of the system memory. One such solution is a hardware implementation
of a type of execute-only memory, which allows instructions stored in memory to be
executed but not manipulated otherwise [Lie et al. 2000]. Side channel attacks can be
prevented by specialized design techniques in ASICs, or using architectural concepts,
such as decay caches [Keramidas et al. 2008], for protection from cache information
leakage, and bus encryption [Best 1981; Kuhn 1997] to protect data exchanged between
the processor and the main memory.
The diversity of the attacks [Ravi et al. 2004] against embedded systems creates
the need for software as well as hardware protection technologies, in order to address
security problems in embedded systems. Hardware modifications are necessary and
sufficient to defend against some of those attacks, as mentioned above. However, for
more complex attacks on highly programmable systems, operating systems (OS) enhancements or other software techniques must be used; importantly, software solutions
to attacks are cost effective, considering the high cost of several hardware solutions.
The availability of such technologies enables the development of trusted computing
platforms for applications and services.
Security in more complex, highly-programmable systems requires development of
methodologies for a wide range of issues, from methodologies for secure bootstrapping,
in order to verify system integrity in every step of the booting procedure [Arbaugh
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et al. 1997] to the use of process isolation and process level attestation techniques
[Microsoft 2011] to protect running processes. Additional OS enhancements include
techniques for context switching, exception handling, inter-process communication
and memory management [Lie et al. 2003; Garfinkel et al. 2003]. Importantly, software
authentication and validation is necessary and feasible, since it can be achieved by
using techniques like oblivious hashing [Chen et al. 2003] or program shepherding
[Kiriansky et al. 2002].
4. NETWORKED EMBEDDED SYSTEMS AND APPLICATION SECURITY
Secure communication is based on the use of encryption and authorization mechanisms
together with a secure routing method across a network. The decision of the encryption
scheme to be used is critical since the encryption complexity defines the level of security
offered by the communication. As increasing computational resources become available
to embedded systems, traditional public key cryptography is becoming a viable option
for some applications, although it is still too demanding computationally for most
embedded applications and services. Elliptic curve cryptography provides a promising
solution to embedded systems, due to its lower computational resources than algebraic
public key cryptography, while providing a high level of security [Miller 1986].
Key management is an important process in the establishment of secure communication as keys are the base of the encryption and decryption mechanisms. Often,
key management constitutes the weak point of a security system, since disclosure or
leakage of keys renders even the strongest cryptosystem ineffective. Global communication keys usually cannot be pre-defined in the networked systems, since the security
of the network is easily compromised. Thus, it is necessary to establish an effective
mechanism of generating and distributing keys. Two effective ways of achieving this
is the use of temporary global keys and random key distribution. With temporary
global keys, a global permanent key is used to establish a main key, and then, the
global key is destroyed in order to avoid key leakage, the main risk with using global
keys [Perrig et al. 2004]. In random key distribution, a large number of keys exist and
communication is accomplished through the use of choosing random subsets of keys.
With appropriate set size choices, communication between all ends of a network can
be accomplished [Chan et al. 2003].
Networked embedded systems constitute distributed systems, since they coordinate
to provide applications and services. As such, they need to defend against known
distributed system attacks at the application layer in addition to the communication
layers they implement. Such attacks include security flaws in the management of
application complexity, distributed denial-of-service, secure upgrading, etc.
Upgrading systems in a networked embedded environment is a necessity for fixing
software issues as well as for implementing new features and services. The increasing
software complexity of systems and services leads to an increased number of software
bugs and therefore, systems need to be updated during their operation. Furthermore,
new features and new services can be implemented by software upgrade on existing
hardware. However, software upgrading leads to security risks, because it offers possibility for malicious software to replace legitimate software. To face these security
risks, it is important to implement defences at both the communication protocols and
the internal organization of the system. The transmission of mobile code, i.e., system
software transmitted over a communication link, must be done with higher security
requirements than normal communication. Moreover, upgradeability can be limited,
or not allowed at all, for the software components that access crucial system resources.
Remote management of networked systems is another example of a service which
provides functional advantages but poses security risks, due to computation and transmission of critical information. Remote management implementations need to be built
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Security Challenges in Embedded Systems 66:7
in combination with an effective intrusion detection system to prevent attacks. Remote
management communication requires implementation with high encryption requirements as well as limitation of the access rights of the remote users.
All networked embedded systems, especially those connected to the Internet, need
to control the communicated data, allowing only appropriate (e.g., authorized) users
and processes to send/receive “legal” data. Firewalls can usually be implemented at
the network and application layers, either in end systems or in the network infrastructure [Bolding 1995]. Since networked embedded systems often have very specific
and well-defined communication needs, firewalls can be strictly configured to allow the
limited type of legitimate communication. The point where the firewall can be implemented, at the network or application layer and in the node or in the network, depends
on the nature of the networked systems, its available resources and the network topology. For instance, ad-hoc networks need to have a level of protection at the node level,
while more centralized systems can more successfully implement network layer and
network level protection [Slijepcevic et al. 2002].
Denial-of-Service (DoS) and Distributed Denial-of-Service (DDoS) attacks are a
prevalent threat against networked embedded systems. Such attacks prevent the system from either working or serving its intended users by overloading one or more
resources of the networked system. The targeted resources, usually, are CPU, memory, and network bandwidth. There are two basic types of DoS attacks [Hussain et al.
2003]. The first type targets specific software and/or hardware vulnerabilities that can
be exploited by sending one or more carefully crafted packets in order to crash the
networked system. The attacks of the second type take the form of massive amounts
of network traffic originating from thousands of compromised systems that mix with
the legitimate traffic and prevent the victim system from serving its legitimate users.
Such attacks are called DDoS attacks.
The first type of attack is yet another manifestation of the ongoing race between
system vulnerabilities and software patches. The second type is the one that cannot be
easily stopped, mainly due to the fact that shared network services can usually be accessed by all systems that are connected to the same network. The current architecture
of IPv4 makes things worse by allowing IP packets to be sent with arbitrary values
on their source IP address field [Wang et al. 2007]. Systems and mechanisms that
deal with DDoS attacks combine intrusion detection and traceback schemes for early
identification, filtering and tracing of a DDoS attack [Peng et al. 2007]. Such mechanisms include ingress/egress filtering for attack prevention [Ferguson and Senie 1998],
signature and anomaly-based detection [Cabrera et al. 2001; Wang et al. 2002], packet
marking [Belenky and Ansari 2003; Savage et al. 2001], and packet logging [Snoeren
et al. 2002] for attack traceback.
Finally, an important category of networked embedded systems that requires special attention is that of sensor networks. Sensor networks differ from other embedded
system networks in that they usually form ad-hoc networks of a large number of
nodes, often with very limited computational resources. As a consequence, protocols
employed for communication between sensor nodes must meet stricter performance
requirements. The limitation lead sensor networks to implement encryption mechanisms at the link layer, typically. A good encryption strategy for sensor networks is
to use encryption mechanisms with different complexity, depending on the value of
information exchanged [Zhu et al. 2003].
5. PRIVACY, SAFETY AND DEPENDABILITY
Privacy protection, safety and dependability are system properties often associated
with security, as mentioned in Section 2. Their importance is growing significantly in
modern environments, due to the high penetration of embedded systems in consumer,
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66:8 D. N. Serpanos and A. G. Voyiatzis
enterprise, and industrial environments. Since their interdependence with security is
strong, as shown in Figure 1, we need to address them, in order to demonstrate their
relationship with security as well as the important technologies they require for the
development of reliable and secure platforms for all emerging services.
5.1. Privacy Protection
Privacy protection is a significant requirement in embedded systems, considering several of the applications areas such as mobile phones, home networks, health systems,
e/m-banking systems, etc. The requirement is to protect sensitive personal information
involved in applications, according to legal directives. Specifically, privacy protection
requires the conditional collection and storage of personal information. Importantly,
privacy requires often the time-limited storage of relative information as well as its
access only by appropriately authorized personnel, when necessary. Such requirements
constitute a significant burden on systems, which do not have to provide simple confidentiality mechanisms (encryption/decryption), but complex access control mechanisms and periodic auditing of systems as well, in order to ensure compliance with the
required policies and laws. Furthermore, the increasing amount of information considered as personal or private leads to a need for development of adaptive and scalable
solutions that can accommodate new requirements as they emerge [Muller 2006]. ¨
5.2. Safety
Safety is a generic term used to describe requirements for information and process
protection against failures that lead to significant risk to life or stability. The generality
of the term originates from the fact that safety is a property of the service (or process)
that is implemented using a computational and communications platform (embedded
in this case). Although the concept of safety is relatively simple and clear in some
cases, e.g., automobile and avionics systems [eSafety Forum 2011; Tadlock 2002], it
can be extended to a wide range of process properties. For example, privacy protection
in several environments can be considered a safety property as well for the persons
involved (as in the case of an electronic transaction, for example, where leakage of
private information can lead to risks of digital fraud and to significant loss of funds,
affecting stability in one’s life).
There are two main problems posed by safety: (i) the development of safe systems
based on limited-resource subsystems, such as embedded systems, and (ii) the verification that the developed system meets the safety requirements placed. Clearly, the
first problem includes addressing several of the security issues addressed above. Importantly, verification is a critical issue here, especially because it is a computationally
hard problem. Although there have been technologies developed for system development, which lead to computationally feasible system verification, such as straight-line
code adoption, etc., there is still significant requirement for new techniques which will
enable safety property verification, especially considering the increasing complexity
and capabilities of deployed embedded systems.
5.3. Dependability
Dependability of computational and communication systems has been a desirable
property for many application domains. As a result, development of dependable
systems has experienced significant advances and has been considered as a mature
field for a long time, using well established and deeply analyzed techniques [Siewiorek
and Swarz 1982]. However, most of the methods developed up to date are based on the
assumption of accidental faults and errors, using appropriate fault models. In contrast,
conventional systems, and especially embedded systems, can be attacked by malicious
attackers inserting faults and causing errors and failures that differ significantly
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Security Challenges in Embedded Systems 66:9
from the models used up to date. Furthermore, the increasing complexity of highly
integrated embedded systems and networks of embedded systems render many of
the existing techniques inappropriate or with limited effectiveness in the emerging
environments. Thus, it is a clear challenge to develop new methods and technologies
that combine security and dependability appropriately, in order to develop the desired
embedded platforms [Serpanos and Henkel 2008].
6. CONCLUSION
Embedded systems security is an emerging field in embedded systems technology,
relevant to all application domains of these systems. In this article, we addressed embedded systems security and identified its relationship to privacy protection, safety
and dependability, which are often considered security properties. We presented security requirements based on the application domains and we described technologies
and methods for autonomous and networked embedded systems. Finally, we presented
the challenges placed by privacy, safety and dependability requirements, which lead to
complementary technical problems than the conventional security ones.
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Received June 2011; accepted November 2011
ACM Transactions on Embedded Computing Systems, Vol. 12, No. 1s, Article 66, Publication date: March 2013.