June 1996
Last Revised June 3, 1996
Four fundamental problems must be overcome if our vision of survivable
networked information systems is to be realized.
For a software engineering methodology to successfully support
the design and analysis of survivable systems, it will have to
effectively address the four fundamental problem areas described
above. We believe that a solution for each of these problems lies
in an architecture-based engineering tradeoff approach to software
analysis and design. The goal is to abstract the fundamental design
principles of survivability into an architectural representation
which makes clear the survivability properties of proposed and
delivered systems. Designers and evaluators must be able to model
the behavior of a system at the abstract architectural level to
provide the leverage necessary to have a sweeping and profound
impact on improving the survivability of delivered systems. This
approach will enhance survivability in the context of appropriate
tradeoffs among the other attributes of software engineering quality.
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We assert that an architecture-based approach to survivability
by design will not only improve survivability in today's unbounded
network environment (e.g., the Internet), but will also successfully
mitigate risks to survival in the most plausible future network
environments. We believe that one such environment will be characterized
by a shift from customer-owned and -controlled computing resources
(communicating over an unbounded network) to a world of autonomous
agents where most of the computing resources will be contained
in the unbounded network infrastructure and will be provided by
a multitude of computing and communications service providers.
In this future environment, most computing and communications
tasks will be accomplished by remote agents on behalf of the customers
they represent. We will show that an architecture-based approach
can extend its survivability design and evaluation capabilities
into this and other plausible future environments.
In this subsection we first define survivable systems as an abstract, idealized concept. We next describe a vision of a future computing environment which we believe will form the context within which future survivable systems will function. Finally, we outline several desirable design capabilities for future survivable systems
In the realm of networked information systems, the natural escalation
of offensive threats versus defensive countermeasures has demonstrated
time and again that no practical systems can be built which are
invulnerable to attack. Affordability will always be a significant
factor in the design, implementation, and maintenance of the systems
we build to support our national infrastructure (such as the power
grid, the public switched communications networks, and the financial
networks) and our national defense. In fact, the trend toward
increased sharing of common infrastructural components in the
interests of economy will ensure that the civilian networked information
infrastructure will always be an inseparable part of our national
defense.
Practical, affordable systems are virtually never 100% customized,
but rather are built from commonly available off-the-shelf components
whose internal structure is no secret to the community at large.
The trend toward building systems through integration and reuse,
rather than through "one-shot," customized design and
coding efforts, is one of the cornerstones of modern software
engineering strategy. Unfortunately, the intellectual complexity
associated with most software design, coding, and testing efforts
ensures that exploitable bugs can and will be discovered in commercial
and public domain products whose internal structures are widely
known. When these products are incorporated as components of larger
systems, those systems become vulnerable to attack strategies
based on the exploitable bugs associated with the component products.
Popular commercial and public domain components offer an attacker
a ubiquitous set of targets, with well-known and typically unvarying
internal structures. This lack of variability among components
(and therefore lack of variability among systems) allows a single
attack strategy to have a wide-ranging and potentially devastating
impact.
The traditional use of the term "computer security"
has a binary implication which suggests that at any moment in
time a system is either safe or is compromised. We believe that
the term "computer security" and the limited viewpoint
it engenders (e.g., largely ignoring the aspects of recovery from
a compromise and of maintaining performance during and after an
intrusion), is inadequate to support the necessary improvement
of the state of the art and the state of the practice of protecting
computer systems from attack. In contrast, the term survivable
systems refers to systems whose components collectively accomplish
their mission even under attack and despite active intrusions
which effectively damage some significant portion of the system.
Accomplishment of mission refers to the ability to deliver essential
functionality in a timely manner, where the definition of essential
and timely are up to the users of the system. This collective
capability should not be dependent upon the survival of a specific
information resource, computation, or communication. In a military
setting, essential might refer to the maintenance of overwhelming
technical superiority, and timely may refer to the ability to
deliver results in less than one decision cycle of the enemy.
In the public sector, essential and timely may refer to some composite
measure of the disruption of stock trades and bank transactions,
representing a threshold that must not be exceeded.
In short, the nature of computing dictates that even hardened
systems can and will be broken. Robustness under attack is at
least as important as hardness or resistance to attack. Hardness
certainly contributes to survivability, but robustness under attack
is the essential characteristic that distinguishes survivability
from traditional computer security. Although the concepts and
techniques associated with system survivability are embryonic,
they include (but are not limited to) traditional areas of software
engineering and computer science such as reliability, testing,
dependability, fault tolerance, verification of correctness, performance,
and computer security.
Survivability requires robustness under conditions of war, terrorism,
crime, and accident. The concept of survivability includes fault
tolerance, but is not equivalent to it. Fault tolerance relates
to the statistical probability of an accidental fault or combination
of faults, not to a malicious attack. For example, an analysis
of a system may determine that the simultaneous occurrence of
the three statistically independent faults f1 to f3 will cause
the system to crash. The probability of the three independent
faults occurring simultaneously by accident may be vanishingly
small, but a malicious intruder with a knowledge of the system's
internals can orchestrate the simultaneous occurrence of the three
faults needed to bring down the system. A fault tolerance system
would not need to address the possibility of the three faults
occurring simultaneously, if the probability of occurrence is
below a threshold of concern, whereas a survivable system would
require a contingency plan to be in place to deal with it.
Redundancy is another factor that can contribute to the survivability
of systems, but redundancy alone is insufficient since multiple
identical backup systems share identical vulnerabilities. A survivable
system would require each backup system to offer equivalent functionality,
but to exhibit significant variance from the others in its implementation.
This would thwart any attempt to compromise the primary system
and all backup systems with a single attack strategy.
In summary, when we discuss system survivability, we are referring
to the collective capability of a system to deliver essential
functionality in a timely manner, in the face of an actual or
threatened attack or accident. This collective capability should
carried out even if a significant portion of the system is incapacitated
by the attack or accident.
As an idealized concept, it is hard to argue with the notion of survivable systems. However to evaluate the prospects for the success of any proposed realization of a survivable system, one must first understand the context (i.e., the computing environment) within which that survivable system will operate. This subsection describes some of the characteristics of what we believe to be the most plausible future networked computing environment: a largely unbounded network domain infrastructure supporting an agent-based computing and communication paradigm.
To better understand the context within which survivable systems
must operate, it is important to distinguish between bounded systems
and unbounded systems. A bounded system is one in which all of
the parts of the system are under a unified administrative control
and can be completely characterized and controlled. At a bare
minimum, the behavior of a bounded system can be understood and
all of its various parts identified. In an unbounded system there
is no unified administrative control over its parts. We use the
term administrative control in the strictest sense, which includes
the power to impose and enforce sanctions, not merely to recommend
an appropriate security policy. It is possible that some parts
of an unbounded system cannot even be identified by an outside
observer, let alone have actions or behaviors that fall within
predictable limits.
Unbounded systems are a significant component of today's computing
environment and will play an even a larger role in the future.
The Internet - a non-hierarchical network of systems under local
administrative control only - is one current example of an unbounded
system. There are conventions that allow the parts of the Internet
to work together, but there is no global administrative control
to assure that these parts are behaving according to these conventions.
Unfortunately, the security problems associated with the operation
of unbounded systems are greatly underestimated.
The architecture of secure bounded systems is built upon the notion
of a security policy - its existence, its enforcement, or its
lack - imposed by the exercise of administrative control. In contrast,
an unbounded system has no administrative control with which to
impose global security policy. For instance, on the Internet today
the backbone architecture is independent of security policy considerations
because there is no global administrative control.
An unbounded system can be composed of bounded and unbounded systems
connected together in a network. Figure 1, below, represents an
unbounded domain consisting of a collection of bounded systems,
where each bounded system is under separate administrative control.
If each bounded system were completely disconnected from the other
systems, it would be possible to fully characterize the security
state of each system in terms of security policies.
Agents are executable software objects whose executions are not
tied to any specific host or computing resource or to any geographical
or logical network location. Agents perform computation and communication
on the behest of a user, but, from the user's perspective, the
execution platforms typically exist somewhere within an unbounded
domain. The conceptual model of agent operation is one where the
agent, at the request of a user, goes to one or more remote hosts
to perform a computation or gather information, and then returns
to the user with the results of that "journey of computation."
An agent's mode of operation may range from partially to fully
autonomous, and the degree to which an agent is autonomous may
vary throughout the life of that agent.
Blurring the line between computation and communication, these
agents provide a means of remote distributed computation that
is radically different from traditional computing paradigms. An
agent has a computational task to fulfill for its user and has
some associated resource budget and authorization and authentication
primitives. Redundancy for agents (through replication) is a lightweight
operation from a resource standpoint at the individual replicant
level, but the ability to spawn an unlimited number of replicants
may forever change the face of computing and resource allocation
as we know it today. For example, supporting survivability through
inference channels would be an efficient strategy in an agent-based
computational environment.
Just as domain name servers advertise hostnames, domain service
servers will advertise services and prices and authorization requirements.
For example, services may be free to educational users, while
others are charged depending on the number of service-based units
of work delivered, or on the more traditional resource-based units
consumed, such as CPU time, storage, etc. Agents may contain built-in
discriminators to prioritize and negotiate selection of the service
offerings, or they may rely on third party broker agents which
do the searching and selecting of services desired by the primary
agent. Natural biological and immunological analogs appear to
apply more readily to agent-based computation than to more traditional
complex process set computation. Much of today's object-oriented
and neural-net research can readily be applied to agents. Analogies
to DNA are strong, and replication strategies that require two
genetically similar but diverse agents to merge prior to replication
provide added fault-tolerance.
Here are some of the salient features of a future agent-based
computing environment:
We believe that an architecture-based approach to the analysis
and design of survivable systems in unbounded network domains
would make possible the following capabilities. Each of these
capabilities would lead to significant improvement in the development
and deployment of large-scale information systems.
Much of today's research and practice in the field of computer
systems security takes a perilously narrow view of the means by
which one can defend against computer intrusions. This narrow
view is dangerously incomplete because it focuses almost exclusively
on hardening a system (e.g., using firewall technology or an Orange
book approach to host protection) to prevent a break-in or other
malicious attack, but says little about how to detect a computer
security incident or what to do once an security incident has
occurred or is under way. The view is accompanied by security
evaluation techniques that limit their focus to the relative hardness
of a system, as opposed to assessing a system's robustness under
attack.
Although the application of the term survivability to computer
systems is relatively new, the practice of survivability is not.
Much of the survivability practice to date has been in the realm
of incident response (IR) teams, and in fact the CERT Coordination
Center has, since its inception, been supporting the survivability
of the Internet community by providing incident response services
(helping organizations respond to and recover from incidents),
and publishing and distributing vulnerability advisories (akin
to public health notices). The CERT/CC has also encouraged variability
by encouraging folks to separate different services onto different
machines. Thus, the CERT/CC (and its IR team in particular) has
always been concerned about survivability (though it has never
called it that) and has been very successful in helping sites
with risk mitigation and recovery. The realm of incident response
is the survivability of deployed systems, and that's the state
of the art for deployed systems.
Our seven years of experience at the CERT Coordination Center
in responding to computer security incidents has shown us that
how organizations respond to (and recover from) computer intrusions
is at least as important as the steps they take to prevent them.
Although the CERT/CC deals with survivability primarily at the
level of the system administrator, which is quite different from
the architectural level which is the focus of this paper, its
experience provides ample evidence of the effectiveness of dealing
with survivability issues on a broad scale. We believe that the
widespread availability and use of survivable systems by the Internet
community and throughout the Internet infrastructure itself will
provide the best hope for the dramatic improvements necessary
to make the Internet a more survivable networked information "system
of systems" that will be viable for commerce, defense, the
conduct of government, and the support of major elements of the
national infrastructure (e.g., power grid, public switched network,
and air traffic control).
Currently, little of the basic technology in security engineering
and system integration applies to unbounded systems, but instead
assumes that the capability exists to identify, define, and characterize
the extent of administrative control over a system, all access
points to that system, and all signals that may appear at those
access points. In unbounded systems such as the current Internet
and the future National Information Infrastructure, these boundary
conditions cannot be fully determined.
On the Internet today, the cornerstone of security is based upon
the notion of a firewall, which is an attempt to create a logically
bounded system within a physically unbounded one. We assert that
"bounded-system thinking" within unbounded domains leads
to security designs and architectures that are fundamentally flawed.
One notable example is the use of a firewall as the basic security
component of the Internet. This approach is severely limited and
can be readily circumvented by exploiting the fundamental differences
between bounded and unbounded systems. Traditional firewalls are
the state of the art for security architectures, but not for survivable
systems, because they are passive, filter-only devices. The evolution
of firewalls into active firewalls, which have detection and response
capability, will allow firewalls to have a role in survivable
systems.
This focus of this section is on how to bridge the gap between our current state and the future promise of survivable systems. We first look at the critical scalability problems facing incident response teams, and then the issues facing designers of survivable systems.
One of the most crucial issues coincident with the exponential
growth of the Internet is how to scale incident response and vulnerability
resolution activities to keep up with the dramatic increase in
intruder activity. Despite heroic efforts by individual response
team members, incident response teams have fallen behind in the
quantity and level of services they offer, and are steadily losing
ground.
A scalable incident response capability requires at least a partially shared knowledge base with extensive automated workflow support. This section gives a brief overview of several of the issues and design considerations for the development of a scalable security information infrastructure. The purpose of this infrastructure is threefold:
The exponential growth of the Internet has produced an unprecedented
rise in the number of computer security incidents, threatening
the capacity and ability of current incident response teams to
successfully intervene. Moreover, the dramatic shift in the character
of the Internet from solely an educational and research network
to a global network supporting corporate, government, national
defense, and other high stakes activities has greatly increased
the incentives for computer intrusions. This shift in the character
of the Internet has also increased the likelihood of more skilled
attention being devoted to the task of breaking network security,
including attacks on the infrastructure of the Internet itself.
The scalable security information infrastructure will provide
for the capture (for later query and analysis) of incident-related
information, comprising an incident response and software vulnerability
knowledge base. This knowledge base will allow researchers, vendors,
software engineering practitioners, educators, and incident response
personnel to learn from real-world evidence (and actual artifacts)
of survivability mistakes and successes, and to take proactive
steps to improve the survivability of networked information systems.
For example, the analysis of the information contained in this
knowledge base will help in the identification of those system
administration and software engineering practices that make sites
vulnerable to attack. Other analyses will provide valuable insights
into attack patterns and incident trends, allowing investigators
to discern the broad patterns shaped by individual incidents.
Researchers will use the "lessons learned" to improve
the survivability of the next generation of networked information
systems. Finally, analyses of data on the workflow interactions
between response teams and their constituencies (and between cooperating
response teams) will contribute to the continuous improvement
of the practice of incident handling itself.
The CERT Coordination Center's collection of software vulnerability
data provides empirical evidence that vendors continue to release
software containing essentially the same classes of security flaws,
over and over again, year by year. The vendor response to these
flaws is typically the issuance of a patch, or add-on, that addresses
the immediate problem at hand, but does not solve the problem
at the design level, which is often the cause of the subsequent
reappearance of the same type of flaw in the same software.
We can no longer afford to ignore the survivability aspects of
the creation and continuous improvement of large-scale systems,
or to treat the security and survivability of such systems in
isolation (i.e., as mere add-ons or patches to existing designs).
Security and survivability must be designed in from the outset,
and must be an integral part of an evolutionary design process
for that system.
Currently, security changes to a software system are rarely made
at the architectural level. Security enhancements or corrections
for security vulnerabilities are typically made in the form of
"patches" and are not reflected in the architectural
description of the system. This "disconnect" between
the architecture and the implementation of a system all too often
has an unexpected and undesirable impact on security, as well
as on the other attributes of software quality. On a daily basis,
the CERT Coordination Center sees the real-world damage (in the
form of system intrusions) caused by the lack of a theoretical
foundation upon which to build sound software engineering practices
for the design, implementation, and maintenance of survivable
systems.
One of the major goals of research into software architectures
for survivable systems is to build a predictive model to describe
the survivability implications of multiple bounded and unbounded
systems that collectively form a large, unbounded domain. Another
major goal is to provide the architectural primitives, techniques,
and the supporting tools necessary to build highly survivable
systems that exist within unbounded domains.
A description of the two major goals of this work is provided
below:
We will create a predictive security and survivability model of
unbounded systems in general and the Internet in particular. The
most salient characteristic of our model is that it will allow
a user to conduct "what-if" analyses of the survivability
characteristics of a particular component, collection of components
or system.
The deliverable is a detailed description of a predictive model
for understanding and evaluating security attributes in an unbounded
system. The predictive nature of the model will allow us to derive
security and survivability characteristics from the specific architectural
descriptions of model elements in a simulation of an unbounded
system (such as the Internet). It is expected that this model
(or modeling technology) will be a step towards creating more
secure architectures for the Internet, the NII, and future unbounded
systems.
Currently, the CERT Coordination Center performs vulnerability
analysis to create and distribute workarounds, and to assist vendors
in creating and distributing patches. During the data analysis
phase of this project, we will abstract the vulnerability record
into classes of architectural and implementation practices that
lead to specific vulnerabilities. This analysis will be linked
with an analysis of intruder behavior to help determine the real-world
security impact of specific architectural and implementation practices.
The abstraction and classification of vulnerabilities and their
links to real-world intrusions will contribute the creation of
our predictive model by allowing us to extrapolate behavior and
system vulnerabilities for future systems.
The aspects of reality that the model must have abstract representations
for include organizational culture (e.g., security practices),
technology, organizational policies, and the intruder environment.
The representation of the model must be reliable (e.g., clearly
mappable to reality), precise, and limited for practicality. The
model should also support fuzzy numbers and fuzzy relationships.
Note that user representation provided by the model may range
from a simple gauge package (e.g., dashboard) to fully sensualized
visualization techniques.
Our view of the most useful structure for the model is one that
would support scenario -based "what-if" analyses, much
like a "spreadsheet for survivability". This spreadsheet
will provides a composite view of a system's survivability in
the context of an organization, and the intruder environment,
and will allow you to ask questions. For example, "An IP-spoofing
attack is directed at X. What's at risk?"
Such a model would be well suited for the kinds of risk-based tradeoff decisions that business executives typically make. This constraint-based propagational model has significant advantages over models that would merely "stamp" a component or system with a numerical survivability rating. Simple survivability rating numbers won't tell you what would happen in a specific attack scenario or how to improve your system's capacity to survive an attack. Moreover, a numerical rating won't tell you how a specific component or system would perform in the context of your organization's unique culture, policies, threats, and vulnerabilities.
This work will enable us to understand and specify the security
and survivability tradeoffs between alternative architectures.
Ultimately it will allow us to understand and specify the tradeoffs
between security and other attributes of software quality (such
as performance, maintainability, and ease-of-use). The architecture
models in conjunction with the unbounded domain model, will enable
us to characterize (and perhaps quantify) the risks associated
with using specific architectures for particular applications
or purposes, in the context of an unbounded domain.
Another benefit of our work will be to produce alternative architectures
to replace the contemporary (flawed) firewall architecture. System
administrators on the Internet are currently employing filtering
routers and application gateways at critical points in the Internet
infrastructure to restrict various services that have been found
to be insecure. This architectural solution does not take into
account the rapid growth of service types and advances that render
firewall architecture ineffective (such as Mobile-IP, which allows
a user to change their access point to the network while keeping
the same source IP address). Our work will produce alternatives
to the existing firewall architecture to allow administrators
to maintain a high state of security at any specified point in
the Internet or any other unbounded system.
The flip side of the problem where designers neglect to consider
security or survivability in their design, is that when security
or survivability actually is considered, it is typically in isolation
from the other attributes of software engineering quality, such
as performance, dependability, modifiability and ease of use.
There is no common model, no common architectural language for
understanding, assessing, and articulating the software engineering
tradeoffs available to designers of survivable systems. The bottom
line is that software designers, their managers, and their customers
need to be able to specify the tradeoffs between enhanced survivability
and the other attributes of software quality (including affordability).
They also need to be able to evaluate the success of competing
designs (and implementations) in achieving their overall specifications,
including survivability.
In the next few years, we expect to see ground-breaking research attempt to bring together the various attributes associated with software engineering into an integrated software evaluation framework. This framework will allow the software design practitioner to consolidate the metrics associated with the individual attributes of software engineering for the purpose of deriving composite measures of software quality. As a result of this work, the practitioner will be able to analyze and evaluate tradeoffs among the various attributes on both a qualitative and quantitative basis. These software engineering attributes currently include:
At present, researchers in each of these separate "ilities" do not share a common language or common methodology, and therefore metrics for the various attributes cannot be combined and tradeoffs cannot be evaluated in any sensible way. The most glaring deficiency seen in the design of software systems today is that survivability is typically not one of the attributes being considered in the set of software quality attributes. Unfortunately, this is not surprising, given the fact that in traditional software engineering the other quality attributes dominate the design process. Survivability and security (if considered at all) are an afterthought. Survivability and security are relegated to a series of add-on patches, which are put in place after problems are discovered and reported by the customer or by an incident response team. Because such patches are typically knee-jerk reactions to an emergency situation, rather than the result of a principled systems engineering design, they do not solve broad classes of problems, but instead only close a small number of holes and leave countless others open. In fact such patches occasionally introduce new security vulnerabilities (or bring old ones back to life).
Even at the time that a patch is being designed, the focus of
"how to patch the problem" is usually on speed and ease
of solution, as well as performance and functionality (e.g., "don't
degrade existing abilities") rather than on designing the
best solution from a survivability or security standpoint. Conversely,
the decision is sometimes made to maximize security and survivability
(i.e., "close the hole") at any cost, leading to a design
effort in which there is no consideration of appropriate tradeoffs.
The characteristics and the dimensions of the design tradeoffs
that will arise once survivability is made an inherent part of
the software engineering process is a fertile area for exploratory
and applied research, with the potential for a very high-impact
payoff in software engineering process improvement. Note that
we will treat key survivability attributes at the architecture
level. For instance we will attempt to model variability at the
component level.
The results of such work will include the development of survivability
metrics, composite metrics, key practices, an understanding of
software engineering tradeoffs at the point of design instead
of as post-design patches. Such work would also support the continuous
improvement of survivable systems, using a principled evolutionary
design process that allowed architecture tradeoff analysis at
every step of the way.