Theres More to Securing Web Services Systems Than WS-Security
This column is about securing systems that are implemented in the Web services paradigm. The scope of this topic is huge and the issues are complex. Further complicating the problem is the fact that we are still very, very early in the life of the paradigm and most of the detail has yet to be worked out. However, since this is an evolutionary paradigm, even if we don't yet have all the specifics, we do know what the general classes of problems are . . . and where to look for them. So for now, in this column, we will be looking at the kinds of controls that will need to be implemented in order to secure systems that are built around Web services. We will focus on issues at the macro level here; there are some problems that exist independently of the choice of Web server, application server, authentication mechanism, etc. These issues, if not addressed, will result in exposed systems, no matter how well the WS* standards are implemented, whether secure programming techniques were employed or how well the rest of the system is done.
What is a Web service?
Whenever one encounters an article or paper on Web services, somewhere early in the document, the author defines the term. Most begin with a disclaimer to the effect that there are as many descriptions as there are describers. I certainly do not want to be the one to break with tradition. However, before we get to that, it is important to have a small sample of what others have to say.
BEA Systems – Characteristics of Web services will be:
- "Loose coupling between the public contract and the underlying implementation
- Asynchronous interaction
- Business-level documents as the unit of communication." 
CIO Magazine – "Web services is an amorphous blob of business apps, and different players define it differently. Ultimately, it can be described as any application delivered over the Internet and accessed by any device, from Pcs to mobile phones. What Web services can offer is a set of shared protocols and standards that permit systems to share data and services without requiring humans to broker the conversation. The result promises to be 'on-the-fly' links between the online processes of different companies." 
HP – "Web Services proposes a service-oriented paradigm for computing in which distributed, loosely coupled services collaboratively provide business services and can be accessed by the Internet and by end customers (possibly consumers)." 
IBM – "Web services are software components that are developed using specific technologies from three primary technology categories:
- An XML-based description format (for example, WSDL)
- An application messaging proctocol (for example, SOAP)
- A collection or transport protocol (for example, HTTP)" 
Microsoft – "XML Web services are built on XML, SOAP, WSDL and UDDI specifications. These constitute a set of baseline specifications that provide the foundation for application integration and aggregation." 
Sun Microsystems – "The foundation of Web services is XML messaging over standard Web protocols such as HTTP." 
World Wide Web Consortium – "A Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP-messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards." 
Web Services from the Information Security Perspective
Of course, the way one talks about Web services depends upon one's focus. So lets take a look at some of the characteristics of Web services that have implications for Information Security practitioners.
- Web services is a set of services that provide a means by which disparate systems can work with each other.
- Web services provide for a loose coupling of systems – i.e. the endpoint systems have no direct knowledge of each other nor any relationship (formal or informal) with each other.
- The endpoint systems were not originally designed as pieces of an integrated system nor were they originally designed to participate in the provision of a service to "strangers." (More on what this means later).
- Web services use a stateless, asynchronous messaging mechanism.
- Web services transactions can span significant amounts of space and time and multiple machines.
- The system architecture is one which:
- is multi-layer
- is distributed
- is implemented using a services-based software architecture
- allows remote procedure calls (RPCs) using Web Services Definition Language (WSDL) as the Interface Definition Language (IDL) (to systems that were not originally designed to provide that service).
Basically, Web services allows us to create new systems by connecting existing systems from (potentially multiple) different security and trust domains into one system . . . over the Internet. Gives me a headache just to think about it . . .
Some Security Implications of This Perspective
Web services systems offer all of the security problems inherent in the distributed computing environment, EAI and Web applications all rolled up in one package. There is not enough time in this column to examine all of them. We will introduce a few here with the intention of exploring them more fully in subsequent columns.
Usually, some of the requirements that determine the design, construction, implementation and operation of systems are intended to prevent, detect or mitigate the (information security) threats and risks that the system is expected to encounter during its lifetime. These known and anticipated threats comprise the design threat model and drive the design control assumptions and the design trust assumptions of the system. The system design and selection of controls, then, are based on the threats and risks that were known or anticipated at the time the system was designed and implemented. (For this discussion, the notion of controls includes business and technical processes and procedures as well as "the usual" physical access controls, firewalls, encryption, etc.). When one inserts Web services between endpoint systems that were not designed with Web services in mind, one violates the design threat models, design control assumptions and the design trust assumptions made by the endpoint systems. This has the effect of introducing threats for which there are no controls in the design control set. One has just introduced a new set of threats and risks. These emergent risks and threats imply:
- New risk models
- New trust models
- New business processes
- New technical processes
- New operations processes
This implies the need for a thorough reassessment of the threats and vulnerabilities to which existing systems will be exposed when they are incorporated into a system that is mediated by Web services.
If done "by the book," an organization's selection of InfoSec controls is based on risk assessments and is a compromise between the cost of protecting its assets and their value. Introducing Web services into a system's environment introduces a set of risks for which there must be controls implemented. Some of these controls should allow organizations to manage end-to-end:
- On the communication channel
- Of confidential information
Audit trails to determine
- Who initiated the transaction
- When did it originate
- From where did it originate
- When did it pass through what process
- What transformations if any were done on the data while it was in transit
- Of the communication channel
- Of the data in flight
- Of the semantic and referential integrity of the data
- Of the transaction (some of which can be very long-running)
- Of the session
- Of the network and the systems
- Of origin – the sender cannot falsely deny having been the originator of the message
- Of receipt – the recipient cannot falsely deny having received the message
The Four Dimensions of Trust
- Identification – establish the identity of the principals participating in the transaction
- Authentication – establish that the principals truly are who they represent themselves to be
- Authorization – determine what functions the authenticated principal has the rights to peform on what objects
- Trust Management – ". . . formulating security policies and security credentials, determining whether particular sets of credentials satisfy the relevant policies, and deferring trust to third parties." 
Interconnection of Systems from Different Trust Domains
A major challenge for the design of systems that incorporate Web services is that the system must securely transport data and mediate between systems that exist in completely different trust domains. A major challenge for any given client (endpoint) of Web services is that it has to do business with other endpoints that are outside its own trust domain and, in the absence of certain controls, can make no assumptions about the provenance, integrity or validity of data that it receives from the Web service. Because of this, trust management will become a major source of risk and have a greater impact on business and technical processes than all other aspects of designing, implementing and managing Web services and their clients. It is hard to overemphasize the importance of understanding the nature of this "impedance mismatch" and allowing for it in the design of Web services systems.
Some Implications for the Organization's Risk Management Process and System Development Life Cycle
With the introduction of Web services comes a change in the way legacy systems interact with their environment. This change has implications for the organization's risk profile and risk management process. It also impacts the system development process and the SDLC. A few of the areas affected are described below. A subsequent column will cover the topic in much more detail.
- System security engineering is especially important when building systems that use Web services because "both sides of the interface" are affected.
- A thorough risk assessment at the beginning of Web services projects is important in order to understand how the introduction of Web services will affect the risk, trust and control assumptions of the systems with which the Web services will interface.
- Having a formal assurance (Certification and Accreditation) process in place is very important. It is imperative that the business owners who are accountable for the performance of the system understand the risks and exposures that Web services create and have a means by which they can be confident that the controls that have been chosen will do the job they are intended to do.
- Clearly defining system ownership and the accountable party is particularly important in this case. It is difficult because of the nature of this kind of system, but that is why it is so important. With so many players involved, it is crucial to successfully managing the risks associated with these systems that clear ownership and accountability be established on a system-by-system basis.
To this point, we have not talked much about Web services, per se. From a high-level software architecture perspective, Web services systems are not any different from any other distributed, multi-tier, services-based architecture. In the simplest case of a services-based architecture, there are three "layers." There is a presentation layer, a layer that provides business processing and data access services to the presentation layer, and a data management/storage layer. So far, the discussion has been around issues that arise because of the nature of the architecture . . . that it is distributed, multi-tier and services-based. There are, though, a set of security issues that are specific to the services layer. Anyone who has built software systems using the DCE (or DCOM/MSDCE), CORBA or SESAME understands them. Naturally, the specific details differ because of the diferences in the environments, but the general problems are the same. For instance, the basic security requirements of authentication, integrity and confidentiality and the need for registration services and naming services exist for each of the environments; and each environment provides a set of security services. Such is the case of Web services. However, there are some aspects of the Web services environment that make meeting these requirements quite challenging . . . Following is a current snapshot of the "State of the Web Services Security Union."
Emerging Standards for Securing Web Services
WS-Security – Web Services Security – An extension of SOAP that provides privacy and integrity mechanisms for parts of SOAP messages. (September, 2002)
SAML – Security Assertion Markup Language – An XML-based framework for exchanging security information. (May, 2002)
XACML – eXtensible Access Control Markup Language – A language for expressing information security policy. (February, 2003)
XKMS – (XML-KISS and XML-KRSS) – Provide access to PKI key management functions. (March 2001)
XMLEnc – Defines a mechanism for encrypting part(s) of an XML document (with potentially different keys). (December, 2002)
WS-Policy – Describes the capabilities and constraints of the security (and other business) policies on intermediaries and endpoints. (May, 2003)
WS Security Specifications in Process
WS-Trust – Will describe a framework for trust models that enables Web services to securly interoperate.
WS-Privacy – Will describe a model for how Web services and requesters state privacy preferences and organizational privacy practice statements.
WS-SecureConversation – Will describe how to manage and authenticate message exchanges between parties including security context exchange and establishing and deriving session keys.
WS-Federation – Will describe how to manage and broker the trust relationships in a heterogeneous federated environment including support for federated identities.
WS-Authorization – Will describe how to manage authorization data and authorization policies.
Much work has been done, but the hard part is yet to come.
Trust Management Revisited
The general problems for trust management are not new. There is a healthy literature on the topic and there exist several subsystems that provide various trust management mechanisms. The problem is that the Web services paradigm is enough of a shift that it serves to broaden the scope of "The Trust Management Problem" significantly. It is the decoupling of the endpoints that creates the problems. The challenge for designers, implementers and risk managers of systems that are built around Web services is that this introduces a whole universe of issues and problems that do not exist in tightly-coupled systems, and most have no experience with trust management issues that arise out of loosely-coupled systems. Complicating the problem is that most of the standards work that addresses trust management issues is still in committee.
In the next few columns, we will go into more detail on the implications described here. None of what we will cover will be rocket science, and I do not expect that it will be earthshaking news to anyone. After all, this is a self-selecting audience, and I expect that the anyone who would be interested in this topic would already be thinking about this. You may already have tried to get the message across to $unwilling_to_hear_anything_manager. For some reason, it is easier to get those folks' attention if the message comes from someone else. My goal in writing this column is to provide those who need it with a message that they can take to $unwilling_to_hear_anything_manager and say: "Look, here is someone else who is saying what I've been saying all along."
 BEA Weblogic Workshop: The Future of Web Services – Here, Today, (note: the following URL is broken) http://www.bea.com/products/weblogic/wl_workshop_tech_wp.pdf
 Emerging Technology – Investing in Web Services, http://www.cio.com/archive/091502/et_pundit.html
 Web services concepts – a technical overview, http://www.hpmiddleware.com/downloads/pdf/web_services_tech_overview.pdf
 Best practices for Web services: Back to the basics, Part 1, http://www-106.ibm.com/developerworks/library/ws-best1
 Web Services Specifications, http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnglobspec/html/wsspecsover.asp
 Sun ONE and Web Services, http://www.sun.com/executives/sunjournal/v5n2/feature6.html
 Web Services Architecture, http://www.w3.org/TR/ws-arch/wsa.pdf
 Decentralized Trust Management, http://www.crypto.com/papers/policymaker.pdf