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Enterprise Web Services Security: A Reference Architecture

Meeting the challenges of security and management

Web services are past the initial marketing hype. Early Web services were part of experimental one-off projects within a single enterprise department. Now, larger Web services deployments are moving outside of the enterprise firewall to better leverage existing business partnerships and value chains.

Larger Web services projects come with a price, however. They are more complicated to implement and more costly to manage. They require careful deployment planning throughout the enterprise based on well-established business processes.

The emerging proliferation of Web services networks presents challenges in terms of security and management. Enterprise deployments of Web services can only be successful if business and information technology (IT) managers are convinced they can systematically control access to Web services, meet customer service requirements, and monitor and meter Web services use.

Need for a Reference Architecture
A large number of vendors have entered the Web services market with products that address one or the other aspect of security and management. However, in the absence of a global view or "big picture," there is a danger that enterprises following opportunistic, product-based implementations may create security architectures that do not scale with the anticipated growth of Web services deployments.

Furthermore, Web services are only one of many resources that need to be managed and secured in the enterprise. A one-off approach will lead to an isolated Web services "island of security" and will require future deployment refactoring and integration with the overall enterprise security system. Using a Web services reference architecture as a guideline, organizations can develop a road map for Web services security and management meeting current enterprise needs and supporting future large-scale deployments of Web services networks.

Web services deployments use various components such as containers, network security, management, and identity platforms that feature complementary, and sometimes overlapping, functionality.

Not surprisingly, all of these components provide some security and management features and they must all work together to provide an efficient and reliable solution. The Web services reference architecture analyzes the role of each component and suggests appropriate integration points between them.

Purpose and Scope
This article discusses an architecture for deploying enterprise Web services focusing on the role of security. Web services security is presented in the context of an overall Web services architecture, with emphasis on manageability and scalability. This article is intended for several audiences.

  • IT management staff:
    -Provide guidance on the types of security relevant to the enterprise
    -Provide high-level requirements that a scalable security architecture should meet
    -Provide a logical model for Web services security deployment
  • Security architects:
    -Describe the relationships between security and the key components of a Web services architecture
    -Provide security deployment models that address a range of enterprise architectures and meet the proposed scalability requirements
  • Independent software vendors (ISV)
    -Describe interface and integration points between the various products in a Web services architecture
Reference Architecture Components
The reference architecture includes the following components:
  • A logical framework describing the concepts and product categories required for a complete Web services solution
  • Architectural guidelines for Web services security
  • Deployment models for a family of enterprise Web services security architectures
Logical Architecture Model
The logical model of the reference architecture describes the various layers of functionality required in terms of:
  • Protection and threat prevention: (network security)
  • Access enablement: Identity and Access Management - IAM
  • Business policy enforcement: Web services management - WSM
Figure 1 shows the Simple Object Access Protocol (SOAP) message flow going in the enterprise to the Web services provider, and the SOAP message flow coming out of the enterprise from the Web services consumer to outside partners.

 

Traditional network firewalls and security appliances make up the protection and threat prevention layer (network security). Identity and access management (IAM) is the access enablement layer, Web services management is the business policy enforcement layer, and Web services are deployed to and hosted by a container or a group of containers.

Several logical architecture layers may be implemented in a single product. For example, security appliances combine network security with some identity services (e.g., authentication). Xontainers often include some components of the WSM and IAM layers. The logical model is consistent with these and other possible implementation realizations.

Protection and Threat Prevention Layer
The protection and threat prevention layer is implemented by traditional network firewalls and network security appliances operating at different levels of the security stack, as shown in Figure 2.

 

Both traditional network firewalls and security appliances may include support for intrusion detection, thus protecting Web resources against HTTP-based threats such as denial of service (sending more requests to a Web service than the system that hosts it can handle), buffer overflow (trying to store a larger amount of data than a temporary storage [or buffer[ can handle), replay attacks (copying a valid message and resending it repeatedly to a Web service to clog its hosting environment), and malicious SQL or JavaScript tampering in business payloads (HTML forms or XML documents).

Traditional Network Firewalls
Traditional network firewalls are devices that enforce access rights between two networks, for example, the Internet and the enterprise. One side of the wall (the enterprise) is protected from the other side of the wall (the Internet) by filtering out undesirable data packets. For example, simple packet-filtering devices check Internet Protocol (IP) addresses and port numbers to allow the authorized data packets to pass through the firewall.

Security Appliances
Security appliances come in various form factors and support analysis of network flows at the upper layers of the security stack. They can be available as software products (also known as XML firewalls), and hardware devices (also known as XML or network appliances).

XML appliances scan the incoming byte streams for specific XML tags. XML appliances originated as XML traffic accelerators providing high-speed XML processing such as parsing of an XML document for well-formedness, validation of an XML document against an XML Schema, format transformation (e.g., between two XML dialects or from XML to HTML).

Over time, XML appliances have added standards-based security features, in particular XML encryption and XML signature, as well as high-level security frameworks such as the Security Assertion Markup Language (SAML). These capabilities provide in the domain of XML the equivalent of what Secure Socket Layer (SSL) concentrators provide for transport-level security.

Some security appliances interface with user stores provided by Lightweight Directory Access Protocol (LDAP) implementations or Relational Database Management Systems (RDBMS), as shown in Figure 3

 

Access Enablement Layer
The protection and threat prevention layer and the enablement layer are distinct. To cover all aspects of security management, an enterprise architecture needs to incorporate both layers as they serve different purposes.

The enablement layer includes identity and access management (IAM).

  • Identity management: The process of capturing and maintaining key security and profile attributes used to recognize users throughout an enterprise's organizational groups and lines of business. Identity management also involves the ability to establish, maintain, and terminate privileges and policies within the access management infrastructure.
  • Access management: The process of authentication (verifying a user's identity based on submitted credentials), authorization (granting access to specific resources based on an authenticated user's entitlements or access rights), audit (examining accesses to protected resources), and sessioning (propagating identities across lines of business or between trusted business partners).
An IAM platform allows the enterprise to manage a single set of identities (user names and profiles or attributes) and maintain an enterprise-wide logical user repository (user information can actually be stored in several user directories using a mix of different technologies, such as Microsoft's Active Directory [AD], LDAP implementations, RDBMS, or mainframe applications).

The IAM platform secures access to corporate resources using centralized access policies based on identities and roles. For example, an access policy may state that users must have certain roles in order to be authorized to access an application.

Unlike the protection and threat prevention layer, an IAM platform knows both identities and Web services. As a result, a wide range of identity-based services can be provided by the enablement layer, in particular access control and management of user attributes for authorization decisions.

IAM platforms can be delivered in different ways. Figure 4 shows an example of an IAM platform that includes an identity management application residing on an application server, and enterprise security policies stored in a policy server that interfaces with the enterprise's multiple user directories (information about a single user may be contained in several user stores that need to be queried to extract relevant user data).

 

In Figure 4, security enforcement is provided by agents that intercept requests to corporate resources. Users can be authenticated against a specific user store and authorized against another user store configured with the policy server, thus leveraging existing user information wherever it is available across the enterprise.

Business Policy Enforcement Layer
The business policy enforcement layer is embodied by Web services management (WSM) systems. Figure 5 shows an example of a typical WSM platform. Various implementations are available from diverse WSM vendors.

 

WSM systems help companies manage the services provided by the applications that enable service-oriented architectures. For example, WSM systems can help the IT department manage the availability of a Web service based on corporate business rules.

Typically, WSM systems support binding of business-level management policies to Web services. Policies may address service-level agreements (SLA) such as:

  • Availability (making sure the application supporting a Web service is up and running)
  • Metering and billing
  • Quality of service (ensuring that certain performance metrics are met by a Web service)
An example of a business policy defined by the WSM platform could be, "No more than 10 purchase orders should be accepted from Partner A during any 10-day period." WSM systems provide runtime enforcement of such policies via a Web service management pipeline or interception point. Identity is an essential prerequisite for the enforcement of business-level policies.

Advanced WSM platforms support a broader range of policies including policies relating to identity and access management. A key proposal in this reference architecture is for the WSM platform to rely on the IAM layer to integrate with the enterprise security infrastructure, thus benefiting from enterprise-wide, identity-bound security services.

Web Services Containers
Web services containers host Web services on industry-standard platforms such as Microsoft .NET and the Java 2 Platform, Enterprise Edition (J2EE). Typically, Web services containers include Web servers, application servers, business-to-business servers, messaging servers, enterprise resource planning (ERP) systems, etc.

Web services containers support Web services development and deployment based on industry standards such as the Extensible Markup Language (XML) Schema, SOAP, the Web Services Definition Language (WSDL), Universal Description, and Discovery Integration (UDDI), and other emerging specifications.

Web services containers include a security layer specific to each vendor's platform, resulting in multiple points of security administration if the enterprise uses multiple Web services container platforms. Most Web services containers interface with the IAM platform by exchanging security information included in standard headers defined by the Web Services Security (WS-Security) specification. WS-Security defines profiles for various security tokens, for example, SAML. In this way, heterogeneous platforms can use standard security credentials to support cross-platform security.

Reference Architecture Guidelines
This section presents the general security properties that should not be overlooked when deploying networks of Web services in the enterprise.

  • Abstracting security from Web services development
  • Complying with Web services standards
  • Relying on centralized, policy-based security management
  • Delivering the IAM layer as a set of shared services
  • Integrating with existing enterprise security systems
Abstracting Security from Web Services Development
A Web service is a program that can be developed in any language, the most prevalent ones being Java and .NET-supported languages, for example, C# or Visual Basic.NET. Most Web services development environments generate the XML document (a WSDL file) that describes how the Web service functions can be accessed by the requesting parties (Web services consumers).

Once developed, a Web service needs to be deployed to its container. Most Web services vendors provide integrated development environments (IDE) or tool kits that allow you to automatically broadcast Web services through SOAP messages, including UDDI requests for publication.

Early attempts to secure access to Web services were achieved programmatically by requiring the Web service consumer to provide a key with each request. Logic in the Web service would process the key and give access to the requester if the appropriate value was passed. This approach did not meet with much success because it was unwieldy and not scalable, but its premises and basic principles are still used by platform vendors to implement security in Web services, with the following drawbacks:

  • The onus is on the developer/deployer to implement security in the Web services environment
  • The security-specific code needs to be duplicated in each Web service implementation, leading to "silos" of application-level security
It is recommended that security logic be externalized outside applications and implemented by security services.

Complying with Web Services Security Standards
Over the past year, a standards-based security model for Web services has been defined and widely endorsed by the industry. In addition to existing transport-level standards such as SSL, the basis for the security model is provided by XML Encryption, XML Signature, WS-Security, and SAML.

XML Content Confidentiality, Integrity, and Authenticity

  • XML Encryption: Represents the encrypted content of XML data, the information that enables a recipient to decrypt it, and a mechanism for conveying encryption-key information to the recipient.
  • XML Signature: Defines the representation of signatures on digital content, and procedures for processing those signatures. XML Signature provides detailed elements supporting data integrity, signature assurance, and non-repudiation for Web services data.
Message Structure
  • WS-Security: Defines how to attach signature and encryption headers to SOAP messages. It also provides profiles that specify how to insert different types of security tokens in WS-Security headers
    -Username/Password digest: Defines how a Web services consumer can supply a username as a credential for authentication. The username can be accompanied by an encrypted password.
    -X.509 certificate: Defines a binding between a public key and a set of attributes used for authentication, e.g., username, certificate issuer, etc.
    -Kerberos ticket: A security token used for authentication.
    -SAML assertion: A security token that can include authentication, attributes, and authorization-decision statements. SAML assertions communicate security information between heterogeneous applications or Web services, within the enterprise or between the enterprise and its trusted partners.
    -XrML document: The Right Expression Language (REL) license tokens inserted in WS-Security headers are used for authorization and are based on the Extensible Rights Markup Language (XrML).
    -XCBF document: Defines how to use the XML Common Biometric Format (XCBF) with the WS-Security specification.
Emerging specifications are based on WS-Security and purport to provide higher-level service interfaces such as authentication, sessioning, and syntax for security metadata. It will take some time before these specifications are adopted as industry standards, but vendors should participate in their definitions and implement them as they mature.

Support for standards such as WS-Security and SAML are key requirements of the reference architecture. However, support for standards alone does not address the need for a scalable and manageable enterprise security infrastructure.

Relying on Centralized, Policy-Based Security Management
Security services enforce and implement security policies uniformly across the enterprise. Security policies apply to both the protection and threat prevention layer (network security) and the access enablement layer (IAM).

Identities should be bound to security policies in the IAM platform. Security policies describe a broad range of identity-based security services such as passwords, credentials, authentication, attributes, and authorization-decision information. For example, a security policy may state that "Only Preferred Partners can use the Inventory Control Web service. Authentication based on WS-Security Digital Signature or WS-Security User Name and Password are acceptable."

Relying on a centralized, policy-based security management approach provides many advantages:

  • Single point of access: All the security information can be concentrated in a centralized point of control, such as a policy server.
  • Scalability: A single set of policies can be managed by delegated administrators. For example, the corporate security administrator can assign administration roles to separate departments or sub-groups.
  • Manageability: Every time a policy needs to be changed, modifications apply to all the applications and Web services that use it, thus minimizing the complexity of propagating and managing the security information of the enterprise.
  • Conformance: Policy-based security management facilitates compliance with established company policies and business processes, as well as industry regulations.
Delivering the IAM Layer as a Set of Shared Services
The IAM layer is used to define rules applicable to services that can be shared throughout the enterprise, such as authentication, user attributes, authorization decision, sessioning, and federation (describing user identities to partners).

IAM gives the enterprise the ability to extend existing authorization models. The IAM layer can map organizational structures and functional responsibilities to create and manage roles. The IAM platform can then bind policies to roles for access control.

When IAM is used as a shared service, other areas of activities can concentrate on their specific functions. For example, providing a shared security service allows applications and Web services developers to concentrate on business logic, thus reducing the complexity of applications and avoiding "silos" of security.

Integrating with Existing Enterprise Security Systems
Very often, companies have to deal with many diverse and heterogeneous systems, each requiring its own administrative interface. Adding security to each of these systems increases cost of ownership and administration complexity.

Summary
Web services are only one of many resources that need to be secured. They should not be dependent on specific security environments and rules but should be managed as part of all of an enterprise's corporate data assets such as Web applications, ERP systems, and in-house applications.

Companies that rely on specific tools or security models to protect access to Web services will later need to integrate their Web services security solution to their overall enterprise infrastructure to both increase security and lower administration costs.

It is recommended that Web services security be integrated with the overall enterprise security infrastructure at the very beginning of the Web services deployment phase.

More Stories By Marc Chanliau

Marc Chanliau has been in the software industry for more than 20 years and is currently a director of product management at Oracle where he is responsible for Identity Management solutions and innovations. He is heavily involved in security and XML standards groups including serving as the first chair person of the OASIS Security Services Technical Committee (SSTC), which culminated in the adoption of SAML as an official OASIS standard, participating on the WS-Security Technical Committee, helping to define the Liberty Alliance 2.0 specifications, and participating in the Java Specification Request (JSR) committee.

More Stories By Prateek Mishra

Prateek Mishra, Ph.D. has more than ten years experience with enterprise-class distributed systems. He is Director of Technology at Netegrity and works on Strategy and Standards. He was an Editor of the SAML 1.0 specification and is co-chair of the SSTC (SAML) Committee and participates in the WSS (WS-Security) Committee.

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