Web services security -- Chapter 8, OWASP Guide to Building Secure Web Applications and Web Services

Web services security issues and how to deal with those issues are addressed in this section of the OWASP Guide to Building Secure Web Applications and Web Services.

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Web Services

This section of the Guide details the common issues facing Web services developers, and
methods to address common issues. Due to the space limitations, it cannot look at all of the
surrounding issues in great detail, since each of them deserves a separate book of its own.
Instead, an attempt is made to steer the reader to the appropriate usage patterns, and warn about
potential roadblocks on the way.

Web Services have received a lot of press, and with that comes a great deal of confusion over
what they really are. Some are heralding Web Services as the biggest technology breakthrough
since the web itself; others are more skeptical that they are nothing more than evolved web
applications. In either case, the issues of web application security apply to web services just as
they do to web applications.

At the simplest level, web services can be seen as a specialized web application that differs mainly
at the presentation tier level. While web applications typically are HTML-based, web
services are XML-based. Interactive users for B2C transactions normally access web
applications, while web services are employed as building blocks by other web applications for
forming B2B chains using the so-called SOA model. Web services typically present a public
functional interface, callable in a programmatic fashion, while web applications tend to deal with
a richer set of features and are content-driven in most cases.

Securing Web Services
Web Service, like other distributed applications, require protection at multiple levels:

  • SOAP messages that are sent on the wire should be delivered confidentially and without
    tampering.
  • The server needs to be confident who it is talking to and what the clients are entitled to.

  • The clients need to know that they are talking to the right server, and not a phishing site
    (see the Phishing chapter for more information)
  • System message logs should contain sufficient information to reliably reconstruct the
    chain of events and track those back to the authenticated callers

Correspondingly, the high-level approaches to solutions, discussed in the following sections,
are valid for pretty much any distributed application, with some variations in the implementation details.

The good news for Web Services developers is that these are infrastructure-level tasks, so,
theoretically, it is only the system administrators who should be worrying about these issues.
However, for a number of reasons discussed later in this chapter, WS developers usually have to
be at least aware of all these risks, and oftentimes they still have to resort to manually coding or
tweaking the protection components.

Communication security
There is a commonly cited statement, and even more often implemented approach – "we are
using SSL to protect all communication, we are secure". At the same time, there have been so
many articles published on the topic of "channel security vs. token security" that it hardly makes
sense to repeat those arguments here. Therefore, listed below is just a brief rundown of most
common pitfalls when using channel security alone:

  • It provides only "point-to-point" security
    Any communication with multiple "hops" requires establishing separate channels (and trusts)
    between each communicating node along the way. There is also a subtle issue of trust
    transitivity, as trusts between node pairs {A,B} and {B,C} do not automatically imply {A,C}
    trust relationship.
  • Storage issue
    After messages are received on the server (even if it is not the intended recipient), they exist
    in the clear-text form, at least – temporarily. Storing the transmitted information at the
    intermediate aggravates the problem or destination servers in log files (where it can be browsed
    by anybody) and local caches.
  • Lack of interoperability
    While SSL provides a standard mechanism for transport protection, applications then have to
    utilize highly proprietary mechanisms for transmitting credentials, ensuring freshness, integrity,
    and confidentiality of data sent over the secure channel. Using a different server, which is
    semantically equivalent, but accepts a different format of the same credentials, would require
    altering the client and prevent forming automatic B2B service chains.

    Standards-based token protection in many cases provides a superior alternative for message-
    oriented Web Service SOAP communication model.

    That said -- the reality is that the most Web Services today are still protected by some form of
    channel security mechanism, which alone might suffice for a simple internal application.
    However, one should clearly realize the limitations of such approach, and make conscious
    trade-offs at the design time, whether channel, token, or combined protection would work better
    for each specific case.

Passing credentials
In order to enable credentials exchange and authentication for Web Services, their developers
must address the following issues.

First, since SOAP messages are XML-based, all passed credentials have to be converted to
text format. This is not a problem for username/password types of credentials, but binary ones
(like X.509 certificates or Kerberos tokens) require converting them into text prior to sending
and unambiguously restoring them upon receiving, which is usually done via a procedure called
Base64 encoding and decoding.

Second, passing credentials carries an inherited risk of their disclosure – either by sniffing
them during the wire transmission, or by analyzing the server logs. Therefore, things like
passwords and private keys need to be either encrypted, or just never sent "in the clear". Usual
ways to avoid sending sensitive credentials are using cryptographic hashing and/or signatures.

Ensuring message freshness
Even a valid message may present a danger if it is utilized in a "replay attack" – i.e. it is sent
multiple times to the server to make it repeat the requested operation. This may be achieved by
capturing an entire message, even if it is sufficiently protected against tampering, since it is the
message itself that is used for attack now (see the XML Injection section of the Interpreter
Injection chapter).

Usual means to protect against replayed messages is either using unique identifiers (nonces)
on messages and keeping track of processed ones, or using a relatively short validity time
window. In the Web Services world, information about the message creation time is usually
communicated by inserting timestamps, which may just tell the instant the message was created,
or have additional information, like its expiration time, or certain conditions.

The latter solution, although easier to implement, requires clock synchronization and is
sensitive to "server time skew," whereas server or clients clocks drift too much, preventing
timely message delivery, although this usually does not present significant problems with
modern-day computers. A greater issue lies with message queuing at the servers, where
messages may be expiring while waiting to be processed in the queue of an especially busy or
non-responsive server.

Protecting message integrity
When a message is received by a web service, it must always ask two questions: "whether I
trust the caller," "whether it created this message." Assuming that the caller trust has been
established one way or another, the server has to be assured that the message it is looking at was
indeed issued by the caller, and not altered along the way (intentionally or not). This may affect
technical qualities of a SOAP message, such as the message's timestamp, or business content,
such as the amount to be withdrawn from the bank account. Obviously, neither change should go
undetected by the server.

In communication protocols, there are usually some mechanisms like checksum applied to
ensure packet's integrity. This would not be sufficient, however, in the realm of publicly exposed
Web Services, since checksums (or digests, their cryptographic equivalents) are easily
replaceable and cannot be reliably tracked back to the issuer. The required association may be
established by utilizing HMAC, or by combining message digests with either cryptographic
signatures or with secret key-encryption (assuming the keys are only known to the two
communicating parties) to ensure that any change will immediately result in a cryptographic error.

Protecting message confidentiality
Oftentimes, it is not sufficient to ensure the integrity – in many cases it is also desirable that
nobody can see the data that is passed around and/or stored locally. It may apply to the entire
message being processed, or only to certain parts of it – in either case, some type of encryption is
required to conceal the content. Normally, symmetric encryption algorithms are used to encrypt
bulk data, since it is significantly faster than the asymmetric ones. Asymmetric encryption is then
applied to protect the symmetric session keys, which, in many implementations, are valid for one
communication only and are subsequently discarded.

Applying encryption requires conducting an extensive setup work, since the communicating
parties now have to be aware of which keys they can trust, deal with certificate and key
validation, and know which keys should be used for communication.

In many cases, encryption is combined with signatures to provide both integrity and
confidentiality. Normally, signing keys are different from the encrypting ones, primarily because
of their different lifecycles – signing keys are permanently associated with their owners, while
encryption keys may be invalidated after the message exchange. Another reason may be
separation of business responsibilities - the signing authority (and the corresponding key) may
belong to one department or person, while encryption keys are generated by the server controlled
by members of IT department.

Access control
After message has been received and successfully validated, the server must decide:

  • Does it know who is requesting the operation (Identification)
  • Does it trust the caller's identity claim (Authentication)
  • Does it allow the caller to perform this operation (Authorization)

There is not much WS-specific activity that takes place at this stage – just several new ways
of passing the credentials for authentication. Most often, authorization (or entitlement) tasks
occur completely outside of the Web Service implementation, at the Policy Server that protects
the whole domain.

There is another significant problem here – the traditional HTTP firewalls do not help at
stopping attacks at the Web Services. An organization would need a XML/SOAP firewall, which
is capable of conducting application-level analysis of the web server's traffic and make
intelligent decision about passing SOAP messages to their destination. The reader would need to
refer to other books and publications on this very important topic, as it is impossible to cover it
within just one chapter.

Audit
A common task, typically required from the audits, is reconstructing the chain of events that
led to a certain problem. Normally, this would be achieved by saving server logs in a secure
location, available only to the IT administrators and system auditors, in order to create what is
commonly referred to as "audit trail". Web Services are no exception to this practice, and follow
the general approach of other types of Web Applications.

Another auditing goal is non-repudiation, meaning that a message can be verifiably traced
back to the caller. Following the standard legal practice, electronic documents now require some
form of an "electronic signature", but its definition is extremely broad and can mean practically
anything – in many cases, entering your name and birthday qualifies as an e-signature.

As far as the WS are concerned, such level of protection would be insufficient and easily
forgeable. The standard practice is to require cryptographic digital signatures over any content
that has to be legally binding – if a document with such a signature is saved in the audit log, it
can be reliably traced to the owner of the signing key.

Web Services Security Hierarchy
Technically speaking, Web Services themselves are very simple and versatile – XML-based
communication, described by an XML-based grammar, called Web Services Description
Language (WSDL, see http://www.w3.org/TR/2005/WD-wsdl20-20050510), which binds abstract
service interfaces, consisting of messages, expressed as XML Schema, and operations, to the
underlying wire format. Although it is by no means a requirement, the format of choice is
currently SOAP over HTTP. This means that Web Service interfaces are described in terms of
the incoming and outgoing SOAP messages, transmitted over HTTP protocol.

Standards committees
Before reviewing the individual standards, it is worth taking a brief look at the organizations,
which are developing and promoting them. There are quite a few industry-wide groups and
consortiums, working in this area, most important of which are listed below.

W3C (see http://www.w3.org) is the most well known industry group,
which owns many Web-related standards and develops them in Working Group format. Of particular
interest to this chapter are XML Schema, SOAP, XML-dsig, XML-enc,
and WSDL standards (called recommendations in the W3C's jargon).

OASIS (see http://www.oasis-open.org) mostly deals with Web Service-specific
standards, not necessarily security-related. It also operates on a committee basis, forming
so-called Technical Committees (TC) for the standards that it is going to be developing.
Of interest for this discussion, OASIS owns WS-Security and SAML standards.

Web Service Interoperability group (WS-I, see http://www.ws-i.org/) was formed to promote general
framework for interoperable Web Services. Mostly its work consists of taking other broadly
accepted standards, and develop so-called profiles, or set of requirements for conforming Web
Service implementations. In particular, its Basic Security Profile (BSP) relies on the OASIS'
WS-Security standard and specifies sets of optional and required security features in Web Services
that claim interoperability.

Liberty Alliance (LA, see http://projectliberty.org) consortium was formed
to develop and promote an interoperable Identity Federation framework. Although this
framework is not strictly Web Service-specific, but rather general, it is important for this topic
because of its close relation with the SAML standard developed by OASIS.

Besides the previously listed organizations, there are other industry associations, both
permanently established and short-lived, which push forward various Web Service security
activities. They are usually made up of software industry's leading companies, such as
Microsoft, IBM, Verisign, BEA, Sun, and others, that join them to work on a particular issue or
proposal. Results of these joint activities, once they reach certain maturity, are often submitted to
standardizations committees as a basis for new industry standards.

SOAP
Simple Object Access Protocol (SOAP, see http://www.w3.org/TR/2003/REC-soap12-part1-20030624/)
provides an XML-based framework for exchanging structured and typed information between
peer services. This information, formatted into Header and Body, can theoretically be transmitted
over a number of transport protocols, but only HTTP binding has been formally defined and is in
active use today. SOAP provides for Remote Procedure Call-style (RPC) interactions, similar to
remote function calls, and Document-style communication, with message contents based
exclusively on XML Schema definitions in the Web Service's WSDL. Invocation results may be
optionally returned in the response message, or a Fault may be raised, which is roughly
equivalent to using exceptions in traditional programming languages.

SOAP protocol, while defining the communication framework, provides no help in terms of
securing message exchanges – the communications must either happen over secure channels, or
use protection mechanisms described later in this chapter.

XML security specifications (XML-dsig & Encryption)
XML Signature (XML-dsig, see http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/),
and XML Encryption (XML-enc, see http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/)
add cryptographic protection to plain XML documents. These specifications add integrity,
message and signer authentication, as well as support for encryption/decryption of whole XML
documents or only of some elements inside them.

The real value of those standards comes from the highly flexible framework developed to
reference the data being processed (both internal and external relative to the XML document),
refer to the secret keys and key pairs, and to represent results of signing/encrypting operations as
XML, which is added to/substituted in the original document.

However, by themselves, XML-dsig and XML-enc do not solve the problem of securing
SOAP-based Web Service interactions, since the client and service first have to agree on the
order of those operations, where do look for the signature, how to retrieve cryptographic tokens,
which message elements should be signed and encrypted, how long a message is considered to
be valid, and so on. These issues are addressed by the higher-level specifications, reviewed in the
following sections.

Security specifications
In addition to the above standards, there is a broad set of security-related specifications being
currently developed for various aspects of Web Service operations.

One of them is SAML, which defines how identity, attribute, and authorization assertions
should be exchanged among participating services in a secure and interoperable way.

A broad consortium, headed by Microsoft and IBM, with the input from Verisign, RSA
Security, and other participants, developed a family of specifications, collectively known as
"Web Services Roadmap". Its foundation, WS-Security, has been submitted to OASIS and
became an OASIS standard in 2004. Other important specifications from this family are still
found in different development stages, and plans for their submission have not yet been
announced, although they cover such important issues as security policies (WS-Policy et al),
trust issues and security token exchange (WS-Trust), establishing context for secure conversation
(WS-SecureConversation). One of the specifications in this family, WS-Federation, directly
competes with the work being done by the LA consortium, and, although it is supposed to be
incorporated into the Longhorn release of Windows, its future is not clear at the moment, since it
has been significantly delayed and presently does not have industry momentum behind it.

WS-Security Standard
WS-Security specification (WSS) was originally developed by Microsoft, IBM, and Verisign
as part of a "Roadmap", which was later renamed to Web Services Architecture, or WSA. WSS
served as the foundation for all other specifications in this domain, creating a basic infrastructure
for developing message-based security exchange. Because of its importance for establishing
interoperable Web Services, it was submitted to OASIS and, after undergoing the required
committee process, became an officially accepted standard. Current version is 1.0, and the work
on the version 1.1 of the specification is under way and is expected to be finishing in the second
half of 2005.

Organization of the standard
The WSS standard itself deals with several core security areas, leaving many details to so-
called profile documents. The core areas, broadly defined by the standard, are:

  • Ways to add security headers (WSSE Header) to SOAP Envelopes
  • Attachment of security tokens and credentials to the message
  • Inserting a timestamp
  • Signing the message
  • Encrypting the message
  • Extensibility

Flexibility of the WS-Security standard lies in its extensibility, so that it remains adaptable to
new types of security tokens and protocols that are being developed. This flexibility is achieved
by defining additional profiles for inserting new types of security tokens into the WSS
framework. While the signing and encrypting parts of the standards are not expected to require
significant changes (only when the underlying XML-dsig and XML-enc are updated), the types
of tokens, passed in WSS messages, and ways of attaching them to the message may vary
substantially. At the high level the WSS standard defines three types of security tokens,
attachable to a WSS Header: Username/password, Binary, and XML tokens. Each of those types
is further specified in one (or more) profile document, which defines additional token's attributes
and elements, needed to represent a particular type of security token.

Figure 2: WSS specification hierarchy

Purpose
The primary goal of the WSS standard is providing tools for message-level communication
protection, whereas each message represents an isolated piece of information, carrying enough
security data to verify all important message properties, such as: authenticity, integrity,
freshness, and to initiate decryption of any encrypted message parts. This concept is a stark
contrast to the traditional channel security, which methodically applies pre-negotiated security
context to the whole stream, as opposed to the selective process of securing individual messages
in WSS. In the Roadmap, that type of service is eventually expected to be provided by
implementations of standards like WS-SecureConversation.

From the beginning, the WSS standard was conceived as a message-level toolkit for securely
delivering data for higher level protocols. Those protocols, based on the standards like
WS-Policy, WS-Trust, Liberty Alliance, rely on the transmitted tokens to implement access
control policies, token exchange, and other types of protection and integration. However, taken
alone, the WSS standard does not mandate any specific security properties, and an ad-hoc
application of its constructs can lead to subtle security vulnerabilities and hard to detect
problems, as is also discussed in later sections of this chapter.

WS-Security Building Blocks
The WSS standard actually consists of a number of documents – one core document, which
defines how security headers may be included into SOAP envelope and describes all high-level
blocks, which must be present in a valid security header. Profile documents have the dual task of
extending definitions for the token types they are dealing with, providing additional attributes,
elements, as well as defining relationships left out of the core specification, such as using
attachments.

Core WSS 1.0 specification, located at
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0,
defines several types of security tokens (discussed later in this section – see 0), ways to reference
them, timestamps, and ways to apply XML-dsig and XML-enc in the security headers – see the
XML Dsig section for more details about their general structure.

How data is passed
WSS security specification deals with two distinct types of data: security information, which
includes security tokens, signatures, digests, etc; and message data, i.e. everything else that is
passed in the SOAP message. Being an XML-based standard, WSS works with textual
information grouped into XML elements. Any binary data, such as cryptographic signatures or
Kerberos tokens, has to go through a special transform, called Base64 encoding/decoding, which
provides straightforward conversion from binary to ASCII formats and back. Example below
demonstrates how binary data looks like in the encoded format:

cCBDQTAeFw0wNDA1MTIxNjIzMDRaFw0wNTA1MTIxNjIzMDRaMG8xCz

After encoding a binary element, an attribute with the algorithm's identifier is added to the
XML element carrying the data, so that the receiver would know to apply the correct decoder to
read it. These identifiers are defined in the WSS specification documents.

Security header's structure
A security header in a message is used as a sort of an envelope around a letter – it seals and
protects the letter, but does not care about its content. This "indifference" works in the other
direction as well, as the letter (SOAP message) should not know, nor should it care about its
envelope (WSS Header), since the different units of information, carried on the envelope and in
the letter, are presumably targeted at different people or applications.

A SOAP Header may actually contain multiple security headers, as long as they are
addressed to different actors (for SOAP 1.1), or roles (for SOAP 1.2). Their contents may also be
referring to each other, but such references present a very complicated logistical problem for
determining the proper order of decryptions/signature verifications, and should generally be
avoided. WSS security header itself has a loose structure, as the specification itself does not
require any elements to be present – so, the minimalist header with an empty message will
look like:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
    <soap:Header>
        <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" soap:mustUnderstand="1">
        
        </wsse:Security>
    </soap:Header>>
    <soap:Body>

    </soap:Body>
</soap:Envelope>

However, to be useful, it must carry some information, which is going to help securing the
message. It means including one or more security tokens (see 0) with references, XML
Signature, and XML Encryption elements, if the message is signed and/or encrypted. So, a
typical header will look more like the following picture:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <wsse:Security xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" soap:mustUnderstand="1">
      <wsse:BinarySecurityToken EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" wsu:Id="aXhOJ5">MIICtzCCAi... 
      </wsse:BinarySecurityToken>
      <xenc:EncryptedKey xmlns:xenc="http://www.w3.org/2001/04/xmlenc#">
        <xenc:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
      <dsig:KeyInfo xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
        <wsse:SecurityTokenReference>
          <wsse:Reference URI="#aXhOJ5" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3"/>
        </wsse:SecurityTokenReference>  
      </dsig:KeyInfo>
      <xenc:CipherData>
          <xenc:CipherValue>Nb0Mf... </xenc:CipherValue>
      </xenc:CipherData>
      <xenc:ReferenceList>
          <xenc:DataReference URI="#aDNa2iD"/>
      </xenc:ReferenceList>
      </xenc:EncryptedKey>
      <wsse:SecurityTokenReference wsu:Id="aZG0sG">
      <wsse:KeyIdentifier ValueType="http://docs.oasis-open.org/wss/2004/XX/oasis-2004XX-wss-saml-token-profile-1.0#SAMLAssertionID" wsu:Id="a2tv1Uz">1106844369755</wsse:KeyIdentifier>
      </wsse:SecurityTokenReference>
      <saml:Assertion AssertionID="1106844369755" IssueInstant="2005-01-27T16:46:09.755Z" Issuer="www.my.com" MajorVersion="1" MinorVersion="1" xmlns:saml="urn:oasis:names:tc:SAML:1.0:assertion">
            ...    
      </saml:Assertion>
      <wsu:Timestamp wsu:Id="afc6fbe-a7d8-fbf3-9ac4-f884f435a9c1">
      <wsu:Created>2005-01-27T16:46:10Z</wsu:Created>
      <wsu:Expires>2005-01-27T18:46:10Z</wsu:Expires>
      </wsu:Timestamp>
      <dsig:Signature xmlns:dsig="http://www.w3.org/2000/09/xmldsig#" Id="sb738c7">
      <dsig:SignedInfo Id="obLkHzaCOrAW4kxC9az0bLA22">
            ...
        <dsig:Reference URI="#s91397860">
            ...         
            <dsig:DigestValue>5R3GSp+OOn17lSdE0knq4GXqgYM=< /dsig:DigestValue>
        </dsig:Reference>
        </dsig:SignedInfo>
        <dsig:SignatureValue Id="a9utKU9UZk">LIkagbCr5bkXLs8l... </dsig:SignatureValue>
        <dsig:KeyInfo>
        <wsse:SecurityTokenReference>
          <wsse:Reference URI="#aXhOJ5" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3"/>
        </wsse:SecurityTokenReference>
        </dsig:KeyInfo>
      </dsig:Signature>
    </wsse:Security>
  </soap:Header>
  <soap:Body xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" wsu:Id="s91397860">
    <xenc:EncryptedData xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" Id="aDNa2iD" Type="http://www.w3.org/2001/04/xmlenc#Content">
      <xenc:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc"/>
      <xenc:CipherData>
      <xenc:CipherValue>XFM4J6C... </xenc:CipherValue>
      </xenc:CipherData>
    </xenc:EncryptedData>
  </soap:Body>
</soap:Envelope>

Types of tokens
A WSS Header may have the following types of security tokens in it:

  • Username token
    Defines mechanisms to pass username and, optionally, a password - the latter is described in
    the username profile document. Unless whole token is encrypted, a message which includes a
    clear-text password should always be transmitted via a secured channel. In situations where the
    target Web Service has access to clear-text passwords for verification (this might not be possible
    with LDAP or some other user directories, which do not return clear-text passwords), using a
    hashed version with nonce and a timestamp is generally preferable. The profile document defines
    an unambiguous algorithm for producing password hash:
    Password_Digest = Base64 ( SHA-1 ( nonce + created + password ) )
    
  • Binary token
    They are used to convey binary data, such as X.509 certificates, in a text-encoded format,
    Base64 by default. The core specification defines BinarySecurityToken element, while profile
    documents specify additional attributes and sub-elements to handle attachment of various tokens.
    Presently, the X.509 profile has been adopted, and work is in progress on the Kerberos profile.
          <wsse:BinarySecurityToken EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3" wsu:Id="aXhOJ5">
            MIICtzCCAi...
          </wsse:BinarySecurityToken>
    
  • XML token
    These are meant for any kind of XML-based tokens, but primarily – for SAML assertions.
    The core specification merely mentions the possibility of inserting such tokens, leaving all
    details to the profile documents. At the moment, SAML 1.1 profile has been accepted by OASIS.
          <saml:Assertion AssertionID="1106844369755" IssueInstant="2005-01-27T16:46:09.755Z" Issuer="www.my.com" MajorVersion="1" MinorVersion="1" xmlns:saml="urn:oasis:names:tc:SAML:1.0:assertion">
                ...    
          </saml:Assertion>
    

Although technically it is not a security token, a Timestamp element may be inserted into a
security header to ensure message's freshness. See the further reading section for a design
pattern on this.

Referencing message parts
In order to retrieve security tokens, passed in the message, or to identify signed and
encrypted message parts, the core specification adopts usage of a special attribute, wsu:Id. The
only requirement on this attribute is that the values of such IDs should be unique within the
scope of XML document where they are defined. Its application has a significant advantage for
the intermediate processors, as it does not require understanding of the message's XML Schema.
Unfortunately, XML Signature and Encryption specifications do not allow for attribute
extensibility (i.e. they have closed schema), so, when trying to locate signature or encryption
elements, local IDs of the Signature and Encryption elements must be considered first.

WSS core specification also defines a general mechanism for referencing security tokens via
SecurityTokenReference element. An example of such element, referring to a SAML assertion in
the same header, is provided below:

      <wsse:SecurityTokenReference wsu:Id="aZG0sGbRpXLySzgM1X6aSjg22">
        <wsse:KeyIdentifier ValueType="http://docs.oasis-open.org/wss/2004/XX/oasis-2004XX-wss-saml-token-profile-1.0#SAMLAssertionID" wsu:Id="a2tv1Uz">
            1106844369755
          </wsse:KeyIdentifier>
        </wsse:SecurityTokenReference>

As this element was designed to refer to pretty much any possible token type (including
encryption keys, certificates, SAML assertions, etc) both internal and external to the WSS
Header, it is enormously complicated. The specification recommends using two of its possible
four reference types – Direct References (by URI) and Key Identifiers (some kind of token
identifier). Profile documents (SAML, X.509 for instance) provide additional extensions to these
mechanisms to take advantage of specific qualities of different token types.

Communication Protection Mechanisms
As was already explained earlier (see 0), channel security, while providing important
services, is not a panacea, as it does not solve many of the issues, facing Web Service
developers. WSS helps addressing some of them at the SOAP message level, using the
mechanisms described in the sections below.

Integrity
WSS specification makes use of the XML-dsig standard to ensure message integrity,
restricting its functionality in certain cases; for instance, only explicitly referenced elements can
be signed (i.e. no Embedding or Embedded signature modes are allowed). Prior to signing an
XML document, a transformation is required to create its canonical representation, taking into
account the fact that XML documents can be represented in a number of semantically equivalent
ways. There are two main transformations defined by the XML Digital Signature WG at W3C,
Inclusive and Exclusive Canonicalization Transforms (C14N and EXC-C14N), which differ in
the way namespace declarations are processed. The WSS core specification specifically
recommends using EXC-C14N, as it allows copying signed XML content into other documents
without invalidating the signature.

In order to provide a uniform way of addressing signed tokens, WSS adds a Security Token
Reference (STR) Dereference Transform option, which is comparable with dereferencing a
pointer to an object of specific data type in programming languages. Similarly, in addition to the
XML Signature-defined ways of addressing signing keys, WSS allows for references to signing
security tokens through the STR mechanism (explained in 0), extended by token profiles to
accommodate specific token types. A typical signature example is shown in an earlier sample in
the section 0.

Typically, a XML signature is applied to secure elements such as SOAP Body and the
timestamp, as well as any user credentials, passed in the request. There is an interesting twist
when a particular element is both signed and encrypted, since these operations may follow (even
repeatedly) in any order, and knowledge of their ordering is required for signature verification.
To address this issue, the WSS core specification requires that each new element is pre-pended to
the security header, thus defining the "natural" order of operations. A particularly nasty problem
arises when there are several security headers in a single SOAP message, using overlapping
signature and encryption blocks, as there is nothing in this case that would point to the right
order of operations.

Confidentiality
For its confidentiality protection, WSS relies on yet another standard, XML Encryption.
Similarly to XML-dsig, this standard operates on selected elements of the SOAP message, but it
then replaces the encrypted element's data with a sub-element carrying
the encrypted bytes. For encryption efficiency, the specification recommends using a unique key,
which is then encrypted by the recipient's public key and pre-pended to the security header in a
element. A SOAP message with encrypted body is shown in the section 0.

Freshness
SOAP messages' freshness is addressed via timestamp mechanism – each security header
may contain just one such element, which states, in UTC time and using the UTC time format,
creation and expiration moments of the security header. It is important to realize that the
timestamp is applied to the WSS Header, not to the SOAP message itself, since the latter may
contain multiple security headers, each with a different timestamp. There is an unresolved
problem with this "single timestampt" approach, since, once the timestamp is created and signed,
it is impossible to update it without breaking existing signatures, even in case of a legitimate
change in the WSS Header.

      <wsu:Timestamp wsu:Id="afc6fbe-a7d8-fbf3-9ac4-f884f435a9c1">
      <wsu:Created>2005-01-27T16:46:10Z</wsu:Created>
      <wsu:Expires>2005-01-27T18:46:10Z</wsu:Expires>
      </wsu:Timestamp>

If a timestamp is included in a message, it is typically signed to prevent tampering and replay
attacks. There is no mechanism foreseen to address clock synchronization issue (which, as was
already point out earlier, is generally not an issue in modern day systems) -- this has to be
addressed out-of-band as far as the WSS mechanics is concerned. See the further reading section
for a design pattern addressing this issue.

Access Control Mechanisms
When it comes to access control decisions, Web Services do not offer specific protection
mechanisms by themselves – they just have the means to carry the tokens and data payloads in a
secure manner between source and destination SOAP endpoints.


Identification
Identification represents a claim to have certain identity, which is expressed by attaching
certain information to the message. This can be a username, a SAML assertion, a Kerberos
ticket, or any other piece of information, from which the service can infer who the caller claims
to be.

WSS represents a very good way to convey this information, as it defines an extensible
mechanism for attaching various token types to a message (see 0). It is the receiver's job to
extract the attached token and figure out which identity it carries, or to reject the message if it
can find no acceptable token in it.

Authentication
Authentication can come in two flavors – credentials verification or token validation. The
subtle difference between the two is that tokens are issued after some kind of authentication has
already happened prior to the current invocation, and they usually contain user's identity along
with the proof of its integrity.

WSS offers support for a number of standard authentication protocols by defining binding
mechanism for transmitting protocol-specific tokens and reliably linking them to the sender.
However, the mechanics of proof that the caller is who he claims to be is completely at the Web
Service's discretion. Whether it takes the supplied username and password's hash and checks it
against the backend user store, or extracts subject name from the X.509 certificate used for
signing the message, verifies the certificate chain and looks up the user in its store – at the
moment, there are no requirements or standards which would dictate that it should be done one
way or another.

Authorization
XACML may be used for expressing authorization rules, but its usage is not Web Service-
specific – it has much broader scope. So, whatever policy or role-based authorization mechanism
the host server already has in place will most likely be utilized to protect the deployed Web
Services deployed as well.

Depending on the implementation, there may be several layers of authorization involved at
the server. For instance, JSRs 224 (JAX-RPC 2.0) and 109 (Implementing Enterprise Web
Services), which define Java binding for Web Services, specify implementing Web Services in
J2EE containers. This means that when a Web Service is accessed, there will be a URL
authorization check executed by the J2EE container, followed by a check at the Web Service
layer for the Web Service-specific resource. Granularity of such checks is implementation-
specific and is not dictated by any standards. In the Windows universe it happens in a similar
fashion, since IIS is going to execute its access checks on the incoming HTTP calls before they
reach the ASP.NET runtime, where SOAP message is going to be further decomposed and
analyzed.

Policy Agreement
Normally, Web Services' communication is based on the endpoint's public interface, defined
in its WSDL file. This descriptor has sufficient details to express SOAP binding requirements,
but it does not define any security parameters, leaving Web Service developers struggling to find
out-of-band mechanisms to determine the endpoint's security requirements.

To make up for these shortcomings, WS-Policy specification was conceived as a mechanism
for expressing complex policy requirements and qualities, sort of WSDL on steroids. Through
the published policy SOAP endpoints can advertise their security requirements, and their clients
can apply appropriate measures of message protection to construct the requests. The general
WS-Policy specification (actually comprised of three separate documents) also has extensions
for specific policy types, one of them – for security, WS-SecurityPolicy.

If the requestor does not possess the required tokens, it can try obtaining them via trust
mechanism, using WS-Trust-enabled services, which are called to securely exchange various
token types for the requested identity.

Figure 3. Using Trust service






Forming Web Service Chains
Many existing or planned implementations of SOA or B2B systems rely on dynamic chains
of Web Services for accomplishing various business specific tasks, from taking the orders
through manufacturing and up to the distribution process.

Figure 4: Service chain



Here is just a few of the issues that hamper Web Services interaction – incompatible
authentication and authorization models for users, amount of trust between services themselves
and ways of establishing such trust, maintaining secure connections, and synchronization of user
directories or otherwise exchanging users' attributes. These issues will be briefly tackled in the
following paragraphs.

Incompatible user access control models
As explained earlier, in section 0, Web Services themselves do not include separate
extensions for access control, relying instead on the existing security framework. What they do
provide, however, are mechanisms for discovering and describing security requirements of a
SOAP service (via WS-Policy), and for obtaining appropriate security credentials via WS-Trust
based services.

Service trust
In order to establish mutual trust between client and service, they have to satisfy each other's
policy requirements. A simple and popular model is mutual certificate authentication via SSL,
but it is not scalable for open service models, and supports only one authentication type. Services
that require more flexibility have to use pretty much the same access control mechanisms as with
users to establish each other's identities prior to engaging in a conversation.

Secure connections
Once trust is established it would be impractical to require its confirmation on each
interaction. Instead, a secure client-server link is formed and maintained all time while client's
session is active. Again, the most popular mechanism today for maintaining such link is SSL, but
it is not a Web Service-specific mechanism, and it has a number of shortcomings when applied
to SOAP communication, as explained in 0.

Synchronization of user directories
This is a very acute problem when dealing with cross-domain applications, as users'
population tends to change frequently among different domains. So, how does a service in
domain B decide whether it is going to trust user's claim that he has been already authenticated
in domain A? There exist different aspects of this problem. First – a common SSO mechanism,
which implies that a user is known in both domains (through synchronization, or by some other
means), and authentication tokens from one domain are acceptable in another. In Web Services
world, this would be accomplished by passing around a SAML or Kerberos token for a user.

Domain federation
Another aspect of the problem is when users are not shared across domains, but merely the
fact that a user with certain ID has successfully authenticated in another domain, as would be the
case with several large corporations, which would like to form a partnership, but would be
reluctant to share customers' details. The decision to accept this request is then based on the
inter-domain procedures, establishing special trust relationships and allowing for exchanging
such opaque tokens, which would be an example of Federation relationships. Of those efforts,
most notable example is Liberty Alliance project, which is now being used as a basis for SAML
2.0 specifications. The work in this area is still far from being completed, and most of the
existing deployments are nothing more than POC or internal pilot projects than to real cross-
companies deployments, although LA's website does list some case studies of large-scale
projects.

Available Implementations
It is important to realize from the beginning that no security standard by itself is going to
provide security to the message exchanges – it is the installed implementations, which will be
assessing conformance of the incoming SOAP messages to the applicable standards, as well as
appropriately securing the outgoing messages.

.NET -- Web Service Extensions
Since new standards are being developed at a rather quick pace, .NET platform is not trying
to catch up immediately, but uses Web Service Extensions (WSE) instead. WSE, currently at the
version 2.0, adds development and runtime support for the latest Web Service security standards
to the platform and development tools, even while they are still "work in progress". Once
standards mature, their support is incorporated into new releases of the .NET platform, which is
what is going to happen when .NET 2.0 finally sees the world. The next release of WSE, 3.0, is
going to coincide with VS.2005 release and will take advantages of the latest innovations of
.NET 2.0 platform in messaging and Web Application areas.

Considering that Microsoft is one of the most active players in the Web Service security area
and recognizing its influence in the industry, its WSE implementation is probably one of the
most complete and up to date, and it is strongly advisable to run at least a quick interoperability
check with WSE-secured .NET Web Service clients. If you have a Java-based Web Service, and
the interoperability is a requirement (which is usually the case), in addition to the questions of
security testing one needs to keep in mind the basic interoperability between Java and .NET Web
Service data structures.

This is especially important since current versions of .NET Web Service tools frequently do
not cleanly handle WS-Security's and related XML schemas as published by OASIS, so some
creativity on the part of a Web Service designer is needed. That said – WSE package itself
contains very rich and well-structured functionality, which can be utilized both with ASP.NET-
based and standalone Web Service clients to check incoming SOAP messages and secure
outgoing ones at the infrastructure level, relieving Web Service programmers from knowing
these details. Among other things, WSE 2.0 supports the most recent set of WS-Policy and
WS-Security profiles, providing for basic message security and WS-Trust with WS-
SecureConversation. Those are needed for establishing secure exchanges and sessions -- similar
to what SSL does at the transport level, but applied to message-based communication.

Java toolkits
Most of the publicly available Java toolkits work at the level of XML security, i.e. XML-dsig
and XML-enc – such as IBM's XML Security Suite and Apache's XML Security project. Java's
JSR 105 and JSR 106 (still not finalized) define Java bindings for signatures and encryption,
which will allow plugging the implementations as JCA providers once work on those JSRs
is completed.

Moving one level up, to address Web Services themselves, the picture becomes muddier -- at
the moment, there are many implementations in various stages of incompleteness. For instance,
Apache is currently working on the WSS4J project, which is moving rather slowly, and there is
commercial software package from Phaos (now owned by Oracle), which suffers from a lot of
implementation problems.

A popular choice among Web Service developers today is Sun's JWSDP, which includes
support for Web Service security. However, its support for Web Service security specifications
in the version 1.5 is only limited to implementation of the core WSS standard with username and
X.509 certificate profiles. Security features are implemented as part of the JAX-RPC framework
and configuration-driven, which allows for clean separation from the Web Service's
implementation.

Hardware, software systems
This category includes complete systems, rather than toolkits or frameworks. On one hand,
they usually provide rich functionality right off the shelf, on the other hand – its usage model is
rigidly constrained by the solution's architecture and implementation. This is in contrast to the
toolkits, which do not provide any services by themselves, but handing system developers
necessary tools to include the desired Web Service security features in their products… or to
shoot themselves in the foot by applying them inappropriately.

These systems can be used at the infrastructure layer to verify incoming messages against the
effective policy, check signatures, tokens, etc, before passing them on to the target Web Service.
When applied to the outgoing SOAP messages, they act as a proxy, now altering the messages to
decorate with the required security elements, sign and/or encrypt them.

Software systems are characterized by significant configuration flexibility, but comparatively
slow processing. On the bright side, they often provide high level of integration with the existing
enterprise infrastructure, relying on the back-end user and policy stores to look at the credentials,
extracted from the WSS header, from the broader perspective. An example of such service is
TransactionMinder from the former Netegrity – a Policy Enforcement Point for Web Services
behind it, layered on top of the Policy Server, which makes policy decisions by checking the
extracted credentials against the configured stores and policies.

For hardware systems, performance is the key – they have already broken gigabyte
processing threshold, and allow for real-time processing of huge documents, decorated according
to the variety of the latest Web Service security standards, not only WSS. The usage simplicity is
another attractive point of those systems - in the most trivial cases, the hardware box may be
literally dropped in, plugged, and be used right away. These qualities come with a price, however
– this performance and simplicity can be achieved as long as the user stays within the pre-
configured confines of the hardware box. The moment he tries to integrate with the back-end
stores via callbacks (for those solutions that have this capability, since not all of them do), most
of the advantages are lost. As an example of such hardware device, DataPower provides a nice
XS40 XML Security Gateway, which acts both as the inbound firewall and the outbound proxy
to handle XML traffic in real time.

Problems
As is probably clear from the previous sections, Web Services are still experiencing a lot of
turbulence, and it will take a while before they can really catch on. Here is a brief look at what
problems surround currently existing security standards and their implementations.

Immaturity of the standards
Most of the standards are either very recent (couple years old at most), or still being
developed. Although standards development is done in committees, which, presumably, reduces
risks by going through an exhaustive reviewing and commenting process, some error scenarios
still slip in periodically, as no theory can possibly match the testing resulting from pounding by
thousands of developers working in the real field.

Additionally, it does not help that for political reasons some of this standards are withheld
from public process, which is the case with many standards from the WSA arena (see 0), or that
some of the efforts are duplicated, as was the case with LA and WS-Federation specifications.

Performance
XML parsing is a slow task, which is an accepted reality, and SOAP processing slows it
down even more. Now, with expensive cryptographic and textual conversion operations thrown
into the mix, these tasks become a performance bottleneck, even with the latest crypto- and
XML-processing hardware solutions offered today. All of the products currently on the market
are facing this issue, and they are trying to resolve it with varying degrees of success.

Hardware solutions, while substantially (by orders of magnitude) improving the performance,
can not always be used as an optimal solution, as they can not be easily integrated with the
already existing back-end software infrastructure, at least – not without making performance
sacrifices. Another consideration whether hardware-based systems are the right solution – they
are usually highly specialized in what they are doing, while modern Application Servers and
security frameworks can usually offer a much greater variety of protection mechanisms,
protecting not only Web Services, but also other deployed applications in a uniform and
consistent way.

Complexity and interoperability
As could be deduced from the previous sections, Web Service security standards are fairly
complex, and have very steep learning curve associated with them. Most of the current products,
dealing with Web Service security, suffer from very mediocre usability due to the complexity of
the underlying infrastructure. Configuring all different policies, identities, keys, and protocols
takes a lot of time and good understanding of the involved technologies, as most of the times
errors that end users are seeing have very cryptic and misleading descriptions.

In order to help administrators and reduce security risks from service misconfigurations,
many companies develop policy templates, which group together best practices for protecting
incoming and outgoing SOAP messages. Unfortunately, this work is not currently on the radar of
any of the standard's bodies, so it appears unlikely that such templates will be released for public
use any time soon. Closest to this effort may be WS-I's Basic Security Profile (BSP), which tries
to define the rules for better interoperability among Web Services, using a subset of common
security features from various security standards like WSS. However, this work is not aimed at
supplying the administrators with ready for deployment security templates matching the most
popular business use cases, but rather at establishing the least common denominator.

Key management
Key management usually lies at the foundation of any other security activity, as most
protection mechanisms rely on cryptographic keys one way or another. While Web Services have
XKMS protocol for key distribution, local key management still presents a huge challenge in
most cases, since PKI mechanism has a lot of well-documented deployment and usability issues.
Those systems that opt to use homegrown mechanisms for key management run significant risks
in many cases, since questions of storing, updating, and recovering secret and private keys more
often than not are not adequately addressed in such solutions.

Further Reading

Associated specifications are: For more complete description of access control tasks, please, refer to other sections of this Guide. Unfortunately, both WS-Policy and WS-Trust specifications have not been submitted for standardization to public bodies, and their development is progressing via private collaboration of several companies, although it was opened up for other participants as well. As a positive factor, there have been several interoperability events conducted for these specifications, so the development process of these critical links in the Web Services' security infrastructure is not a complete black box. This is in theory. In practice, there are a lot of obstacles hidden among the way, and one of the major ones among them – security concerns about publicly exposing processing functions to intra- or Internet-based clients.

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