The default behavior of XML serialization through XmlSerializer can take you through many cases, but sometimes you need to take more control over the way in which your objects are serialized. The class provided here illustrates two potential problems commonly encountered when working with XML serialization of objects:. In the first case, you have a StringCollection that represent names or abbreviations of the states for example, North Carolina.
There are a number of ways that you could serialize such a list but which one should the XmlSerializer choose? In this case, you would need to provide more information. The second issue is a bit trickier because one of the members of the class is of the System. Uri type, and the System. Uri class has no default constructor a public constructor that accepts no parameters.
Classes that have no such constructor cannot be deserialized from XML because the XmlSerializer class does not know how to instantiate one, and if it can't be deserialized, XmlSerializer will also refuse to serialize it.
Because of this, the XmlSerializer would also fail to serialize the MyService class because ServiceUri happens to be a public member.
The default behavior of XML serialization can take you through most cases, but sometimes you need to take more control over the way in which objects are serialized.
In addition to handling messages as raw XML, as you saw previously, there are three other methods at your disposal that you can use to shape the content of your Web services messages: attributes, IXmlSerializable, and IXmlElement. Implementing IXmlElement. If you want, you can implement IXmlElement in your own classes and achieve much the same results as if you had derived from OpenElement. If you do choose to implement IXmlElement yourself instead of deriving from a WSE base class, you must remember to provide a special constructor for your object.
This special constructor must take as a parameter an instance of the XmlElement class, the same parameter that you would pass to the IXmlElement.
LoadXml method. This constructor can be nothing more than a pass-through to the LoadXml method, as shown here:. Without this constructor, your message objects will appear to be serialized to XML successfully, but they will fail when WSE attempts to deserialize them. Instead of creating the object with a default empty constructor and calling LoadXml, it simply constructs the object using the XmlElement.
There are times, of course, when you need to step in and control the behavior of the serialization process. The first method that a developer can use to control the XML serialization process is through attributes. Serialization namespace provides a number of attributes that can be used to change normal serialization behavior. Figure 3 lists a few of the more common attributes and the effect that each of them has on the serialization process.
The full list is available in the documentation for the System. Serialization namespace. Whenever a class that uses one or more of these attributes is serialized, the XmlSerializer class uses reflection to determine what attributes are being used and how they should be interpreted.
In the class shown here, I have added attributes to the previously defined class in order to declaratively shape the XML serialization, as shown in Figure 4.
The XmlRootAttribute describes the MyService class as a whole, the XmlArrayAttribute describes how the States collection should be serialized as a list of items, and the XmlIgnoreAttribute states that the Uri member ServiceUri should be skipped completely during serialization.
With those changes, the class can now be successfully passed to XmlSerializer. The XML that is created when this object is serialized looks like this:. Although using the attributes to shape XML messages is straightforward, it cannot fix every problem. Most obviously, there is still no serialized information about the Uri member, which may or may not be a problem depending on the class design.
On the other hand, suppose you did not like how the States list was serialized. You may still be unable to get what you want from attributes. For example, there are no attributes available that can tell the XmlSerializer to write an array of strings as an XML type of xsd:list, as shown in the following:. This is a common enough construct in XML and one that is well suited to representing a list of items, but that does not require the overhead of an element for each item.
For that reason, elements of the xsd:list type appear in many of the messages used for the latest Web services specifications, like WS-Discovery. In order to format the list of strings as an xsd:list, you would need to take more control over the XML serialization process. Another way to take more control is by implementing the IXmlSerializable interface, which is a mechanism for overriding the XML serialization process:. The IXmlSerializable interface has three methods: two for serialization and one for schema generation.
The WriteXml and ReadXml methods are invoked whenever the object in question is being serialized or deserialized. These methods take an XmlWriter instance and XmlReader instance respectively, types which are probably familiar to developers who have been working with XML in the. NET Framework. Just as you could use familiar XmlDocument methods when working at the SoapEnvelope level, you can rely on methods like XmlWriter. WriteStartElement in order to create your message. The GetSchema method returns an instance of System.
XmlSchema, the. This method is called whenever schema information is needed about the object, such as during the construction of WSDL for the Web service.
Using this method takes care of some of the issues you might encounter with XML serialization. For example, if your object implements IXmlSerializable, you can provide public properties and members that are non-serializable data types without generating exceptions from XmlSerializer. This would allow you to take care of the Uri member that you could not serialize earlier.
Figure 5 shows an example of how you can serialize a collection of strings as an xsd:list type. This code is written to serialize a generic list, but the SerializableList class can serve as a base class, and by initializing the ListName and ListNamespace members in the constructor, you can further customize your serialization. It would seem that implementing the three methods of IXmlSerializable would take care of every issue developers might have with their messages, but there are a couple of problems that prevent that from being the case.
Originally, the IXmlSerializable interface was not recommended for use outside of the. NET Framework classes. The documentation stated that it was meant for internal use only, and so developers who chose to use it knew that it was a technical risk. Since then, it has become clear that its use will be supported in the.
That's why the AuthenticateToken overridable method on the UsernameTokenManager class requires you to return the stored password instead of a simpler Boolean value. This is a hazard from a general security standpoint. If the server is ever hacked, the attacker can just sit and see clear passwords pass by. This approach can produce severe damage in the case of a compromised server and lays the server open to replay attacks. A replay attack is when someone sees a valid token pass and just inserts the same token in another message.
A way to alleviate such risks consists of using and storing pre-hashed versions of the password created using an algorithm such as SHA-1 on which both the sender and the receiver agree.
Basically, you transmit the hashed password either further hashed or clear , rather than the original, and make AuthenticateToken return the hashed password stored in the database.
Of course, the hash algorithm must be the same. This trick alone doesn't make the Web service call inherently more secure. Once the system is hacked, it doesn't make any difference if the attacker replays the call using the original or the hashed password. However, at least the hacker doesn't know the original password and can't cause more severe damage to the system.
If custom user authentication is your choice, then an even more secure approach involves adding a signature to the SOAP message along with the user token. If you choose to indicate a digital signature, you can avoid sending the password altogether. The signature built on the client from user name and password the user token represents your proof of possession. By doing so, you instruct Web Services Enhancements 2.
The WSE 2. The property gets and sets the IPrincipal object for the security token. The principal is set in the AuthenticateToken method immediately after retrieving the password for the specified user. In this case, the principal is a WindowsPrincipal object. Its IsInRole method will look up the user in the list of Windows accounts.
If you're using a custom user name token class, you must add some code to your override of AuthenticateToken to generate a new GenericPrincipal object with its own list of roles:.
Just adding credentials to a SOAP message doesn't make it inherently more secure. At a minimum, you need to add some encryption to ensure that the contents of the SOAP message are not intelligible as they travel over the network. SOAP messages, in fact, are by default plain text and thus can be read by any recipient. An encrypted SOAP message is cryptographically encoded, so that only the owner of the proper decryption key can read the contents of the message. SSL is easy to set up and works well; however, it has been designed mainly for Web sites and is not always optimal for Web services.
The first aspect to consider is that Web services are often implemented over HTTP, but that is neither necessary nor is it the only option.
This might not seem like a significant issue but consider what happens when a message is routed through multiple locations. Should you permit sensitive data to be visible to everybody who has something to process in the message? It's a good rule to make critical data available to the fewest number of people, so an all-or-nothing encryption scheme may pose a severe security issue. Another point to consider might be performance. Data confidentiality doesn't come for free and encrypting and decrypting the whole message is, computationally speaking, more costly than protecting only portions of it.
WS-Security provides you with a flexibility that you just can't obtain out of SSL or similar protocols. Asymmetric encryption allows the client of a Web service to encrypt the message using the public key of an X. Subsequently, only the owner of the private key can decrypt the message. Symmetric encryption requires that a Web service and client already share a secret key. The information collected through the WSE 2.
As for digital signing, by default WSE 2. You can change these default settings programmatically or by editing the assertions in the policy file. The first relates to the secure sending of the bytes over the network. While encryption guarantees that the message is not viewed, you also need to ensure that nothing got modified along the way. Digital signatures are one way of providing such integrity as well as providing the identity of the sender. The second aspect relates to the binding between the sender and the requested function.
Authentication checks the credentials of the sender whom he or she claims to be while authorization verifies whether the sender has rights to perform the operation. The third level of security gets closer to the usage and logic of the Web service and, among other things, is aimed at ensuring that methods are invoked according to accepted predicates. Web services policies are extremely important to this level of security. WS-Policy provides a flexible grammar for expressing the capabilities and requirements of WebMethods.
A policy is a collection of one or more policy assertions. Some assertions specify requirements related to the wire such as the authentication scheme or transport protocol while others specify requirements critical to the service usage such as the privacy policy.
WS-Policy provides a unified grammar to allow assertions to be defined consistently. The WS-PolicyAssertions standard provides an initial set of assertions to address some common needs of Web services. Built-in assertions include the required character set, language, and specification version. A fourth type of assertion is MessagePredicate. MessagePredicate lets you assert arbitrary expressions to implement predicates and preconditions to which each message must conform.
For example, these assertions can implement business rules. The three aspects of Web services security I've examined form a defense perimeter only within the Web service application, which is where most of today's attacks occur. Nowadays, a traditional firewall is not necessarily an effective barrier because attacks tend to look like legitimate traffic to firewalls, which often only inspect the outermost envelope such as the headers of communication packets.
To be effective, firewalls must start looking into the actual payload to figure out the intent of a request.
ISA Server belongs to this new breed of firewalls and has the ability to do deep content inspection and analysis for many popular Internet protocols. ISA Server is the next version of the Internet firewall, offering improved network security and performance. Designed from the ground up to provide efficient application-layer filtering, ISA Server is a rule-based firewall and a powerful HTTP proxy and caching server. Of the several enhancements that the newest version of ISA brings to the table, the one that is most relevant here is the extensible application-layer filtering.
In Figure 7 , you can see the overall architecture of ISA Server where the custom application filter components stand out. In this way, ISA is capable of blocking any non-HTTP traffic, any contents brought by packets that look suspicious, and anything else that violates rules and policies that are set administratively. A fine-grained set of security policies give administrators the means to configure the level of security as they want.
Application filters play a key role in the architecture shown in Figure 7. They act like protocol handlers by filtering both outgoing access and incoming published traffic. As I mentioned earlier in this article, this portion of the architecture is extensible and based on a plug-in model.
In other words, third-party vendors can extend ISA with ad hoc filters performing particular tasks. Several companies offer application filters for ISA Server that enhance security and interoperability for different protocols and types of traffic a list is available at Partner Application Filters.
I'd like to focus on one of these products in particular—Forum XWall for ISA Server—because of its unique Web service-specific features to protect against intrusions. Forum XWall an ISA Server plug-in works as a kind of smart firewall that implements certain security features specifically targeted at Web services. Usually calls to Web services pass through ports that firewalls leave open, like port 80 and All of the policies in the figure are created by navigating the Publish Web Service or Publish Secure Service menu items in the task list.
This is similar to the Publish Web Server and Publish Secure Server tasks that exist in the traditional firewall policies. In doing so, it can check the conformance of the call to the WSDL of the service and prevent parameters tampering, buffer overflows, replay attacks, command injection, and more.
It also enforces data integrity by checking raw XML against approved schemas, user-defined data validation rules, and a set of Intrusion Detection and Prevention IDP rules such as giving packets a maximum length and a maximum number of nested tags.
In some cases, you can also use XWall to virtualize the WSDL based on ACLs so that any call that the authenticated user attempts to direct at a certain WebMethod is blocked at the gate if the user is not authorized.
Access to Web service methods can be controlled either by mapping each method to a group of users for authorization or just by turning off access to every method. The Divide method doesn't require any authorization and thus anyone can invoke it. Finally, access to Multiply and Subtract are simply turned off so no one can access these operations. As an increasingly essential piece of the enterprise architecture, Web services shouldn't simply be considered a sort of Web site accessed by software rather than humans.
To deliver appropriate security to a Web service-based environment, you need to remember that Web services are first and foremost "services" and not necessarily tied to HTTP. Hence, secure transportation layers like SSL are certainly an option, but not the only one, and not necessarily the best one. Clash of Clans. Subway Surfers. TubeMate 3. Google Play.
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