Security Considerations with Metadata
When using the metadata features in Windows Communication Foundation (WCF), consider the security implications of publishing, retrieving, and using service metadata.
When to Publish Metadata
WCF services do not publish metadata by default. To publish metadata for a WCF service you must explicitly enable metadata publishing by adding metadata endpoints to your service (see Publishing Metadata). Leaving metadata publishing disabled reduces the attack surface for your service and lowers the risk of unintentional information disclosure. Not all services must publish metadata. If you do not have to publish metadata, consider leaving it turned off. Note that you can still generate metadata and client code directly from your service assemblies using the ServiceModel Metadata Utility Tool (Svcutil.exe). For more information about using Svcutil.exe to export metadata, see How to: Use Svcutil.exe to Export Metadata from Compiled Service Code.
Publishing Metadata Using a Secure Binding
The default metadata bindings that WCF provides are not secure and they allow anonymous access to the metadata. The service metadata that a WCF service publishes contains a detailed description about the service and may intentionally or unintentionally contain sensitive information. For example, service metadata may contain information about infrastructure operations that was not intended to be broadcast publicly. To protect service metadata from unauthorized access, you can use a secure binding for your metadata endpoint. Metadata endpoints respond to HTTP/GET requests that can use Secure Sockets Layer (SSL) to secure the metadata. For more information, see How to: Secure Metadata Endpoints.
Securing your metadata endpoints also provides a way for requesters to securely retrieve service metadata without the risk of tampering or spoofing.
Using Only Trusted Metadata
You can use service metadata to automatically construct the run-time components required to call the service. You can also use metadata at design time to develop a client application or at runtime to dynamically update the binding a client uses to call a service.
Service metadata can be tampered with or spoofed when retrieved in an insecure manner. Tampered metadata can redirect your client to a malicious service, contain compromised security settings, or contain malicious XML structures. Metadata documents can be large and are frequently saved to the file system. To protect against tampering and spoofing, use a secure binding to request service metadata when one is available.
Using Safe Techniques for Processing Metadata
Service metadata is frequently retrieved from a service over a network using standardized protocols such as WS-MetadataExchange (MEX). Many metadata formats include referencing mechanisms for pointing to additional metadata. The MetadataExchangeClient type automatically processes references for you in Web Services Description Language (WSDL) documents, XML Schema, and MEX documents. The size of the MetadataSet object created from the retrieved metadata is directly proportional to the MaximumResolvedReferences value for the MetadataExchangeClient instance that is used and the MaxReceivedMessageSize value for the binding being used by that MetadataExchangeClient instance. Set these quotas to appropriate values as dictated by your scenario.
In WCF, service metadata is processed as XML. When processing XML documents, applications should protect themselves against malicious XML structures. Use the XmlDictionaryReader with appropriate quotas when processing XML and also set the ProhibitDtd property on the XmlReaderSettings object for your XmlReader instance to true.
The metadata system in WCF is extensible and metadata extensions can be registered in your application configuration file (see Extending the Metadata System). Metadata extensions can run arbitrary code, so you should protect your application configuration file with appropriate access control lists (ACLs) and register only trusted metadata extension implementations.
Validating Generated Clients
When generating client code from metadata retrieved from a source that is not trusted, validate the generated client code to ensure that the generated client conforms to your client applications security policies. You can use a validating behavior to check settings on your client binding or visually inspect code generated by tools. For an example of how to implement a client that validates behaviors, see Client Validation.
Protecting Application Configuration Files
A service's application configuration file may control how and if metadata is published. It is a good idea to protect the application configuration file with appropriate access control lists (ACLs) to ensure an attacker cannot modify such settings.