The only way to truly keep a web service secure is to host it on a web server and then turn off the server.

In real life no web service is 100% secure but there are methodologies for increasing the security around web services. In order for consumers of a web service they must adhere to the service’s Service-Level Agreement (SLA). 

An SLA is a digital contract between a web service and its consumer. This contract defines what methods and protocols must be used to access the web service along with the defined data formats for sending and receiving data through the service. If either part does not abide by the contract then the service will not be accessible for consumption.

Common areas for securing web services:

• Universal Discovery Description Integration  (UDDI)
• Web Service Description Language  (WSDL)
• Application Level
• Network Level

“UDDI is a specification for maintaining standardized directories of information about web services, recording their capabilities, location and requirements in a universally recognized format.” (UDDI, 2010)

WSDL on the other hand is a standardized format for defining a web service. A WSDL describes the allowable methods for accessing the web service along with what operations it performs.

Web services in the Application Level can control access to what data is available by implementing its own security through various methodologies but the most common method is to have a consumer pass in a token along with a system identifier so that they system can validate the users access to any data or actions that they may be requesting.

Security restrictions can also be applied to the host web server of the service by restricting access to the site by IP address or login credentials. Furthermore, companies can also block access to a service by using firewall rules and only allowing access to specific services on certain ports coming from specific IP addresses. This last methodology may require consumers to obtain a static IP address and then register it with the web service host so that they will be provide access to the information they wish to obtain.

It is important to note that these areas can be secured in any combination based on the security level tolerance dictated by the publisher of the web service. This being said, the bare minimum security implantation must be in the Application Level within the web service itself. Typically I create a security layer within a web services exposed Internet that requires a consumer identifier and a consumer token. This information is then used to authenticate the requesting consumer before the actual request is performed.

Refernece:
UDDI. (2010). Retrieved 11 13, 2011, from LooselyCoupled.com: http://www.looselycoupled.com/glossary/UDDI
Service-Level Agreement (SLA). (n.d.). Retrieved 11 13, 2011, from SearchITChannel: http://searchitchannel.techtarget.com/definition/service-level-agreement