The coupling of Web services has an inherent security risk, in that a chain of Web services is as strong as its weakest link. The Web service that has the weakest link may not even belong to your organization, but its vulnerabilities will have significant consequences. A breach of the weakest link can compromise the Web service in several ways:
Allowing interception of data that flows between services, to provide information about servers, usernames, passwords, or personal, financial, medical or other sensitive information
Allowing alteration of data that flows between services, to return incorrect results or redirect the flow to other services.
Simply shutting down the service itself, so that other dependent services can no longer function, disrupting multiple users from multiple access points.