Most of the time, In most instances it is necessary to secure the Security Token Service. According the Trust Brokering model defined in the WS-Trust specification, the subject (user) should authenticate himself to the STS before obtaining a token. STS may use this authentication information when constructing the security token. For example, STS may populate the required claims based on the user name provided by the subject. You can apply a security policy for STS by clicking on the "Apply Security Policy" link.
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