If a user initiates an authentication request through ADFS by using an ADFSclient" such as a supported browser, ADFS first verifies that the user credentials enable successful authentication to ActiveDirectory. After successful ActiveDirectory authentication, the SecurityTokenService (STS) component of ADFS server then issues a security token which is used to authenticate the user to the other service, such as Office365. Office 365 implicitly trusts the token issuer, in this case ActiveDirectory.