Abstract: Embodiments of the invention employ a KUSO (Kerio Unity Sign On) server to work with different web services (which offer online service via to users via user accounts) to offer single sign-on capability to different services. With the use of the KUSO server, a user only has to authenticate with one of the web services in order to have authenticated access to all web services. After the first successful authentication at one of the web services, the web server that successfully authenticates the user communicates the successful authentication with the KUSO server using a special channel and a special token. Subsequently authentication verification is performed transparently by the KUSO server if the user wishes to access any of the other web services. Safeguards for various edge conditions during sign-on and sign-offs are provided to improve security.