Hi!
Forms authentication is supported by Cassini. We don't have a formal list of differences between Cassini and IIS, but to tell whether a feature is supported by Cassini or not one has to look whether the feature is provided by ASP.NET or by IIS. If the feature is a part of ASP.NET, as forms authentication, then it is supported by Cassini. If it's an IIS feature - like SSL support, integrated auth, some IIS-specific server varibales - then the feature is not likely to be in Cassini.
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Regards,
UltiDev Team.
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