FedAuth, FedAuth1 and .ASPXAUTH are cookies connected to Claims and Forms Authentication. To secure these cookies you need to first secure the Sitefinity backend with SSL. You could find additional information regarding the configurations in our Sitefinity documentation and the following blog post. Note that all backend pages should require SSL and everything should be configured strictly.
Then you need to change the following lines in your web.config file:
RequireSsl should be set to true. The line could be found under:
FedAuth and FedAuth1 will be secured after the property requireSsl is set to true.
Then you need to add this line:
//The above line should be placed here
The cookie .ASPXAUTH will be secured after the above line is added.
Then restart your project by making a dummy change in your web.config file. Run the project and clear all browser cookies.