From CORS specification:
If the resource supports credentials add a single Access-Control-Allow-Origin header, with the value of the Origin header as value, and add a single Access-Control-Allow-Credentials header with the case-sensitive string "true" as value.
Otherwise, add a single Access-Control-Allow-Origin header, with either the value of the Origin header or the string "*" as value.
The string "*" cannot be used for a resource that supports credentials.
This is also noted in CouchDB docs.