CouchDB version 3.0.0 shipped with a new configuration setting that governs access control to the entire database server called `require_valid_user_except_for_up`. It was meant as an extension to the long standing setting `require_valid_user`, which in turn requires that any and all requests to CouchDB will have to be made with valid credentials, effectively forbidding any anonymous requests. The new `require_valid_user_except_for_up` is an off-by-default setting that was meant to allow requiring valid credentials for all endpoints except for the `/_up` endpoint. However, the implementation of this made an error that lead to not enforcing credentials on any endpoint, when enabled. CouchDB versions 3.0.1[1] and 3.1.0[2] fix this issue.

Reference:
https://docs.couchdb.org/en/master/cve/2020-1955.html