Blog author (and OCI maintainer) here. The request to get a bearer token sends the password or PAT using the basic auth header, base64 encoded, but otherwise clear-text. That's the request the www-authenticate header is triggering. Once the token is received, the registry uses that to verify access, and that eventually expires. But the attacker isn't getting the token, they are requesting the credentials that would be used to acquire a bearer auth token.