PostgreSQL versions before 9.2.22, 9.3.18, 9.4.13, 9.5.8 and 9.6.4 are vulnerable to authorization flaw allowing remote authenticated attackers to retrieve passwords from the user mappings defined by the foreign server owners without actually having the privileges to do so.
https://www.postgresql.org/about/news/1772/
https://security.gentoo.org/glsa/201710-06
https://access.redhat.com/errata/RHSA-2017:2728
https://access.redhat.com/errata/RHSA-2017:2678
https://access.redhat.com/errata/RHSA-2017:2677
http://www.securitytracker.com/id/1039142
http://www.securityfocus.com/bid/100275