This indicates an attack attempt to exploit a Security Policy Bypass vulnerability in PostgreSQL database
The vulnerability is due to improper authentication of user in the libpq module. A remote attacker may be able to exploit this to access/modify data through bypassing the authentication filter via crafted packets.
PostgreSQL PostgreSQL 9.2.x prior to 9.2.22
PostgreSQL PostgreSQL 9.3.x prior to 9.3.18
PostgreSQL PostgreSQL 9.4.x prior to 9.4.13
PostgreSQL PostgreSQL 9.5.x prior to 9.5.8
PostgreSQL PostgreSQL 9.6.x prior to 9.6.4
Security Bypass: Remote attackers can bypass security checks of vulnerable systems.
Apply the most recent upgrade or patch from the vendor