In PostgreSQL 9.3.x before 9.3.17, 9.4.x before 9.4.12, 9.5.x before 9.5.7, and 9.6.x before 9.6.3, it was found that the PGREQUIRESSL environment variable was no longer enforcing a SSL/TLS connection to a PostgreSQL server. An active Man-in-the-Middle attacker could use this flaw to strip the SSL/TLS protection from a connection between a client and a server.