|PostgreSQL 8.3.7 Documentation|
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While the server is running, it is not possible for a malicious user to interfere with client/server communications. However, when the server is down it is possible for a local user to spoof the normal server by starting their own server. The spoof server could read passwords and queries sent by clients, but could not return any data because the PGDATA directory would still be secure because of directory permissions. Spoofing is possible because any user can start a database server; a client cannot identify an invalid server unless it is specially configured.
The simplest way to prevent invalid servers for local connections is to use a Unix domain socket directory (unix_socket_directory) that has write permission only for a trusted local user. This prevents a malicious user from creating their own socket file in that directory. If you are concerned that some applications might still reference /tmp for the socket file and hence be vulnerable to spoofing, during operating system startup create symbolic link /tmp/.s.PGSQL.5432 that points to the relocated socket file. You also might need to modify your /tmp cleanup script to preserve the symbolic link.
For TCP connections the server must accept only hostssl connections (Section 21.1) and have SSL server.key (key) and server.crt (certificate) files (Section 17.8). The TCP client must connect using sslmode='require' (Section 30.1) and have a ~/.postgresql/root.crt SSL certificate (Section 30.16).