Extension script @substitutions@ within quoting allow SQL injection (CVE-2023-39417)

An extension script is vulnerable if it uses @extowner@, @extschema@, or @extschema:...@ inside a quoting construct (dollar quoting, '', or ""). No bundled extension is vulnerable. Vulnerable uses do appear in a documentation example and in non-bundled extensions. Hence, the attack prerequisite is an administrator having installed files of a vulnerable, trusted, non-bundled extension. Subject to that prerequisite, this enables an attacker having database-level CREATE privilege to execute arbitrary code as the bootstrap superuser. PostgreSQL will block this attack in the core server, so there’s no need to modify individual extensions.