An authenticated user could create a stack-based buffer overflow by changing their own password to a purpose-crafted value. In addition to the ability to crash the PostgreSQL server, this could be further exploited to execute arbitrary code as the PostgreSQL operating system account.
Additionally, a rogue server could send a specifically crafted message during the SCRAM authentication process and cause a libpq-enabled client to either crash or execute arbitrary code as the client's operating system account. (CVE-2019-10164)
More than 25 other bugs have been fixed too, see referenced release notes.