Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
It was discovered that the PostgresQL planner could create incorrect plans in certain circumstances. A remote attacker could use this issue to cause PostgreSQL to crash, resulting in a denial of service, or possibly obtain sensitive information from memory. This issue only affected Ubuntu 20.04 LTS and Ubuntu 21.04. (CVE-2021-3677)
It was discovered that PostgreSQL incorrectly handled certain SSL renegotiation ClientHello messages from clients. A remote attacker could possibly use this issue to cause PostgreSQL to crash, resulting in a denial of service. (CVE-2021-3449)