Internet-Security.com TCP port 9303 details

Elasticsearch uses TCP 9300 for its internal transport; if that port is occupied it auto-increments to 9301, 9302, 9303, etc., so 9303 commonly appears for additional Elasticsearch nodes or multi-instance setups.
OpenSearch (the Elasticsearch fork) uses the same transport mechanism and port behavior, so 9303 is likewise seen in real deployments.
Managed offerings (e.g., Amazon OpenSearch Service) use the transport port internally; self-managed clusters frequently show listeners on 9303 when running multiple nodes on one host.
Security note: exposing the Elasticsearch/OpenSearch transport port publicly has been abused for unauthorized cluster joining or disruption, so it should be restricted or secured with TLS.

TCP 9303