A security analyst completes a vulnerability scan and identifies multiple vulnerabilities across the organization's web servers. The scan reveals CVE-2023-1234 with a CVSS score of 9.8 (critical) affecting a development server, CVE-2023-5678 with a CVSS score of 7.5 (high) affecting a production server, CVE-2023-9012 with a CVSS score of 8.1 (high) affecting a test environment server, and CVE-2023-3456 with a CVSS score of 6.5 (medium) affecting a production server. Which vulnerability should be prioritized for immediate remediation?