CVE-2026-91768 [MEDIUM] – The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 1

The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 16-byte IPv6 address, so listen.allowed_clients matches on a /96 prefix instead of the exact address. An attacker who can source an address sharing the first 96 bits with an allowed one passes the check and reaches the FastCGI endpoint.