Poor input validation in databases typically leads to SQL Injection (SQLi) vulnerabilities, where attackers manipulate input fields to execute arbitrary SQL commands and gain unauthorized data access or control. XSS (A) affects web applications' output rendering, command injection (B) affects OS commands, and buffer overflow (C) affects memory management, so they don't directly relate to database input validation. SQLi is a critical vulnerability extensively covered in the Application Security domain#6:Chapter 6 CompTIA Security+ Study Guide#.