SQL injection vulnerabilities occur when an application includes untrusted input in a command or query without proper validation or escaping, allowing an attacker to interfere with the execution of predetermined SQL commands. In this scenario, the lack of input sanitization in a search function suggests that an attacker could insert malicious SQL statements, leading to an SQL injection vulnerability. Other options are less likely because the question specifically indicates the problem arises from the handling of user input within a search function, which is a common scenario for SQL injections.