Validation controls are used to check the input data from the user before processing it on the server. If the validation controls are moved from the server side to the browser, it means that the user can modify or bypass them using tools such as browser developer tools, JavaScript console, or proxy tools. This would increase the risk of a successful attack by structured query language (SQL) injection, which is a technique that exploits a security vulnerability in an application's software layer that allows an attacker to execute arbitrary SQL commands on the underlying database. SQL injection can result in data theft, data corruption, or unauthorized access to the system.
Buffer overflow, denial of service (DoS), and phishing are not directly related to the validation controls in a web application. Buffer overflow is a type of attack that exploits a memory management flaw in an application or system that allows an attacker to write data beyond the allocated buffer size and overwrite adjacent memory locations. DoS is a type of attack that prevents legitimate users from accessing a service or resource by overwhelming it with requests or traffic. Phishing is a type of attack that uses fraudulent emails or websites to trick users into revealing sensitive information or installing malware.
References:
Client-side form validation - Learn web development | MDN
JavaScript: client-side vs. server-side validation - Stack Overflow
SQL Injection - OWASP