The most effective method to mitigate Cross-Site Scripting (XSS) attacks is to use whitelist input validation. XSS attacks occur when an attacker injects malicious code, usually in the form of a script, into a web application that is then executed by the browser of an unsuspecting user. XSS attacks can compromise the confidentiality, integrity, and availability of the web application and the user’s data. Whitelist input validation is a technique that checks the user input against a predefined set of acceptable values or characters, and rejects any input that does not match the whitelist. Whitelist input validation can prevent XSS attacks by filtering out any malicious or unexpected input that may contain harmful scripts. Whitelist input validation should be applied at the point of entry of the user input, and should be combined with output encoding or sanitization to ensure that any input that is displayed back to the user is safe and harmless. Use Software as a Service (SaaS), require client certificates, and validate data output are not the most effective methods to mitigate XSS attacks, although they may be related or useful techniques. Use Software as a Service (SaaS) is a model that delivers software applications over the Internet, usually on a subscription or pay-per-use basis. SaaS can provide some benefits for web security, such as reducing the attack surface, outsourcing the maintenance and patching of the software, and leveraging the expertise and resources of the service provider. However, SaaS does not directly address the issue of XSS attacks, as the service provider may still have vulnerabilities or flaws in their web applications that can be exploited by XSS attackers. Require client certificates is a technique that uses digital certificates to authenticate the identity of the clients who access a web application. Client certificates are issued by a trusted certificate authority (CA), and contain the public key and other information of the client. Client certificates can provide some benefits for web security, such as enhancing the confidentiality and integrity of the communication, preventing unauthorized access, and enabling mutual authentication. However, client certificates do not directly address the issue of XSS attacks, as the client may still be vulnerable to XSS attacks if the web application does not properly validate and encode the user input. Validate data output is a technique that checks the data that is sent from the web application to the client browser, and ensures that it is correct, consistent, and safe. Validate data output can provide some benefits for web security, such as detecting and correcting any errors or anomalies in the data, preventing data leakage or corruption, and enhancing the quality and reliability of the web application. However, validate data output is not sufficient to prevent XSS attacks, as the data output may still contain malicious scripts that can be executed by the client browser. Validate data output should be complemented with output encoding or sanitization to ensure that any data output that is displayed to the user is safe and harmless.