Sanitize Client-Side: Why Server-Side HTML Sanitization is Doomed to Fail

The battle against Cross-Site Scripting (XSS) has been ongoing for years, with developers constantly seeking new methods to protect web applications from malicious code injection. One common approach is using HTML sanitizers on the server side, which manipulate untrusted user input in a smart way to prevent unwanted markup. However, this strategy often falls short due to limitations and discrepancies in parsing algorithms across different environments. The complexity of HTML parsing can lead to vulnerabilities, as variations in parsing algorithms make it difficult for server-side sanitization to guarantee consistent parsing amongst various endpoints. As a best practice, developers should implement client-side sanitization to ensure that untrusted input is processed in a controlled and secure manner, reducing the risk of a sanitizer bypass.