An application was recently compromised after some malformed data came in via web form. Which of the following would MOST likely have prevented this?

Input validation is the process of ensuring that data submitted through web forms conforms to expected types, formats, and ranges before it is processed by the application. This directly prevents security vulnerabilities such as SQL injection, cross-site scripting (XSS), or buffer overflows by rejecting or sanitizing any malformed or malicious data, thereby stopping attacks at the earliest possible stage.

A proxy server acts as an intermediary for network requests, providing benefits like caching or basic filtering, but it does not inherently perform deep application-level validation of data content within web form submissions to prevent malformed input attacks.

Stress testing evaluates an application's performance and stability under extreme load, focusing on reliability and bottlenecks rather than preventing specific security vulnerabilities caused by malformed input data.

Encoding transforms data into a different format for storage or safe transmission, but it does not prevent malformed or malicious input from initially entering an application, nor does it validate the correctness of the input itself.