Which of the following open web application security threats occurs when a suspicious data in an application is sent to the web browser without proper validation?

The situation described involves sending suspicious data to a web browser without proper validation, which aligns perfectly with the definition of Cross-Site Scripting (XSS).

XSS vulnerabilities arise when an application includes unvalidated user input in web pages that are served to users. This can allow an attacker to inject malicious scripts into content that gets delivered to other users. When the browser renders this malicious script, it can perform actions on behalf of the user or steal sensitive information such as cookies or session tokens.

The key characteristic of XSS is that it leverages the trust a user has in a particular website, meaning that if the application does not sanitize the input before sending it to the web browser, it opens a door for attacks that exploit this trust. Proper input validation and output encoding are critical measures in preventing XSS vulnerabilities. By focusing on these validation processes, applications can safeguard against the execution of malicious scripts from untrusted sources.