User-provided data, such as URL parameters, POST data payloads, or cookies, should always be considered untrusted and tainted. Furthermore, when processing an HTTP request, a web server may copy user-provided data into the body of the HTTP response that is sent back to the user. This behavior is called a "reflection". Endpoints reflecting tainted data could allow attackers to inject code that would eventually be executed in the user's browser. This could enable a wide range of serious attacks like accessing/modifying sensitive information or impersonating other users.

Typically, the solution is one of the following:

* Validate user-provided data based on a whitelist and reject input that is not allowed.
* Sanitize user-provided data from any characters that can be used for malicious purposes.
* Encode user-provided data when it is reflected back in the HTTP response. Adjust the encoding to the output context so that, for example, HTML encoding is used for HTML content, HTML attribute encoding is used for attribute values, and JavaScript encoding is used for server-generated JavaScript.

When sanitizing or encoding data, it is recommended to only use libraries specifically designed for security purposes. Also, make sure that the library you are using is being actively maintained and is kept up-to-date with the latest discovered vulnerabilities.
