User provided data, such as URL parameters, POST data payloads, or cookies, should always be considered untrusted and tainted. Endpoints reflecting back 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's not whitelisted.
* Sanitize user provided data from any characters that can be used for malicious purposes.
* Encode user provided data being reflected as output. 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.
