Summary by mewtw0
It is encountered in cases where any input from the user is printed directly on the screen. Since the code received from the user as input is not written to the database, it is presented to the user only once by the internet browser (think of clicking on a link address (URL) sent to you via e-mail or chat program).
Output encoding is the only ultimate solution. The purpose of output encoding is to convert untrusted input into a secure format where it is displayed as data to the user without running it as code or simple words in the browser.
Such vulnerabilities occur due to insufficient filtering of the inputs received by the user. For this reason, any input received by the user must be filtered. You can benefit from additional references in the "Additional Information" field.