Cross-site Scripting (XSS) attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user in the output it generates without validating or encoding it.

https://www.owasp.org/index.php/Testing_for_Cross_site_scripting

https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet

Inject this string, and in most cases where a script is vulnerable with no special XSS vector requirements the word "XSS" will pop up. Use this URL encoding calculator to encode the entire string. Tip: if you're in a rush and need to quickly check a page, often times injecting the depreciated "<PLAINTEXT>" tag will be enough to check to see if something is vulnerable to XSS by messing up the output appreciably:

';alert(String.fromCharCode(88,83,83))//';alert(String.fromCharCode(88,83,83))//";
alert(String.fromCharCode(88,83,83))//";alert(String.fromCharCode(88,83,83))//--
></SCRIPT>">'><SCRIPT>alert(String.fromCharCode(88,83,83))</SCRIPT>