Rule Definition
An attacker can use XSS to send a malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by the browser and used with that site. These scripts can even rewrite the content of the HTML page.

Remediation
Ensure you don't write dynamic code in a DOM callback event such as onmouseover

Violation Code Sample

<div onmouseover="alert(@Model.UserMessage);"> MOUSE OVER ME !!1</div>  // VIOLATION

<div onmouseover="alert('@Model.UserMessage');"> MOUSE OVER ME !!2</div>  // VIOLATION

<div onmouseover="let msg = @Model.UserMessage"> MOUSE OVER ME !!3</div>  // VIOLATION

Fixed Code Sample

<div onmouseover="alert(hello);"> MOUSE OVER ME !!1</div>  // NO VIOLATION

<div onmouseover="alert('hello');"> MOUSE OVER ME !!2</div>  // NO VIOLATION

<div onmouseover="let msg = hello"> MOUSE OVER ME !!3</div>  // NO VIOLATION

CWE-83: Improper Neutralization of Script in Attributes in a Web Page https://cwe.mitre.org/data/definitions/83.html OWASP Cheat Sheet Series - DOM based XSS Prevention RULE #3 - Be Careful when Inserting Untrusted Data into the Event Handler and JavaScript code Subcontexts within an Execution Context https://cheatsheetseries.owasp.org/cheatsheets/DOM_based_XSS_Prevention_Cheat_Sheet.html#rule-3-be-careful-when-inserting-untrusted-data-into-the-event-handler-and-javascript-code-subcontexts-within-an-execution-context