Description
This script is possibly vulnerable to Cross Site Scripting (XSS) attacks.
Cross site scripting (also referred to as XSS) is a vulnerability that allows an attacker to send malicious code (usually in the form of Javascript) to another user.
Because a browser cannot know if the script should be trusted or not, it will execute the script in the user context allowing the attacker to access any cookies or session tokens retained by the browser.
While a traditional cross-site scripting vulnerability occurs on the server-side code, document object model based cross-site scripting is a type of vulnerability which affects the script code in the client's browser.
Remediation
Your script should filter metacharacters from user input.
References
Acunetix Cross Site Scripting Attack
VIDEO: How Cross-Site Scripting (XSS) Works
DOM based XSS Prevention Cheat Sheet