How to avoid cross side scripting in my entire solution.

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Javascript

I have one existing application which is in production.
user can able to enter these types of alerts in any of my text box.


〈script〉alert('hi there')〈/script >

as this is existing application i canot go each page and edit particular textbox.
i want a generic solution which wil support for all my aspx pages

Thanks in advance.

What I have tried:

have tried adding

<META http-equiv="Content-Security-Policy" content="script-src https://code.jquery.com 'self';">

after adding this application got struck