Protonmail Stored XSS

I found out that attachments containing Javascript or HTML will get executed in the browser once you open in a new tab. The attachment can be anything simple like .html, .svg containing our payload.

  prompt('XSS by Osanda Malith'); 

Once you open the attachment the code gets interpreted.

However due to the same origin policy this bug can’t do much damage. If you do a alert(window.sessionStorage.protonmail_pw) it would result “undefined”.

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