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.
<html>
<script>
prompt('XSS by Osanda Malith');
</script>
</html>
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”.
email attachment with malicious js !!! is it fixed now ? 😉
They won’t fix it I guess 🙂