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Cross-site Scripting occurrences in Hello.js
This affects the package hellojs before 1.18.6. The code get the param oauth_redirect from url and pass it to location.assign without any check and sanitisation. So we can simply pass some XSS payloads into the url param oauth_redirect, such as javascript:alert(1). (2020-10-06, CVE-2020-7741)
Why Cross-site Scripting can be dangerous
Cross site scripting is an attack where a web page executes code that is injected by an adversary. It usually appears, when users input is presented. This attack can be used to impersonate a user, take over control of the session, or even steal API keys.
The attack can be executed e.g. when you application injects the request parameter directly into the HTML code of the page returned to the user:
https://server.com/confirmation?message=Transaction+Complete
what results in:
<span>Confirmation: Transaction Complete</span>
In that case the message can be modified to become a valid Javascript code, e.g.:
https://server.com/confirmation?message=<script>dangerous javascript code here</script>
and it will be executed locally by the user's browser with full access to the user's personal application/browser data:
<span>Confirmation: <script>dangerous javascript code here</script></span>