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Cross-site Scripting occurrences in Obsidian
The feature to preview a website in Plesk Obsidian 18.0.0 through 18.0.32 on Linux is vulnerable to reflected XSS via the /plesk-site-preview/ PATH, aka PFSI-62467. The attacker could execute JavaScript code in the victim's browser by using the link to preview sites hosted on the server. Authentication is not required to exploit the vulnerability. (2021-09-10, CVE-2021-35976)
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>