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Cross-site Scripting occurrences in Hapi Fhir
XSS exists in the HAPI FHIR testpage overlay module of the HAPI FHIR library before 3.8.0. The attack involves unsanitized HTTP parameters being output in a form page, allowing attackers to leak cookies and other sensitive information from ca/uhn/fhir/to/BaseController.java via a specially crafted URL. (This module is not generally used in production systems so the attack surface is expected to be low, but affected systems are recommended to upgrade immediately.) (2019-06-05, CVE-2019-12741)
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>