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Cross-site Scripting occurrences in Backdrop Core
In Backdrop CMS 1.12.x before 1.12.8 and 1.13.x before 1.13.3, some menu links within the administration bar may be crafted to execute JavaScript when the administrator is logged in and uses the search functionality. (This issue is mitigated by the attacker needing permissions to create administrative menu links, such as by creating a content type or layout. Such permissions are usually restricted to trusted or administrative users.) (2019-08-08, CVE-2019-14770)
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>