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Cross-site Scripting occurrences in Groovy Postbuild
A persisted cross-site scripting vulnerability exists in Jenkins Groovy Postbuild Plugin 2.3.1 and older in various Jelly files that allows attackers able to control build badge content to define JavaScript that would be executed in another user's browser when that other user performs some UI actions. (2018-06-05, CVE-2018-1000202)
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>