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Cross-site Scripting occurrences in Json-sanitizer
OWASP json-sanitizer before 1.2.1 allows XSS. An attacker who controls a substring of the input JSON, and controls another substring adjacent to a SCRIPT element in which the output is embedded as JavaScript, may be able to confuse the HTML parser as to where the SCRIPT element ends, and cause non-script content to be interpreted as JavaScript. (2020-06-09, CVE-2020-13973)
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>