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Cross-site Scripting occurrences in Jsoup
jsoup is a Java HTML parser, built for HTML editing, cleaning, scraping, and cross-site scripting (XSS) safety. jsoup may incorrectly sanitize HTML including `javascript:` URL expressions, which could allow XSS attacks when a reader subsequently clicks that link. If the non-default `SafeList.preserveRelativeLinks` option is enabled, HTML including `javascript:` URLs that have been crafted with control characters will not be sanitized. If the site that this HTML is published on does not set a Content Security Policy, an XSS attack is then possible. This issue is patched in jsoup 1.15.3. Users should upgrade to this version. Additionally, as the unsanitized input may have been persisted, old content should be cleaned again using the updated version. To remediate this issue without immediately upgrading: - disable `SafeList.preserveRelativeLinks`, which will rewrite input URLs as absolute URLs - ensure an appropriate [Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) is defined. (This should be used regardless of upgrading, as a defence-in-depth best practice.) (2022-08-29, CVE-2022-36033)
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>