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Cross-site Scripting occurrences in Broken Link Checker
A reflected XSS vulnerability was found in includes/admin/table-printer.php in the broken-link-checker (aka Broken Link Checker) plugin 1.11.8 for WordPress. This allows unauthorized users to inject client-side JavaScript into an admin-only WordPress page via the wp-admin/tools.php?page=view-broken-links s_filter parameter in a search action. (2019-10-18, CVE-2019-17207)
The broken-link-checker plugin through 1.11.8 for WordPress (aka Broken Link Checker) is susceptible to Reflected XSS due to improper encoding and insertion of an HTTP GET parameter into HTML. The filter function on the page listing all detected broken links can be exploited by providing an XSS payload in the s_filter GET parameter in a filter_id=search request. NOTE: this is an end-of-life product. (2019-10-16, CVE-2019-16521)
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>