The Calendar plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'event_desc' parameter in all versions up to, and including, 1.3.16 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with Contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page, granted they can convince an administrator to enable lower privilege users to manage calendar events via the plugin settings.
CWE-79