PPWP <= 1.9.26 - Unauthenticated Stored Cross-Site Scripting via '[ppwp]' Shortcode 'desc_below_form' Attribute via Placeholder Marker Collision

The PPWP – Password Protect Pages plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'comment / shortcode attribute desc_below_form (and placeholder as the marker-smuggling helper)' parameter in all versions up to, and including, 1.9.26 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. This is exploitable by unauthenticated attackers because WordPress's comment_content filter leaves bare shortcode text (containing no HTML tags) untouched, and the comment_whitelist setting auto-approves subsequent comments from a previously approved name and email without administrator intervention.