Executive brief
The Ultimate Post plugin for WordPress, used for creating advanced post grids and search blocks, contains a security flaw that allows users with contributor-level access or higher to inject malicious scripts into website pages. These scripts will automatically run in the browser of any visitor who views the affected page. This could lead to unauthorized actions being performed on behalf of site administrators or the theft of sensitive session information.
Technical details
The Ultimate Post plugin for WordPress is vulnerable to Stored Cross-Site Scripting (XSS) due to insufficient input sanitization and output escaping in the Advanced_Search::content() render callback. Specifically, the 'moreResultsText' block attribute is filtered using wp_kses(), which fails to escape HTML special characters like double quotes. The resulting value is then concatenated directly into the 'data-viewmoretext' HTML attribute without using esc_attr(). This allows authenticated attackers with contributor-level permissions or higher to break out of the attribute and inject arbitrary JavaScript. The vulnerability is present in versions up to and including 5.0.31.
Affected products
- wpxpo Post Grid Gutenberg Blocks – PostX (Ultimate Post) up to and including 5.0.31
Timeline
- 2026-07-09: disclosed
- 2026-07-09: advisory
References
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.27/blocks/Advanced_Search.php
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.27/blocks/Advanced_Search.php
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.27/classes/Blocks.php
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.28/blocks/Advanced_Search.php
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.28/blocks/Advanced_Search.php
- https://plugins.trac.wordpress.org/browser/ultimate-post/tags/5.0.28/classes/Blocks.php
- https://plugins.trac.wordpress.org/changeset?reponame=&old=3598669%40ultimate-post&new=3598669%40ultimate-post