Executive brief
GutenKit is a WordPress plugin that provides block-based content editing for the Gutenberg editor. The plugin fails to properly validate or escape CSS style settings, allowing users with Contributor role and above to inject arbitrary CSS into pages. This enables defacement and interface manipulation visible to other users and site visitors.
Technical details
The vulnerability is a stored CSS injection in the GutenKit WordPress plugin (plugin slug: gutenkit-blocks-addon). The plugin accepts style settings via post meta fields (postBodyCss) and block attributes (blocksCSS) without validation or HTML escaping before outputting them into a <style> element in the page head. An authenticated user with Contributor role or above can inject arbitrary CSS by modifying post metadata or block attributes via the WordPress REST API. While JavaScript execution is prevented at this privilege level by WordPress's content filtering, attackers can perform CSS-based attacks including page defacement, UI redressing, forcing external resource loads, and information disclosure through CSS selectors. The vulnerability was patched in version 2.5.1.
Affected products
- GutenKit GutenKit before 2.5.1
Timeline
- 2026-09-01: disclosed
- 2026-09-01: patched: version 2.5.1 released