A stored Cross-site scripting (XSS) issue in Text Editors and Formats in Backdrop CMS before 1.24.2 allows remote attackers to inject arbitrary web script or HTML via the name parameter. When a user is editing any content type (e.g., page, post, or card) as an admin, the stored XSS payload is executed upon selecting a malicious text formatting option. NOTE: the vendor disputes the security relevance of this finding because "any administrator that can configure a text format could easily allow Full HTML anywhere."

Backdrop CMS 1.12.x before 1.12.8 and 1.13.x before 1.13.3 doesn't sufficiently filter output when displaying certain block labels created by administrators. An attacker could potentially craft a specialized label, then have an administrator execute scripting when administering a layout. (This issue is mitigated by the attacker needing permission to create custom blocks on the site, which is typically an administrative permission.)

jQuery before 3.4.0, as used in Drupal, Backdrop CMS, and other products, mishandles jQuery.extend(true, {}, ...) because of Object.prototype pollution. If an unsanitized source object contained an enumerable __proto__ property, it could extend the native Object.prototype.