Backdrop CMS before 1.27.3 and 1.28.x before 1.28.2 does not sufficiently sanitize field labels before they are displayed in certain places. This vulnerability is mitigated by the fact that an attacker must have a role with the "administer fields" permission.

A stored cross-site scripting (XSS) vulnerability in the Add Link function of BackdropCMS v1.21.1 allows attackers to execute arbitrary web scripts or HTML.

DISPUTED A Cross Site Request Forgery (CSRF) vulnerability exists in Backdrop CMS 1.20, which allows Remote Attackers to gain Remote Code Execution (RCE) on the Hosting Webserver via uploading a maliciously add-on with crafted PHP file. NOTE: the vendor disputes this because the attack requires a session cookie of a high-privileged authenticated user who is entitled to install arbitrary add-ons.

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.)