An XSS issue was discovered in Backdrop CMS 1.28.x before 1.28.5 and 1.29.x before 1.29.3. It does not sufficiently validate uploaded SVG images to ensure they do not contain potentially dangerous SVG tags. SVG images can contain clickable links and executable scripting, and using a crafted SVG, it is possible to execute scripting in the browser when an SVG image is viewed. This issue is mitigated by the attacker needing to be able to upload SVG images, and that Backdrop embeds all uploaded SVG images within <img> tags, which prevents scripting from executing. The SVG must be viewed directly by its URL in order to run any embedded scripting.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.