Backdrop CMS Core 1.35.x before 1.35.2 and 1.34.x before 1.34.6 does not sufficiently validate uploaded SVG images to ensure they lack potentially dangerous SVG tags. SVG images can contain clickable links or executable scripting. By uploading 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 script from executing. The SVG must be viewed directly by its URL in order to run any embedded scripting.
This issue is mitigated by the attacker needing to be able to upload SVG images.
Update your site to the most recent version of Backdrop core. Download available on the Backdrop CMS 1.35.2 release page https://github.com/backdrop/backdrop/releases/tag/1.35.6 . See the update instructions https://docs.backdropcms.org/documentation/core-updates .