Backdrop CMS Core 1.35.x before 1.35.2 and 1.34.x before 1.34.6 mishandles tokens. A token is a small placeholder string (like [node:title] or [user:name]) used to automatically insert dynamic data into text. A browser is provided for selecting tokens by type. The product doesn't sufficiently limit access to the types of tokens that are available in the token browser. The names of the individual tokens are adequately protected.
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/1.35.2 . See the update instructions https://docs.backdropcms.org/documentation/core-updates .