An XSS issue was discovered in the Flag module before 1.x-3.6.2 for Backdrop CMS. Flag is a module that allows flags to be added to nodes, comments, users, and any other type of entity. It doesn't verify flag links before performing the flag action, or verify that the response returned was provided by the flag module. This can allow crafted HTML to result in Cross Site Scripting. This is mitigated by the fact that an attacker must have a role with permission to create links on the website, for example: create or edit comments or content with a filtered text format.
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.