Impact The vulnerability allows any registered and authenticated user to accept or reject any amendments. The impact is on any users who have created proposals where the amendments feature is enabled. This also elevates the user accepting the amendment as the author of the original proposal as people amending proposals are provided coauthorship on the coauthorable resources.

The only check done when accepting or rejecting amendments is whether the amendment reactions are enabled for the component: https://github.com/decidim/decidim/blob/9d6c3d2efe5a83bb02e095824ff5998d96a75eb7/decidim-core/app/permissions/decidim/permissions.rb#L107

The permission checks have been changed at 1b99136 which was introduced in released version 0.19.0. I have not investigated whether prior versions are also affected.

Patches

Not available

Workarounds Disable amendment reactions for the amendable component (e.g. proposals).