Decidim: Push subscriptions can be abused for server-side requests

Decidim is a participatory democracy framework. Prior to 0.30.9, from 0.31.0 before 0.31.5, and in 0.32.0.rc1 before 0.32.0.rc2, when VAPID delivery is enabled, the notification subscription flow stores a client-supplied push endpoint without validating that it belongs to an approved push service, and SendPushNotification later passes that endpoint to WebPush.payload_send, allowing an authenticated user to create stored, mostly blind server-side requests to arbitrary reachable HTTPS endpoints. This issue is fixed in versions 0.30.9, 0.31.5, and 0.32.0.rc2.