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

Description

The push-subscription endpoint stores an attacker-controlled delivery URL, and the notification send path becomes an outbound-request sink when VAPID delivery is enabled. The practical result is an authenticated, stored, mostly blind SSRF primitive to arbitrary HTTPS endpoints reachable from the app server.

Technical description

When VAPID delivery is enabled, the notification subscription flow stores the client-supplied push endpoint without checking that it belongs to an approved push service. The send path later passes that stored URL to WebPush.payload_send, turning the subscription into an attacker-controlled outbound request destination. This is the source-to-sink chain:

1. Source: attacker-controlled subscription.endpoint in the JSON body posted to POST /notifications_subscriptions.
2. Persistence: params[:endpoint] is stored under user.notification_settings["subscriptions"].
3. Retrieval: user.notifications_subscriptions.values returns that stored endpoint later.
4. Sink: build_payload sets endpoint: subscription["endpoint"].
5. Outbound request: WebPush.payload_send(**payload) uses the attacker-supplied endpoint as the destination.

One spec asserts that the endpoint is persisted exactly as supplied:

# decidim-core/spec/services/decidim/notifications_subscriptions_persistor_spec.rb
expect(user.notifications_subscriptions["auth_code_121"]["endpoint"]).to eq(params[:endpoint])

Another spec asserts that SendPushNotification passes the stored endpoint into WebPush.payload_send:

# decidim-core/spec/services/decidim/send_push_notification_spec.rb
first_notification_payload = {
  message:,
  endpoint: subscriptions["auth_key_1"]["endpoint"],
  p256dh: subscriptions["auth_key_1"]["p256dh"],
  auth: subscriptions["auth_key_1"]["auth"],
  vapid: a_hash_including(...)
}
expect(WebPush).to receive(:payload_send).with(first_notification_payload)

Impact

• In a configured deployment, an authenticated user can register an attacker-controlled or otherwise unauthorized HTTPS URL.
• The server then sends outbound POST requests there whenever a notification is pushed.
• This is a stored, mostly blind SSRF primitive: useful for outbound interaction with attacker infrastructure and, where routable, internal HTTPS services.
• Notification metadata is disclosed to the supplied endpoint through the encrypted web push request path.