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. Patches See https://github.com/decidim/decidim/pull/16714. Workarounds Disable the push notifications feature by removing the VAPID keys in the server. Resource SSRF Credits This issue was discovered in a security audit organized by the "Decidim Association" (https://decidim.org) and made by "Radically Open Security" (https://www.radicallyopensecurity.com/) against Decidim financed by "NGI" (https://ngi.eu/).