Guard push-subscription endpoints against SSRF

Web push delivery POSTed to the endpoint URL a user supplied when
registering a subscription, with no scheme, host, or private-network
check -- unlike the OpenGraph unfurl path, which already routes through
the shared SSRF address policy (surfguard). Any authenticated user could
register a subscription whose endpoint pointed at an internal address and
have the server fetch it on every chat message: blind SSRF for internal
recon and reachability probing, plus a thread-pool DoS on the delivery
pool.

Validate the endpoint when the subscription is saved: it must be HTTPS,
its host must belong to a known browser push service (allowlist), and it
must resolve to a public IP. On every delivery, re-resolve the host and
pin the connection to that public IP so a later DNS rebind can't redirect
the request to an internal address. If no public IP resolves at delivery
time -- a rebind, or a subscription that predates this validation --
delivery is skipped rather than falling back to re-resolving the raw
host.

Adds model, controller, and delivery-pinning tests, plus a DNS stub
helper for deterministic resolution in tests.

app/controllers/users/push_subscriptions_controller.rb

@@ -7,11 +7,11 @@ class Users::PushSubscriptionsController < ApplicationController
   def create
     if subscription = @push_subscriptions.find_by(push_subscription_params)
       subscription.touch
+      head :ok
     else
-      @push_subscriptions.create! push_subscription_params.merge(user_agent: request.user_agent)
+      subscription = @push_subscriptions.create push_subscription_params.merge(user_agent: request.user_agent)
+      head subscription.persisted? ? :ok : :unprocessable_entity
     end
-
-    head :ok
   end
 
   def destroy

app/models/push/subscription.rb

@@ -1,7 +1,73 @@
 class Push::Subscription < ApplicationRecord
+  # Web push endpoints only ever point at a browser vendor's push service. An
+  # allowlist keeps a user-supplied endpoint from turning delivery into an SSRF
+  # sink, and pinning the resolved public IP on every delivery closes the
+  # DNS-rebinding gap the way Opengraph::Fetch does for unfurls.
+  PERMITTED_ENDPOINT_HOSTS = %w[
+    jmt17.google.com
+    fcm.googleapis.com
+    updates.push.services.mozilla.com
+    web.push.apple.com
+    notify.windows.com
+  ].freeze
+
   belongs_to :user
 
+  validates :endpoint, presence: true
+  validate :validate_endpoint_url
+
   def notification(**params)
-    WebPush::Notification.new(**params, badge: user.memberships.unread.count, endpoint: endpoint, p256dh_key: p256dh_key, auth_key: auth_key)
+    WebPush::Notification.new(**params, badge: user.memberships.unread.count, endpoint: endpoint, endpoint_ip: resolved_endpoint_ip, p256dh_key: p256dh_key, auth_key: auth_key)
   end
+
+  # The public address to pin this delivery to, or nil when the endpoint is not a
+  # permitted push service or doesn't resolve to a public IP. Enforced here, not
+  # only at save time, so a row that predates validation (or was inserted around
+  # it) still can't drive delivery at a non-allowlisted or private target.
+  # Re-resolved on every call so each delivery pins a freshly looked-up address
+  # rather than trusting the host to still resolve the way it did at sign-up.
+  def resolved_endpoint_ip
+    Surfguard.resolve_public_ips(endpoint_uri.host).first if permitted_endpoint_uri?
+  rescue Surfguard::Unresolvable
+    # A host that resolves to nothing has no usable public IP -- the same outcome
+    # as one whose only addresses are blocked: no endpoint IP to pin, which fails
+    # endpoint validation. Push has no lookup-failed surface to distinguish.
+    nil
+  end
+
+  private
+    # The full shape a deliverable endpoint must have: HTTPS on the default port,
+    # pointing at a permitted push service. Gating resolution on this (not just
+    # the host) keeps a row that slipped past save-time validation from driving
+    # delivery to an odd port or scheme on a permitted vendor's address.
+    def permitted_endpoint_uri?
+      endpoint_uri&.scheme == "https" && endpoint_uri.port == 443 && permitted_endpoint_host?
+    end
+
+    def endpoint_uri
+      URI.parse(endpoint) if endpoint.present?
+    rescue URI::InvalidURIError
+      nil
+    end
+
+    def validate_endpoint_url
+      if endpoint_uri.nil?
+        errors.add(:endpoint, "is not a valid URL")
+      elsif endpoint_uri.scheme != "https"
+        errors.add(:endpoint, "must use HTTPS")
+      elsif endpoint_uri.port != 443
+        errors.add(:endpoint, "must use the default HTTPS port")
+      elsif !permitted_endpoint_host?
+        errors.add(:endpoint, "is not a permitted push service")
+      elsif resolved_endpoint_ip.nil?
+        errors.add(:endpoint, "resolves to a private or invalid IP address")
+      end
+    end
+
+    def permitted_endpoint_host?
+      host = endpoint_uri&.host&.downcase
+      host.present? && PERMITTED_ENDPOINT_HOSTS.any? do |permitted|
+        host == permitted || host.end_with?(".#{permitted}")
+      end
+    end
 end

config/initializers/web_push.rb

@@ -17,7 +17,18 @@ end
 
 module WebPush::PersistentRequest
   def perform
-    if @options[:connection]
+    if endpoint_ip = @options[:endpoint_ip]
+      # Pin the connection to the public IP resolved (and guarded) by
+      # Push::Subscription so delivery can't be rebound to a private address
+      # between resolution and connect. Bypasses the shared persistent pool,
+      # which would re-resolve the host itself.
+      http = Net::HTTP.new(uri.host, uri.port)
+      http.ipaddr = endpoint_ip
+      http.use_ssl = true
+      http.ssl_timeout = @options[:ssl_timeout] unless @options[:ssl_timeout].nil?
+      http.open_timeout = @options[:open_timeout] unless @options[:open_timeout].nil?
+      http.read_timeout = @options[:read_timeout] unless @options[:read_timeout].nil?
+    elsif @options[:connection]
       http = @options[:connection]
     else
       http = Net::HTTP.new(uri.host, uri.port, *proxy_options)

lib/web_push/notification.rb

@@ -1,16 +1,23 @@
 class WebPush::Notification
-  def initialize(title:, body:, path:, badge:, endpoint:, p256dh_key:, auth_key:)
+  def initialize(title:, body:, path:, badge:, endpoint:, endpoint_ip:, p256dh_key:, auth_key:)
     @title, @body, @path, @badge = title, body, path, badge
-    @endpoint, @p256dh_key, @auth_key = endpoint, p256dh_key, auth_key
+    @endpoint, @endpoint_ip, @p256dh_key, @auth_key = endpoint, endpoint_ip, p256dh_key, auth_key
   end
 
+  # @endpoint_ip is the public address Push::Subscription resolved and guarded for
+  # this delivery. When it is nil the host resolved to nothing or to a blocked
+  # (private) address, so we skip delivery rather than let the request fall back
+  # to re-resolving the raw host -- which is what would reopen the SSRF for a
+  # subscription that slipped in before endpoint validation existed.
   def deliver(connection: nil)
-    WebPush.payload_send \
-      message: encoded_message,
-      endpoint: @endpoint, p256dh: @p256dh_key, auth: @auth_key,
-      vapid: vapid_identification,
-      connection: connection,
-      urgency: "high"
+    if @endpoint_ip
+      WebPush.payload_send \
+        message: encoded_message,
+        endpoint: @endpoint, endpoint_ip: @endpoint_ip, p256dh: @p256dh_key, auth: @auth_key,
+        vapid: vapid_identification,
+        connection: connection,
+        urgency: "high"
+    end
   end
 
   private

test/controllers/users/push_subscriptions_controller_test.rb

@@ -3,10 +3,11 @@ require "test_helper"
 class Users::PushSubscriptionsControllerTest < ActionDispatch::IntegrationTest
   setup do
     sign_in :david
+    stub_web_push_dns_resolution
   end
 
   test "create new push subscription" do
-    subscription_params = { "endpoint" => "https://apple", "p256dh_key" => "123", "auth_key" => "456" }
+    subscription_params = { "endpoint" => "https://fcm.googleapis.com/fcm/send/abc123", "p256dh_key" => "123", "auth_key" => "456" }
 
     post user_push_subscriptions_url,
       params: { push_subscription: subscription_params }, headers: { "HTTP_USER_AGENT" => "Mozilla/5.0" }
@@ -29,6 +30,27 @@ class Users::PushSubscriptionsControllerTest < ActionDispatch::IntegrationTest
     assert_response :ok
   end
 
+  test "rejects subscription with non-permitted endpoint" do
+    subscription_params = { "endpoint" => "https://attacker.example.com/steal", "p256dh_key" => "123", "auth_key" => "456" }
+
+    assert_no_difference -> { Push::Subscription.count } do
+      post user_push_subscriptions_url, params: { push_subscription: subscription_params }
+    end
+
+    assert_response :unprocessable_entity
+  end
+
+  test "rejects subscription with endpoint resolving to a private IP" do
+    stub_dns_resolution("169.254.169.254")
+    subscription_params = { "endpoint" => "https://fcm.googleapis.com/fcm/send/abc123", "p256dh_key" => "123", "auth_key" => "456" }
+
+    assert_no_difference -> { Push::Subscription.count } do
+      post user_push_subscriptions_url, params: { push_subscription: subscription_params }
+    end
+
+    assert_response :unprocessable_entity
+  end
+
   test "destroy a push subscription via dev mode" do
     assert_difference -> { Push::Subscription.count }, -1 do
       delete user_push_subscription_url(push_subscriptions(:david_chrome))

test/lib/web_push/persistent_request_test.rb

@@ -0,0 +1,26 @@
+require "test_helper"
+
+class WebPush::PersistentRequestTest < ActiveSupport::TestCase
+  ENDPOINT = "https://fcm.googleapis.com/fcm/send/test123"
+
+  # The delivery must connect to the public IP resolved and guarded by
+  # Push::Subscription, never re-resolve the raw endpoint host at connect time --
+  # otherwise a rebind between resolution and delivery reopens the SSRF. An empty
+  # message keeps the request past encryption and onto the socket we assert on.
+  test "pins delivery to endpoint_ip instead of re-resolving the host" do
+    host = URI(ENDPOINT).host
+    WebMock.disable_net_connect! allow: [ host ]
+
+    TCPSocket.expects(:open).with { |*args, **| args.first == host }.never
+    TCPSocket.expects(:open).with { |*args, **| args.first == DnsTestHelper::WEB_PUSH_PUBLIC_TEST_IP && args[1] == 443 }.throws(:pinned_to_ip)
+
+    assert_throws :pinned_to_ip do
+      WebPush.payload_send \
+        message: "",
+        endpoint: ENDPOINT,
+        endpoint_ip: DnsTestHelper::WEB_PUSH_PUBLIC_TEST_IP,
+        p256dh: "", auth: "", vapid: {},
+        urgency: "high"
+    end
+  end
+end

test/models/push/subscription_test.rb

@@ -0,0 +1,145 @@
+require "test_helper"
+
+class Push::SubscriptionTest < ActiveSupport::TestCase
+  setup do
+    stub_web_push_dns_resolution
+  end
+
+  test "valid subscription with permitted endpoint" do
+    assert build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123").valid?
+  end
+
+  test "rejects endpoint with non-https scheme" do
+    subscription = build_subscription(endpoint: "http://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "must use HTTPS"
+  end
+
+  test "rejects endpoint with non-permitted host" do
+    subscription = build_subscription(endpoint: "https://attacker.example.com/webhook")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "is not a permitted push service"
+  end
+
+  test "rejects endpoint whose host only suffix-matches a permitted host" do
+    subscription = build_subscription(endpoint: "https://evilfcm.googleapis.com.attacker.example/webhook")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "is not a permitted push service"
+  end
+
+  test "rejects blank endpoint" do
+    subscription = build_subscription(endpoint: "")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "can't be blank"
+  end
+
+  test "rejects endpoint on a non-default port" do
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com:8443/fcm/send/abc123")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "must use the default HTTPS port"
+  end
+
+  test "rejects endpoint that resolves to private IP" do
+    stub_dns_resolution("192.168.1.1")
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "resolves to a private or invalid IP address"
+  end
+
+  test "rejects endpoint that resolves to loopback IP" do
+    stub_dns_resolution("127.0.0.1")
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "resolves to a private or invalid IP address"
+  end
+
+  test "rejects endpoint that resolves to link-local IP (AWS IMDS)" do
+    stub_dns_resolution("169.254.169.254")
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "resolves to a private or invalid IP address"
+  end
+
+  test "rejects endpoint whose host resolves to nothing without raising" do
+    stub_dns_failure
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_nil subscription.resolved_endpoint_ip
+    assert_not subscription.valid?
+    assert_includes subscription.errors[:endpoint], "resolves to a private or invalid IP address"
+  end
+
+  test "resolved_endpoint_ip returns the pinned public IP" do
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_equal DnsTestHelper::WEB_PUSH_PUBLIC_TEST_IP, subscription.resolved_endpoint_ip
+  end
+
+  test "notification carries the resolved endpoint IP for delivery pinning" do
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+    notification = subscription.notification(title: "t", body: "b", path: "/")
+
+    assert_equal DnsTestHelper::WEB_PUSH_PUBLIC_TEST_IP, notification.instance_variable_get(:@endpoint_ip)
+  end
+
+  test "delivery is skipped when the endpoint no longer resolves to a public IP" do
+    stub_dns_resolution("10.0.0.5") # host now answers with a private address
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    assert_nil subscription.resolved_endpoint_ip
+    WebPush.expects(:payload_send).never
+    subscription.notification(title: "t", body: "b", path: "/").deliver
+  end
+
+  test "delivery is skipped for a non-permitted host even when it resolves publicly" do
+    # A row that predates endpoint validation: its host resolves to a public IP,
+    # but it is not a permitted push service, so delivery must not proceed.
+    subscription = build_subscription(endpoint: "https://attacker.example.com/collect")
+
+    assert_nil subscription.resolved_endpoint_ip
+    WebPush.expects(:payload_send).never
+    subscription.notification(title: "t", body: "b", path: "/").deliver
+  end
+
+  test "delivery is skipped for a permitted host on a non-default port" do
+    # A legacy/bypassed row: permitted host, resolves publicly, but port 22.
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com:22/fcm/send/abc123")
+
+    assert_nil subscription.resolved_endpoint_ip
+    WebPush.expects(:payload_send).never
+    subscription.notification(title: "t", body: "b", path: "/").deliver
+  end
+
+  test "delivery sends with the pinned endpoint_ip" do
+    subscription = build_subscription(endpoint: "https://fcm.googleapis.com/fcm/send/abc123")
+
+    WebPush.expects(:payload_send).with(has_entry(endpoint_ip: DnsTestHelper::WEB_PUSH_PUBLIC_TEST_IP))
+    subscription.notification(title: "t", body: "b", path: "/").deliver
+  end
+
+  test "accepts all permitted push service domains" do
+    [
+      "https://fcm.googleapis.com/fcm/send/token123",
+      "https://jmt17.google.com/fcm/send/token123",
+      "https://updates.push.services.mozilla.com/wpush/v2/token123",
+      "https://web.push.apple.com/QaBC123",
+      "https://wns2-db5p.notify.windows.com/w/?token=abc123"
+    ].each do |endpoint|
+      subscription = build_subscription(endpoint: endpoint)
+      assert subscription.valid?, "Expected #{endpoint} to be valid, got: #{subscription.errors.full_messages}"
+    end
+  end
+
+  private
+    def build_subscription(endpoint:)
+      Push::Subscription.new(user: users(:david), endpoint: endpoint, p256dh_key: "test_key", auth_key: "test_auth")
+    end
+end

test/models/room/push_test.rb

@@ -3,6 +3,10 @@ require "test_helper"
 class Room::PushTest < ActiveSupport::TestCase
   include ActiveJob::TestHelper
 
+  setup do
+    stub_web_push_dns_resolution
+  end
+
   test "deliver new message to other room users with push subscriptions" do
     task_count = Push::Subscription.count - users(:david).push_subscriptions.count
     perform_enqueued_jobs only: Room::PushMessageJob do

test/test_helper.rb

@@ -17,7 +17,7 @@ class ActiveSupport::TestCase
   # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
   fixtures :all
 
-  include SessionTestHelper, MentionTestHelper, TurboTestHelper
+  include SessionTestHelper, MentionTestHelper, TurboTestHelper, DnsTestHelper
 
   setup do
     ActionCable.server.pubsub.clear

test/test_helpers/dns_test_helper.rb

@@ -0,0 +1,21 @@
+module DnsTestHelper
+  WEB_PUSH_PUBLIC_TEST_IP = "142.250.185.206" # stable public IP for web push DNS stubs in tests
+
+  private
+    # Surfguard resolves through Resolv.getaddresses, which honours /etc/hosts and
+    # search domains and returns every address a host answers with.
+    def stub_dns_resolution(*ips)
+      Resolv.stubs(:getaddresses).returns(ips.map(&:to_s))
+    end
+
+    # A host that resolves to nothing: the resolver errors (timeout/NXDOMAIN),
+    # which Surfguard catches and reports as Unresolvable, distinct from a host
+    # that resolves only to blocked addresses.
+    def stub_dns_failure(error = Resolv::ResolvError)
+      Resolv.stubs(:getaddresses).raises(error)
+    end
+
+    def stub_web_push_dns_resolution
+      stub_dns_resolution(WEB_PUSH_PUBLIC_TEST_IP)
+    end
+end