8 files changed, 69 insertions, 9 deletions
diff --git a/Gemfile b/Gemfile
index da5fc2f38..e2b3b1971 100644
--- a/Gemfile
+++ b/Gemfile
@@ -96,6 +96,10 @@ group :development, :test do
   gem 'rspec-rails', '~> 3.7'
 end
 
+group :production, :test do
+  gem 'private_address_check', '~> 0.4.1'
+end
+
 group :test do
   gem 'capybara', '~> 2.15'
   gem 'climate_control', '~> 0.2'
diff --git a/Gemfile.lock b/Gemfile.lock
index 65a0dfabf..8293977d8 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -376,6 +376,7 @@ GEM
     premailer-rails (1.10.1)
       actionmailer (>= 3, < 6)
       premailer (~> 1.7, >= 1.7.9)
+    private_address_check (0.4.1)
     pry (0.11.3)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
@@ -683,6 +684,7 @@ DEPENDENCIES
   pghero (~> 1.7)
   pkg-config (~> 1.2)
   premailer-rails
+  private_address_check (~> 0.4.1)
   pry-rails (~> 0.3)
   puma (~> 3.10)
   pundit (~> 1.1)
diff --git a/app/lib/exceptions.rb b/app/lib/exceptions.rb
index b2489711d..95e3365c2 100644
--- a/app/lib/exceptions.rb
+++ b/app/lib/exceptions.rb
@@ -4,6 +4,7 @@ module Mastodon
   class Error < StandardError; end
   class NotPermittedError < Error; end
   class ValidationError < Error; end
+  class HostValidationError < ValidationError; end
   class RaceConditionError < Error; end
 
   class UnexpectedResponseError < Error
diff --git a/app/lib/request.rb b/app/lib/request.rb
index 7671f4ffc..5776b3d78 100644
--- a/app/lib/request.rb
+++ b/app/lib/request.rb
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require 'ipaddr'
+require 'socket'
+
 class Request
   REQUEST_TARGET = '(request-target)'
 
@@ -8,7 +11,7 @@ class Request
   def initialize(verb, url, **options)
     @verb    = verb
     @url     = Addressable::URI.parse(url).normalize
-    @options = options
+    @options = options.merge(socket_class: Socket)
     @headers = {}
 
     set_common_headers!
@@ -87,4 +90,18 @@ class Request
   def http_client
     HTTP.timeout(:per_operation, timeout).follow(max_hops: 2)
   end
+
+  class Socket < TCPSocket
+    class << self
+      def open(host, *args)
+        address = IPSocket.getaddress(host)
+        raise Mastodon::HostValidationError if PrivateAddressCheck.private_address? IPAddr.new(address)
+        super address, *args
+      end
+
+      alias new open
+    end
+  end
+
+  private_constant :Socket
 end
diff --git a/app/lib/sidekiq_error_handler.rb b/app/lib/sidekiq_error_handler.rb
new file mode 100644
index 000000000..23785cf05
--- /dev/null
+++ b/app/lib/sidekiq_error_handler.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class SidekiqErrorHandler
+  def call(*)
+    yield
+  rescue Mastodon::HostValidationError => e
+    Rails.logger.error "#{e.class}: #{e.message}"
+    Rails.logger.error e.backtrace.join("\n")
+    # Do not retry
+  end
+end
diff --git a/config/environments/development.rb b/config/environments/development.rb
index 59bc2c3e2..2da407c32 100644
--- a/config/environments/development.rb
+++ b/config/environments/development.rb
@@ -85,3 +85,9 @@ Rails.application.configure do
 end
 
 ActiveRecordQueryTrace.enabled = ENV.fetch('QUERY_TRACE_ENABLED') { false }
+
+module PrivateAddressCheck
+  def self.private_address?(*)
+    false
+  end
+end
diff --git a/config/initializers/sidekiq.rb b/config/initializers/sidekiq.rb
index b70784d79..f875fbd95 100644
--- a/config/initializers/sidekiq.rb
+++ b/config/initializers/sidekiq.rb
@@ -9,6 +9,10 @@ end
 
 Sidekiq.configure_server do |config|
   config.redis = redis_params
+
+  config.server_middleware do |chain|
+    chain.add SidekiqErrorHandler
+  end
 end
 
 Sidekiq.configure_client do |config|
diff --git a/spec/lib/request_spec.rb b/spec/lib/request_spec.rb
index 782f14b18..dc7daa52c 100644
--- a/spec/lib/request_spec.rb
+++ b/spec/lib/request_spec.rb
@@ -38,17 +38,32 @@ describe Request do
   end
 
   describe '#perform' do
-    before do
-      stub_request(:get, 'http://example.com')
-      subject.perform
-    end
+    context 'with valid host' do
+      before do
+        stub_request(:get, 'http://example.com')
+        subject.perform
+      end
+
+      it 'executes a HTTP request' do
+        expect(a_request(:get, 'http://example.com')).to have_been_made.once
+      end
 
-    it 'executes a HTTP request' do
-      expect(a_request(:get, 'http://example.com')).to have_been_made.once
+      it 'sets headers' do
+        expect(a_request(:get, 'http://example.com').with(headers: subject.headers)).to have_been_made
+      end
     end
 
-    it 'sets headers' do
-      expect(a_request(:get, 'http://example.com').with(headers: subject.headers)).to have_been_made
+    context 'with private host' do
+      around do |example|
+        WebMock.disable!
+        example.run
+        WebMock.enable!
+      end
+
+      it 'raises Mastodon::ValidationError' do
+        allow(IPSocket).to receive(:getaddress).with('example.com').and_return('0.0.0.0')
+        expect{ subject.perform }.to raise_error Mastodon::ValidationError
+      end
     end
   end
 end