Downloads::File: fix SSRF when following redirects (#2498).

Fixes the banned IP check not being applied when following redirects:

  http://danbooru.donmai.us/uploads/new?url=http://httpbin.org/redirect-to%3Furl=http://127.0.0.1/test.jpg

app/logical/downloads/file.rb

@@ -9,7 +9,6 @@ class Error < Exception ; end
     attr_reader :url, :referer
 
     validate :validate_url
-    validate :validate_local_hosts
 
     # Prevent Cloudflare from potentially mangling the image. See issue #3528.
     def self.uncached_url(url, headers = {})
@@ -34,13 +33,11 @@ def self.is_cloudflare?(url, headers = {})
     def initialize(url, referer=nil)
       @url = Addressable::URI.parse(url) rescue nil
       @referer = referer
+      validate!
     end
 
     def size
-      validate!
-      options = { timeout: 3, headers: strategy.headers }.deep_merge(Danbooru.config.httparty_options)
-
-      res = HTTParty.head(strategy.file_url, options)
+      res = HTTParty.head(strategy.file_url, **httparty_options, timeout: 3)
 
       if res.success?
         res.content_length
@@ -50,7 +47,6 @@ def size
     end
 
     def download!(tries: 3, **options)
-      validate!
       url = self.class.uncached_url(strategy.file_url, strategy.headers)
 
       Retriable.retriable(on: RETRIABLE_ERRORS, tries: tries, base_interval: 0) do
@@ -59,13 +55,6 @@ def download!(tries: 3, **options)
       end
     end
 
-    def validate_local_hosts
-      ip_addr = IPAddr.new(Resolv.getaddress(url.hostname))
-      if Danbooru.config.banned_ip_for_download?(ip_addr)
-        errors[:base] << "Downloads from #{ip_addr} are not allowed"
-      end
-    end
-
     def validate_url
       errors[:base] << "URL must not be blank" if url.blank?
       errors[:base] << "'#{url}' is not a valid url" if !url.host.present?
@@ -74,9 +63,8 @@ def validate_url
 
     def http_get_streaming(url, file: Tempfile.new(binmode: true), headers: {}, max_size: Danbooru.config.max_file_size)
       size = 0
-      options = { stream_body: true, timeout: 10, headers: headers }
 
-      res = HTTParty.get(url, options.deep_merge(Danbooru.config.httparty_options)) do |chunk|
+      res = HTTParty.get(url, httparty_options) do |chunk|
         size += chunk.size
         raise Error.new("File is too large (max size: #{max_size})") if size > max_size && max_size > 0
 
@@ -94,5 +82,28 @@ def http_get_streaming(url, file: Tempfile.new(binmode: true), headers: {}, max_
     def strategy
       @strategy ||= Sources::Strategies.find(url.to_s, referer)
     end
+
+    def httparty_options
+      {
+        timeout: 10,
+        stream_body: true,
+        headers: strategy.headers,
+        connection_adapter: ValidatingConnectionAdapter,
+      }.deep_merge(Danbooru.config.httparty_options)
+    end
+  end
+
+  # Hook into HTTParty to validate the IP before following redirects.
+  # https://www.rubydoc.info/github/jnunemaker/httparty/HTTParty/ConnectionAdapter
+  class ValidatingConnectionAdapter < HTTParty::ConnectionAdapter
+    def self.call(uri, options)
+      ip_addr = IPAddr.new(Resolv.getaddress(uri.hostname))
+
+      if Danbooru.config.banned_ip_for_download?(ip_addr)
+        raise Downloads::File::Error, "Downloads from #{ip_addr} are not allowed"
+      end
+
+      super(uri, options)
+    end
   end
 end

test/unit/downloads/file_test.rb

@@ -11,12 +11,27 @@ class FileTest < ActiveSupport::TestCase
       context "for a banned IP" do
         should "prevent downloads" do
           Resolv.expects(:getaddress).returns("127.0.0.1")
-          assert_raise(ActiveModel::ValidationError) { Downloads::File.new("http://evil.com").download! }
+          assert_raise(Downloads::File::Error) { Downloads::File.new("http://evil.com").download! }
         end
 
         should "prevent fetching the size" do
           Resolv.expects(:getaddress).returns("127.0.0.1")
-          assert_raise(ActiveModel::ValidationError) { Downloads::File.new("http://evil.com").size }
+          assert_raise(Downloads::File::Error) { Downloads::File.new("http://evil.com").size }
+        end
+
+        should "not follow redirects to banned IPs" do
+          url = "http://httpbin.org/redirect-to?url=http://127.0.0.1"
+          stub_request(:get, url).to_return(status: 301, headers: { "Location": "http://127.0.0.1" })
+
+          assert_raise(Downloads::File::Error) { Downloads::File.new(url).download! }
+        end
+
+        should "not follow redirects that resolve to a banned IP" do
+          url = "http://httpbin.org/redirect-to?url=http://127.0.0.1.nip.io"
+          stub_request(:get, url).to_return(status: 301, headers: { "Location": "http://127.0.0.1.xip.io" })
+          Resolv.expects(:getaddress).returns("127.0.0.1")
+
+          assert_raise(Downloads::File::Error) { Downloads::File.new(url).download! }
         end
       end