CVE-2009-3086

@@ -140,7 +140,7 @@ def unmarshal(cookie)
         data, digest = cookie.split('--')
 
         # Do two checks to transparently support old double-escaped data.
-        unless digest == generate_digest(data) || digest == generate_digest(data = CGI.unescape(data))
+        unless secure_compare(digest, generate_digest(data)) || secure_compare(digest, generate_digest(data = CGI.unescape(data)))
           delete
           raise TamperedWithCookie
         end
@@ -164,4 +164,17 @@ def write_cookie(options)
     def clear_old_cookie_value
       @session.cgi.cookies[@cookie_options['name']].clear
     end
+    
+    # constant-time comparison algorithm to prevent timing attacks
+    def secure_compare(a, b)
+      if a.length == b.length
+        result = 0
+        for i in 0..(a.length - 1)
+          result |= a[i] ^ b[i]
+        end
+        result == 0
+      else
+        false
+      end
+    end
 end

@@ -25,10 +25,10 @@ def initialize(secret, digest = 'SHA1')
     
     def verify(signed_message)
       data, digest = signed_message.split("--")
-      if digest != generate_digest(data)
-        raise InvalidSignature
-      else
+      if secure_compare(digest, generate_digest(data))
         Marshal.load(ActiveSupport::Base64.decode64(data))
+      else
+        raise InvalidSignature
       end
     end
     
@@ -38,6 +38,19 @@ def generate(value)
     end
     
     private
+      # constant-time comparison algorithm to prevent timing attacks
+      def secure_compare(a, b)
+        if a.length == b.length
+          result = 0
+          for i in 0..(a.length - 1)
+            result |= a[i] ^ b[i]
+          end
+          result == 0
+        else
+          false
+        end
+      end
+
       def generate_digest(data)
         require 'openssl' unless defined?(OpenSSL)
         OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new(@digest), @secret, data)

@@ -1,5 +1,7 @@
 *2.3.0 [Edge]*
 
+* Added ActiveSupport::MessageVerifier to aid users who need to store signed messages. [Koz]
+
 * Added ActiveSupport::BacktraceCleaner to cut down on backtrace noise according to filters and silencers [DHH]
 
 * Added Object#try. ( Taken from http://ozmm.org/posts/try.html ) [Chris Wanstrath]

@@ -0,0 +1,45 @@
+module ActiveSupport
+  # MessageVerifier makes it easy to generate and verify messages which are signed
+  # to prevent tampering.
+  # 
+  # This is useful for cases like remember-me tokens and auto-unsubscribe links where the
+  # session store isn't suitable or available.
+  #
+  # Remember Me:
+  #   cookies[:remember_me] = @verifier.generate_message([@user.id, 2.weeks.from_now])
+  # 
+  # In the authentication filter:
+  #
+  #   id, time = @verifier.verify_message(cookies[:remember_me])
+  #   if time < Time.now
+  #     self.current_user = User.find(id)
+  #   end
+  # 
+  class MessageVerifier
+    class InvalidSignature < StandardError; end
+    
+    def initialize(secret, digest = 'SHA1')
+      @secret = secret
+      @digest = digest
+    end
+    
+    def verify_message(signed_message)
+      data, digest = signed_message.split("--")
+      if digest != generate_digest(data)
+        raise InvalidSignature
+      else
+        Marshal.load(ActiveSupport::Base64.decode64(data))
+      end
+    end
+    
+    def generate_message(value)
+      data = ActiveSupport::Base64.encode64s(Marshal.dump(value))
+      "#{data}--#{generate_digest(data)}"
+    end
+    
+    private
+      def generate_digest(data)
+        OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new(@digest), @secret, data)
+      end
+  end
+end
\ No newline at end of file

@@ -56,6 +56,7 @@
 require 'active_support/time_with_zone'
 
 require 'active_support/secure_random'
+require 'active_support/message_verifier'
 
 require 'active_support/rescuable'
 

@@ -0,0 +1,25 @@
+require 'abstract_unit'
+
+class MessageVerifierTest < Test::Unit::TestCase
+  def setup
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = {:some=>"data", :now=>Time.now}
+  end
+  
+  def test_simple_round_tripping
+    message = @verifier.generate_message(@data)
+    assert_equal @data, @verifier.verify_message(message)
+  end
+  
+  def test_tampered_data_raises
+    data, hash = @verifier.generate_message(@data).split("--")
+    assert_not_verified("#{data.reverse}--#{hash}")
+    assert_not_verified("#{data}--#{hash.reverse}")
+  end
+  
+  def assert_not_verified(message)
+    assert_raises(ActiveSupport::MessageVerifier::InvalidSignature) do
+      @verifier.verify_message(message)
+    end
+  end
+end