diff --git a/config/config.example.yml b/config/config.example.yml
index f49c5211..772ec4ec 100644
--- a/config/config.example.yml
+++ b/config/config.example.yml
@@ -534,10 +534,14 @@ log:
 #downcase_username: true
 
 # If you'd like to limit the service hosts that can use CAS for authentication,
-# add the individual IPs and IP ranges in CIDR notation below. Leaving this
-# setting blank will allow any server to authenticate users via the CAS server
+# you can either limit specific service URIs (validating the service parameter),
+# or add the individual IPs and IP ranges in CIDR notation below. Leaving these
+# settings blank will allow any server to authenticate users via the CAS server
 # and potentially harvest sensitive user information.
 
+#allowed_service_uris:
+#  - https://myservice.com/login/cas
+
 #allowed_service_ips:
 #  - 127.0.0.1
 #  - 192.168.0.0/24
diff --git a/lib/casserver/server.rb b/lib/casserver/server.rb
index 14a04bda..c03f0d98 100644
--- a/lib/casserver/server.rb
+++ b/lib/casserver/server.rb
@@ -322,7 +322,11 @@ def self.init_database!
 
       begin
         if @service
-          if @renew
+          if !service_allowed?(@service)
+            $LOG.warn("Disallowed service '#{@service}'.")
+            @message = {:type => 'mistake',
+                        :message => t.error.disallowed_service}
+          elsif @renew
             $LOG.info("Authentication renew explicitly requested. Proceeding with CAS login for service #{@service.inspect}.")
           elsif tgt && !tgt_error
             $LOG.debug("Valid ticket granting ticket detected.")
@@ -409,6 +413,13 @@ def self.init_database!
         @username.downcase!
       end
 
+      if !@service.blank? && !service_allowed?(@service)
+        @message = {:type => 'mistake',
+                    :message => t.error.disallowed_service}
+        status 400
+        return render @template_engine, :login
+      end
+
       if error = validate_login_ticket(@lt)
         @message = {:type => 'mistake', :message => error}
         # generate another login ticket to allow for re-submitting the form
@@ -765,6 +776,12 @@ def compile_template(engine, data, options, views)
       super engine, data, options, views
     end
 
+    def service_allowed?(service)
+      allowed_uris = Array(settings.config[:allowed_service_uris])
+
+      allowed_uris.empty? || allowed_uris.include?(service)
+    end
+
     def ip_allowed?(ip)
       require 'ipaddr'
 
diff --git a/locales/en.yml b/locales/en.yml
index 99eb6094..889d07f1 100644
--- a/locales/en.yml
+++ b/locales/en.yml
@@ -9,6 +9,7 @@ error:
   invalid_target_service: "The target service your browser supplied appears to be invalid. Please contact your system administrator for help."
   unable_to_authenticate: "The client and server are unable to negotiate authentication. Please try logging in again later."
   no_service_parameter_given: "The server cannot fulfill this gateway request because no service parameter was given."
+  disallowed_service: "The target service is not allowed to authenticate against this CAS server. Please contact your system administrator for help."
 
 notice:
   logged_in_as: "You are currently logged in as '%1'. If this is not you, please log in below."
diff --git a/spec/casserver_spec.rb b/spec/casserver_spec.rb
index b49c1a03..ba81a740 100644
--- a/spec/casserver_spec.rb
+++ b/spec/casserver_spec.rb
@@ -36,6 +36,18 @@
       page.should have_content("You have successfully logged in")
     end
 
+    it "auto-forwards when you're already logged in" do
+      visit "/login"
+
+      fill_in 'username', :with => VALID_USERNAME
+      fill_in 'password', :with => VALID_PASSWORD
+      click_button 'login-submit'
+
+      visit "/login?service="+CGI.escape(@target_service)
+
+      page.current_url.should =~ /^#{Regexp.escape(@target_service)}\/?\?ticket=ST\-[1-9rA-Z]+/
+    end
+
     it "fails to log in with invalid password" do
       visit "/login"
       fill_in 'username', :with => VALID_USERNAME
@@ -92,6 +104,38 @@
       #page.should have_xpath("<script>alert(32)</script>")
     end
 
+    describe 'service_uri validation' do
+      let(:service) { 'http://imposter.com/' }
+
+      it "doesn't redirect back to untrusted services" do
+        visit "/login?service="+CGI.escape(service)
+
+        page.should have_content("The target service is not allowed")
+
+        fill_in 'username', :with => VALID_USERNAME
+        fill_in 'password', :with => VALID_PASSWORD
+
+        click_button 'login-submit'
+
+        page.should have_content("The target service is not allowed")
+      end
+
+      it "doesn't redirect back when already logged in" do
+        visit "/login"
+
+        fill_in 'username', :with => VALID_USERNAME
+        fill_in 'password', :with => VALID_PASSWORD
+
+        click_button 'login-submit'
+
+        visit "/login?service="+CGI.escape(service)
+
+        page.should have_content("The target service is not allowed")
+      end
+
+    end
+
+
   end # describe '/login'
 
 
diff --git a/spec/config/default_config.yml b/spec/config/default_config.yml
index 90a83914..ede1abc9 100644
--- a/spec/config/default_config.yml
+++ b/spec/config/default_config.yml
@@ -50,4 +50,7 @@ enable_single_sign_out: true
 #downcase_username: true
 
 allowed_service_ips:
-  - 127.0.0.0/24
\ No newline at end of file
+  - 127.0.0.0/24
+
+allowed_service_uris:
+  - http://my.app.test
\ No newline at end of file