Cross-Site-Scripting in the Online Citizen Card Environment

The Austrian Citizen Card (Bürgkerkarte) is a smart card which allows citizens to authenticate against E-Government (web-)applications, sign online forms, log in to online banking and even sign banking transactions. In order to use the Bürgerkarte a BKU software (Bürgerkartenumgebung, Citizen Card Environment) is needed. The BKU provides a web server with a standardized programming interface. To make the BKU software platform independent and more convenient to use, the Technische Universität Graz implemented the MOCCA OnlineBKU1. A few months ago I have found a Cross-Site-Scripting vulnerability in this software, allowing an attacker to inject JavaScript code into the browser origin of the OnlineBKU installation and consequently in the origin of many web applications using the OnlineBKU. This issue has been fixed after it has been reported to the developers. Updated and properly configured applications are not vulnerable anymore.

The vulnerability

The OnlineBKU consists of a Java Applet that runs on the clients browser, a Java Servlet component and the actual application. The Applet is used to communicate with the smart card and the user, while the server component handles the communication to the application. The application is the component that then processes the data signed by the users citizen card. A successful signing procedure happens as following:

  1. The clients browser sends (HTTP POST) a request to a servlet. This requests contains the request to the Citizen Card Environment (XMLRequest), some Applet parameters (appletGuiStyle, appletWidth, appletHeight, etc.) and the DataURL.
  2. The Servlet responds with an HTML document that embeds the Java Applet. This document is normally shown in an iframe.
  3. The clients browser loads the Applet. The Applet then communicates with a Servlet.
  4. The user interacts with the Applet (e.g. signes a document).
  5. The Applet sends the signed data to a Servlet. The Servlet posts the signed data to the DataURL defined by the application (see step 1).

The application now has several options to respond to the DataURL request 2. Depending on the HTTP status code and the response data, the Citizen Card Environment behaves differently. This allows the application to e.g. send consequent XMLRequests. One of the options available is to return plain HTML with status code 200. The Citizen Card Environment should in this case show the document in the clients browser. Therefore, the Applet redirects the browser to a Servlet that responds the HTML document. As the document originates from a Servlet in the Citizen Card Environment, JavaScript on that page also runs in the context of the Citizen Card Environment. As many providers install the Citizen Card Environment on the same host as the actual application (often as https://server/bkuonline), this flaw also allows to steal session cookies of many applications. One problem may still be obvious: the victim has to sign a document. However, the specification of the Citizen Card Environment defines aNullOperationRequest, that only sends aNullOperationResponseto the DataURL → no user interaction needed to inject JavaScript code (XSS).

The attack

An attacker can do the following:

  1. The victim visits an attacker controlled web page while being logged in on an application that uses a vulnerable Citizen Card Environment.
  2. The attacker prepares (and submits using JavaScript) an HTML form that sends a NullOperationRequest with a forged DataURL to the this Citizen Card Environment.
  3. The clients browser shows the response document with the Java Applet (e.g. hidden in an invisible iframe), the Servlet send the NullOperationResponse to the attacker controlled DataURL.
  4. The attackers server responds to this request with an HTML document containing a malicious JavaScript.
  5. The Applet running on the victims browser redirects to the HTML document (retrieved from the attacker) that is now shown in the origin of the Citizen Card Environment. The JavaScript is executed by the victims browser and may now steal session cookies (Cross-Site-Scripting).

The proof of concept

To demonstrate this issue I have written a tiny HTML page that demonstrates this issue: It sends a forged request to a specified OnlineBKU installation and prints cookies of the that are visible in the origin of the OnlineBKU installation. As many installations have been updated now (and the proof of concept does not work any more on those installations) i have made a short video demonstrating the issue:

The fix

Beginning with version 1.3.8 of the MOCCA OnlineBKU4 the allowed DataURLs have to be specified in a configuration file (as regular expressions). When configured correctly, that fixes this issue.