Web Security : Attacking AJAX - Intercepting and Modifying Server Responses, Subverting AJAX with Injected Data

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1. Intercepting and Modifying Server Responses

1.1. Problem

You want to test your client-side code and see how it handles spurious responses from the server. The server might not always send perfect data, so your client-side code needs at least rudimentary error recovery. Sometimes the request-modification technique in this article is too difficult because the requests are sent in a binary or opaque format that is hard to modify. If you change the client’s state by tampering with server responses, you can let the client-side code generate bad requests for you.

1.2. Solution

We will continue with our WordPress example from this article. Configure your web browser to use WebScarab . Start up WebScarab and click on the Proxy tab. Choose the Manual Edit pane and look for the check box labeled Intercept requests. Also check the box labeled Intercept responses.

When your request appears, just click the Accept Changes button and let it go. The next window to pop up will be the response to that request as shown in Figure 1.

Figure 1. Intercepted response in WebScarab

As with the intercepted request, you can modify any part of the response. You can change header names, header values, or the content of the response.

1.3. Discussion

It is especially handy to enter a careful expression in the “Include Paths matching” box so that you’re only intercepting requests and responses related to just the AJAX requests you’re interested in. In an active AJAX application, you’re going to have lots of requests going back and forth, and this could cause lots of dialog boxes to pop up from WebScarab. It interferes with the application’s functionality and it interferes with your ability to do specific tests if you cannot isolate the requests and responses.

As in the case of an intercepted request, it is useful to change values to include cross-site scripting values. Another really useful method is to identify values that cause the client to change its notion of state. For example, the authors have seen an application that managed records in a database and the response from the server included both record identifiers (e.g., records identified with numbers) and the permissions associated with each record. By modifying the response from the server, the client-side code could be tricked into believing that the user had permission to delete the folder with ID 12345, when in fact they should not have that permission. Clicking “delete” on the record generated a correctly formatted AJAX request to delete the record, and it succeeded. In this particular application, the browser requests were very complicated and difficult to manipulate. Server responses, however, were in easy-to-read XML. The interception and modification of server responses was far easier than formatting a correct request. By changing the XML from the server, the client was tricked into sending delete requests for records that it should not delete, and those requests were honored by the server. Oops!

Notice that both the intercepted request and intercepted response dialog boxes (Figure 10-5) have checkboxes at the top labeled Intercept requests and Intercept responses. Frequently you want to perform a single interception, observe the results, and then stop intercepting. Uncheck these boxes before clicking the Accept Changes button and WebScarab will stop intercepting requests and/or responses. To enable interception again, go back to the Proxy pane and click the boxes again.

2. Subverting AJAX with Injected Data

2.1. Problem

If your application uses AJAX, the server will probably deliver data in a format that client-side JavaScript can parse and use. By crafting injection strings to break that format, one can inject arbitrary content onto a page. Worse yet, existing input validation checking for HTML or JavaScript injection may not detect the same injection in a new data format. This recipe discusses non-structured text that might confuse the browser. XREF and XREF discuss injecting data serialized in XML and JSON, respectively.

2.2. Solution

Several common data formats used for AJAX include raw text, HTML, XML, or JSON. There are ways to escape and abuse each format. The steps to inject the data are the same, but the format is somewhat different.

Imagine a web application that uses AJAX to implement an online chat. Every 10 seconds the browser’s JavaScript calls out to the server and retrieves whatever chat messages have been posted. The response comes back in HTML that looks like Example 1.

Example 1. HTML source of AJAX-based chat
<tr><th>jsmith</th><td>are you going to the show?</td></tr>
<tr><th>mjones</th><td>yeah, mike's driving</td></tr>
<tr><th>jsmith</th><td>can I hitch a ride?</td></tr>
<tr><th>mjones</th><td>sure. be at mike's at 6</td></tr>

The user IDs (jsmith and mjones) are values that the users theoretically can control. When they sign up, or perhaps after they have signed up, they can set their user ID. Now further imagine that the application, on the sign-up page, safely displays the user ID, but allows dangerous characters. That is, if the user types jsmith<hr> as their user ID, the system will display jsmith%3chr%3e, which is safe. However, the application stores the value as jsmith<hr> in the database. In this sort of situation, our test will work well.

The first step of a general test method is to identify data that is retrieved via an AJAX call, rather than data that is delivered when the page first loads. To identify the data AJAX retrieves. Typically such data is the result of other forms of application input—either user input, external RSS feeds, or data delivered via an external API. The easiest case is when a user can submit data through a normal form and that data is stored in the database, then retrieved and delivered via an AJAX call in another place in the application. In our case, it is the polling request for recent chat lines.

The next step is to examine the source code of the web page and see how the returned data is used. In this recipe, we’re discussing plain text or HTML. In our example, the latest chat messages come to us formatted in HTML.

You’ll need to identify this input source and submit a string that breaks your particular data format. Each data format requires a different form of injection string. You’ll recognize that your injection test is successful when you see page content in places it shouldn’t be or entirely new page elements.

Injecting raw text

Raw text is the easiest format into which one can inject data. It’s also identical to HTML injection and so potentially already caught by existing input validation, but you should check anyway. If your AJAX returns raw text and displays it directly on screen, try inserting any normal HTML tag. The <hr> tag works in a pinch; it’s short and easily visible. On the other hand, <script>alert('this is an xss attack');</script> is a slightly more malicious example.

Injecting HTML

HTML injection is fundamentally equivalent to raw text injection, with the one exception that your injection string might arrive as an attribute within an HTML tag. In this case, inspect the AJAX response to determine where your data arrives, and then include the appropriate HTML escape characters. For example, if your AJAX response returns your email address in this HTML: <href="mailto:YOUR_STRING_HERE">Email Address</href>, then you’ll need to include the characters "> prior to normal HTML injection.

2.3. Discussion

Because data serialization can be a ripe area for attack within AJAX applications, avoid writing your own data serialization code. Whenever possible, use standard JSON or XML parsing libraries. They are available for most languages.

A common maxim is that one must make tradeoffs between security and convenience. With the proper libraries, one doesn’t have to sacrifice the convenience of the JSON format for security’s sake, although there’s danger in evaluated JSON code or, returning JSON data without checking for proper authentication.
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