Agile Computing Authors: Pat Romanski, Elizabeth White, Liz McMillan, Yeshim Deniz, Andy Thurai

Related Topics: Agile Computing, Machine Learning

Agile Computing: Article

JavaScript Hijacking: Only 1 Out 12 Popular AJAX Frameworks Prevents It

The first class of vulnerability specific to rich Web apps

Declining Malicious Requests
From the server's perspective, a JavaScript Hijacking attack looks like an attempt at cross-site request forgery, and defenses against cross-site request forgery will also defeat JavaScript Hijacking attacks.

To make it easy to detect malicious requests, every request should include a parameter that is hard for an attacker to guess. One approach is to add the session cookie to the request as a parameter. When the server gets such a request, it can check to be certain the session cookie matches the value in the request parameter. Malicious code does not have access to the session cookie (cookies are also subject to the Same Origin Policy), so there is no easy way for the attacker to craft a request that will pass this test. A different secret can also be used in place of the session cookie. As long as the secret is hard to guess and appears in a context that is accessible to the legitimate application and not accessible from a different domain, it will prevent an attacker from making a valid request.

Some frameworks run only on the client side. In other words, they are written entirely in JavaScript and have no knowledge of the workings of the server. This implies that they do not know the name of the session cookie. Even without knowing the name of the session cookie, they can participate in a cookie-based defense by adding all of the cookies to each request to the server. The following JavaScript fragment outlines this "blind client" strategy:

var httpRequest = new XMLHttpRequest();
var cookies="cookies="+escape(document.cookie);
http_request.open(‘POST', url, true);

The server could also check the HTTP referrer header to make sure the request has originated from the legitimate application and not from a malicious application. Historically speaking, the referrer header has not been reliable, so we don't recommend using it as the basis for any security mechanism.

A server can mount a defense against JavaScript Hijacking by responding only to HTTP POST requests and not responding to HTTP GET requests. This is a defensive technique because the <script> tag always uses GET to load JavaScript from external sources. This defense is also error-prone. The use of GET for better performance is encouraged by Web application experts from Sun and elsewhere. Even frameworks that use POST requests internally, such as GWT, document the steps necessary to support GET requests without mentioning any potential security ramifications. This missing connection between the choice of HTTP methods and security means that, at some point, a programmer may mistake this lack of functionality for an oversight rather than a security precaution and modify the application to respond to GET requests.

Preventing Direct Execution of the Response
To make it impossible for a malicious site to execute a response that includes JavaScript, the legitimate client application can take advantage of the fact that it is allowed to modify the data it receives before executing it, while a malicious application can only execute it using a <script> tag. When the server serializes an object, it should include a prefix (and potentially a suffix) that makes it impossible to execute the JavaScript using a <script> tag. The legitimate client application can remove this extraneous data before running the JavaScript. There are many possible implementations of this approach. We will outline two.

First, the server could prefix each message with the statement:


Unless the client removes this prefix, evaluating the message will send the JavaScript interpreter into an infinite loop. This is the technique Google used to fix the vulnerability Grossman identified. The client searches for and removes the prefix like this:

var object;
var req = new XMLHttpRequest();
req.open("GET", "/object.json",true);
req.onreadystatechange = function () {
if (req.readyState == 4) {
var txt = req.responseText;
if (txt.substr(0,9) == "while(1);") {
txt = txt.substring(10);
object = eval("(" + txt + ")");
req = null;

Second, the server can include comment characters around the JavaScript that have to be removed before the JavaScript is sent to eval(). The following JSON object has been enclosed in a block comment:

[{"fname":"Brian", "lname":"Chess", "phone":"6502135600",
"purchases":60000.00, "email":"[email protected]" }

The client can search for and remove the comment characters like this:

var object;
var req = new XMLHttpRequest();
req.open("GET", "/object.json",true);
req.onreadystatechange = function () {
if (req.readyState == 4) {
var txt = req.responseText;if (txt.substr(0,2) == "/*") {
txt = txt.substring(2, txt.length - 2);
object = eval("(" + txt + ")");
req = null;

Any malicious site that retrieves the sensitive JavaScript via a <script> tag will not gain access to the data it contains.

Conclusion and Recommendations
The JavaScript implementations contained in popular Web browsers have been the source of numerous security problems, so JavaScript Hijacking follows a distinguished list of older attacks, including cross-site scripting and cross-site request forgery. (Bugs in JavaScript implementations have also led to no shortage of browser vulnerabilities.) The fundamental issue that allows JavaScript Hijacking, an eccentricity in the Web browser Same Origin Policy, is also used by benevolent programmers to create legitimate advertisements and application mashups, so it is unlikely that the Same Origin Policy will change in the near future.

The solutions and best practices proposed in this article have been cobbled together in much the same way that rich Web interfaces and mashup concepts have been cobbled together. The Web programming community has stretched the existing technology well past its original purposes, so it is not surprising that we occasionally encounter an unexpected side effect. These workarounds and hacks will be necessary until popular Web browsers support standards that allow for things like secure cross-domain requests and JavaScript sandboxing. Until then, we need to make programmers aware of the risks inherent in communicating confidential data via JavaScript.

To that end, we recommend that all programs that communicate using JavaScript take the following defensive measures:

  • Include a hard-to-guess identifier, such as the session identifier, as part of each request that will return JavaScript. This defeats cross-site request forgery attacks by allowing the server to validate the origin of the request.
  • Include characters in the response that prevent it from being successfully handed off to a JavaScript interpreter without modification. This prevents an attacker from using a <script> tag to witness the execution of the JavaScript.

We need to encourage Web application frameworks to be secure by default. For server-integrated toolkits, this is a matter of changing both the server-side and client-side components. Purely client-side libraries cannot defend against JavaScript Hijacking without support from server-side code, but they can still improve security by making the JavaScript Hijacking risk clear to developers and by including features that make it easy to implement secure server-side code.

If programmers want to create applications that can participate as part of a mashup, they should be required to explicitly disable a security constraint, and the framework should make them aware of the consequences of their actions.

We would like to extend special thanks to a number of people for their feedback on early versions of this article. We thank Jeremiah Grossman for his valuable comments on the technical content of the report, Joe Walker - for a great discussion of DWR security, Alex Russell - for a discussion of the Dojo framework and for highlighting important points missing from the original draft, and Bob Ippolito - for a discussion of the MochiKit framework and suggestions on how to address the issue. Eddie Lee, Adam Murray, and Erik Cabetas helped us proofread the draft and provided insight into their experience with AJAX and AJAX frameworks.

More Stories By Yekaterina Tsipenyuk O'Neil

Yekaterina Tsipenyuk O'Neil is the founding member of the Security Research Group at Fortify Software. She is responsible for performing code audits, identifying and analyzing insecure coding patterns, providing security content for Fortify's software security products, and researching ways to improve the quality of the tools. Yekaterina has a B.S. and an M.S. in computer science from the University of California, San Diego. Her thesis work focused on mobile agent security.

More Stories By Brian Chess

Brian Chess is a founder of Fortify Software and serves as Fortify's chief scientist, where his work focuses on practical methods for creating secure systems. His book, Secure Programming with Static Analysis, shows how static source code analysis is an indispensable tool for getting security right. Brian holds a Ph.D. in computer engineering from the University of California at Santa Cruz. Before settling on security, Brian spent a decade in Silicon Valley working at huge companies and small startups. He has done research on a broad set of topics, ranging from integrated circuit design all the way to delivering software as a service.

More Stories By Jacob West

Jacob West manages Fortify Software’s Security Research Group, which is responsible for building security knowledge into Fortify's products. He brings expertise in numerous programming languages, frameworks and styles together with knowledge about how real-world systems can fail. In addition, he recently co-authored a book, "Secure Programming with Static Analysis," which was published in June 2007. Before joining Fortify, Jacob worked with Professor David Wagner, at the University of California at Berkeley, to develop MOPS (MOdel Checking Programs for Security properties), a static analysis tool used to discover security vulnerabilities in C programs. When he is away from the keyboard, Jacob spends time speaking at conferences and working with customers to advance their understanding of software security.

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.

IoT & Smart Cities Stories
To Really Work for Enterprises, MultiCloud Adoption Requires Far Better and Inclusive Cloud Monitoring and Cost Management … But How? Overwhelmingly, even as enterprises have adopted cloud computing and are expanding to multi-cloud computing, IT leaders remain concerned about how to monitor, manage and control costs across hybrid and multi-cloud deployments. It’s clear that traditional IT monitoring and management approaches, designed after all for on-premises data centers, are falling short in ...
DXWordEXPO New York 2018, colocated with CloudEXPO New York 2018 will be held November 11-13, 2018, in New York City and will bring together Cloud Computing, FinTech and Blockchain, Digital Transformation, Big Data, Internet of Things, DevOps, AI, Machine Learning and WebRTC to one location.
We are seeing a major migration of enterprises applications to the cloud. As cloud and business use of real time applications accelerate, legacy networks are no longer able to architecturally support cloud adoption and deliver the performance and security required by highly distributed enterprises. These outdated solutions have become more costly and complicated to implement, install, manage, and maintain.SD-WAN offers unlimited capabilities for accessing the benefits of the cloud and Internet. ...
Business professionals no longer wonder if they'll migrate to the cloud; it's now a matter of when. The cloud environment has proved to be a major force in transitioning to an agile business model that enables quick decisions and fast implementation that solidify customer relationships. And when the cloud is combined with the power of cognitive computing, it drives innovation and transformation that achieves astounding competitive advantage.
DXWorldEXPO LLC announced today that "IoT Now" was named media sponsor of CloudEXPO | DXWorldEXPO 2018 New York, which will take place on November 11-13, 2018 in New York City, NY. IoT Now explores the evolving opportunities and challenges facing CSPs, and it passes on some lessons learned from those who have taken the first steps in next-gen IoT services.
SYS-CON Events announced today that Silicon India has been named “Media Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Published in Silicon Valley, Silicon India magazine is the premiere platform for CIOs to discuss their innovative enterprise solutions and allows IT vendors to learn about new solutions that can help grow their business.
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
Founded in 2000, Chetu Inc. is a global provider of customized software development solutions and IT staff augmentation services for software technology providers. By providing clients with unparalleled niche technology expertise and industry experience, Chetu has become the premiere long-term, back-end software development partner for start-ups, SMBs, and Fortune 500 companies. Chetu is headquartered in Plantation, Florida, with thirteen offices throughout the U.S. and abroad.
The standardization of container runtimes and images has sparked the creation of an almost overwhelming number of new open source projects that build on and otherwise work with these specifications. Of course, there's Kubernetes, which orchestrates and manages collections of containers. It was one of the first and best-known examples of projects that make containers truly useful for production use. However, more recently, the container ecosystem has truly exploded. A service mesh like Istio addr...