The Web Application Hacker's Handbook

Finding and Exploiting Security Flaws

Dafydd Stuttard, Marcus Pinto

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781118175248
• Edition: 2
• Topic: Ciencia de la computación
• Subtopic: Ciberseguridad

The highly successful security book returns with a new edition, completely updated

Web applications are the front door to most organizations, exposing them to attacks that may disclose personal information, execute fraudulent transactions, or compromise ordinary users. This practical book has been completely updated and revised to discuss the latest step-by-step techniques for attacking and defending the range of ever-evolving web applications. You'll explore the various new technologies employed in web applications that have appeared since the first edition and review the new attack techniques that have been developed, particularly in relation to the client side.

• Reveals how to overcome the new technologies and techniques aimed at defending web applications against attacks that have appeared since the previous edition
• Discusses new remoting frameworks, HTML5, cross-domain integration techniques, UI redress, framebusting, HTTP parameter pollution, hybrid file attacks, and more
• Features a companion web site hosted by the authors that allows readers to try out the attacks described, gives answers to the questions that are posed at the end of each chapter, and provides a summarized methodology and checklist of tasks

Focusing on the areas of web application security where things have changed in recent years, this book is the most current resource on the critical topic of discovering, exploiting, and preventing web application security flaws.

Chapter 1
Web Application (In)security

There is no doubt that web application security is a current and newsworthy subject. For all concerned, the stakes are high: for businesses that derive increasing revenue from Internet commerce, for users who trust web applications with sensitive information, and for criminals who can make big money by stealing payment details or compromising bank accounts. Reputation plays a critical role. Few people want to do business with an insecure website, so few organizations want to disclose details about their own security vulnerabilities or breaches. Hence, it is not a trivial task to obtain reliable information about the state of web application security today.

This chapter takes a brief look at how web applications have evolved and the many benefits they provide. We present some metrics about vulnerabilities in current web applications, drawn from the authors' direct experience, demonstrating that the majority of applications are far from secure. We describe the core security problem facing web applications — that users can supply arbitrary input — and the various factors that contribute to their weak security posture. Finally, we describe the latest trends in web application security and how these may be expected to develop in the near future.

The Evolution of Web Applications

In the early days of the Internet, the World Wide Web consisted only of web sites. These were essentially information repositories containing static documents. Web browsers were invented as a means of retrieving and displaying those documents, as shown in Figure 1.1. The flow of interesting information was one-way, from server to browser. Most sites did not authenticate users, because there was no need to. Each user was treated in the same way and was presented with the same information. Any security threats arising from hosting a website were related largely to vulnerabilities in web server software (of which there were many). If an attacker compromised a web server, he usually would not gain access to any sensitive information, because the information held on the server was already open to public view. Rather, an attacker typically would modify the files on the server to deface the web site's contents or use the server's storage and bandwidth to distribute “warez.”

Figure 1.1 A traditional website containing static information

Today, the World Wide Web is almost unrecognizable from its earlier form. The majority of sites on the web are in fact applications (see Figure 1.2). They are highly functional and rely on two-way flow of information between the server and browser. They support registration and login, financial transactions, search, and the authoring of content by users. The content presented to users is generated dynamically on the fly and is often tailored to each specific user. Much of the information processed is private and highly sensitive. Security, therefore, is a big issue. No one wants to use a web application if he believes his information will be disclosed to unauthorized parties.

Figure 1.2 A typical web application

Web applications bring with them new and significant security threats. Each application is different and may contain unique vulnerabilities. Most applications are developed in-house — many by developers who have only a partial understanding of the security problems that may arise in the code they are producing. To deliver their core functionality, web applications normally require connectivity to internal computer systems that contain highly sensitive data and that can perform powerful business functions. Fifteen years ago, if you wanted to make a funds transfer, you visited your bank, and the teller performed the transfer for you; today, you can visit a web application and perform the transfer yourself. An attacker who compromises a web application may be able to steal personal information, carry out financial fraud, and perform malicious actions against other users.

Common Web Application Functions

Web applications have been created to perform practically every useful function you could possibly implement online. Here are some web application functions that have risen to prominence in recent years:

• Shopping (Amazon)
• Social networking (Facebook)
• Banking (Citibank)
• Web search (Google)
• Auctions (eBay)
• Gambling (Betfair)
• Web logs (Blogger)
• Web mail (Gmail)
• Interactive information (Wikipedia)

Applications that are accessed using a computer browser increasingly overlap with mobile applications that are accessed using a smartphone or tablet. Most mobile applications employ either a browser or a customized client that uses HTTP-based APIs to communicate with the server. Application functions and data typically are shared between the various interfaces that the application exposes to different user platforms.

In addition to the public Internet, web applications have been widely adopted inside organizations to support key business functions. Many of these provide access to highly sensitive data and functionality:

• HR applications allowing users to access payroll information, give and receive performance feedback, and manage recruitment and disciplinary procedures.
• Administrative interfaces to key infrastructure such as web and mail servers, user workstations, and virtual machine administration.
• Collaboration software used for sharing documents, managing workflow and projects, and tracking issues. These types of functionality often involve critical security and governance issues, and organizations often rely completely on the controls built into their web applications.
• Business applications such as enterprise resource planning (ERP) software, which previously were accessed using a proprietary thick-client application, can now be accessed using a web browser.
• Software services such as e-mail, which originally required a separate e-mail client, can now be accessed via web interfaces such as Outlook Web Access.
• Traditional desktop office applications such as word processors and spreadsheets have been migrated to web applications through services such as Google Apps and Microsoft Office Live.

In all these examples, what are perceived as “internal” applications are increasingly being hosted externally as organizations move to outside service providers to cut costs. In these so-called cloud solutions, business-critical functionality and data are opened to a wider range of potential attackers, and organizations are increasingly reliant on the integrity of security defenses that are outside of their control.

The time is fast approaching when the only client software that most computer users will need is a web browser. A diverse range of functions will have been implemented using a shared set of protocols and technologies, and in so doing will have inherited a distinctive range of common security vulnerabilities.

Benefits of Web Applications

It is not difficult to see why web applications have enjoyed such a dramatic rise to prominence. Several technical factors have worked alongside the obvious commercial incentives to drive the revolution that has occurred in how we use the Internet:

• HTTP, the core communications protocol used to access the World Wide Web, is lightweight and connectionless. This provides resilience in the event of communication errors and avoids the need for the server to hold open a network connection to every user, as was the case in many legacy client/server applications. HTTP can also be proxied and tunneled over other protocols, allowing for secure communication in any network configuration.
• Every web user already has a browser installed on his computer and mobile device. Web applications deploy their user interface dynamically to the browser, avoiding the need to distribute and manage separate client software, as was the case with pre-web applications. Changes to the interface need to be implemented only once, on the server, and take effect immediately.
• Today's browsers are highly functional, enabling rich and satisfying user interfaces to be built. Web interfaces use standard navigational and input controls that are immediately familiar to users, avoiding the need to learn how each individual application functions. Client-side scripting enables applications to push part of their processing to the client side, and browsers' capabilities can be extended in arbitrary ways using browser extension technologies where necessary.
• The core technologies and languages used to develop web applications are relatively simple. A wide range of platforms and development tools are available to facilitate the development of powerful applications by relative beginners, and a large quantity of open source code and other resources is available for incorporation into custom-built applications.

Web Application Security

As with any new class of technology, web applications have brought with them a new range of security vulnerabilities. The set of most commonly encountered defects has evolved somewhat over time. New attacks have been conceived that were not considered when existing applications were developed. Some problems have become less prevalent as awareness of them has increased. New technologies have been developed that have introduced new possibilities for exploitation. Some categories of flaws have largely gone away as the result of changes made to web browser software.

The most serious attacks against web applications are those that expose sensitive data or gain unrestricted access to the back-end systems on which the application is running. High-profile compromises of this kind continue to occur frequently. For many organizations, however, any attack that causes system downtime is a critical event. Application-level denial-of-service attacks can be used to achieve the same results as traditional resource exhaustion attacks against infrastructure. However, they are often used with more subtle techniques and objectives. They may be used to disrupt a particular user or service to gain a competitive edge against peers in the realms of financial trading, gaming, online bidding, and ticket reservations.

Throughout this evolution, compromises of prominent web applications have remained in the news. Th...