Brian Dilllard's "Ajile Ajax" Blog

Reviewers overuse the phrase "required reading," but no other description fits the new book "Ajax Security" (2007, Addison Wesley, 470p). This exhaustive tome from Billy Hoffman and Bryan Sullivan places the specific security concerns of the Ajax programming model in historical perspective. It demonstrates not only new security threats that are unique to Ajax, but established threats that have gained new traction in the Web 2.0 era. It then details both the specific technical solutions and - more importantly - the mindset that are necessary to combat such threats. If you call yourself a professional web developer, you need this book.

Because so many developers have historically overlooked the importance of security, the authors approach their topic for what it is: a remedial subject. They take pains to explain the basic mechanisms by which hackers have exploited insecure web applications over the last decade: cross-site request forgeries, denial of service attacks, cross-site scripting and SQL injection. Then they explain how those mechanisms have changed thanks to the rise of xmlHttpRequest, public APIs, mash-ups and aggregators. If you've ever read a Douglas Crockford rant about the "brokenness" of the web security model and wondered why the guy was such an alarmist, Hoffman and Sullivan are only too happy to provide you with a much-needed wake-up call.

"Ajax Security" is written in a clear, direct style that mixes compelling narrative examples with both high-level technical discussions and granular programming how-tos. The authors even fashion Chapter 2, "The Heist," into a miniature techno-thriller, walking us through a day in the life of a fictitious hacker named Eve who practically cackles like a "Mission: Impossible" villain when she discovers the holes in an Ajax web app. The book's mixture of intro-level concepts, real-world analogies and advanced code examples should be jarring, but isn't, thanks to its conversational tone. "Ajax Security" should therefore prove useful to a broad range of readers:

• Developers who either have failed to develop a working knowledge of application security, or whose knowledge has become outdated after recent technological advances.
• Security professionals who haven't kept pace with the rapidly evolving world of Ajax and other client-side technologies.
• Technology managers who need to understand the risks as well as the rewards of the new technologies their engineers are pitching.
• Business users who seek a better understanding of the technologies that power the web and how they can be exploited for malicious ends.

In order to serve all of these readers, "Ajax Security" spends a few chapters establishing the basics of traditional web security. It's worth slogging through these chapters even if you think you're a hardened veteran. The authors get to their central thesis pretty quickly, during a discussion of the "attack surface" of Ajax applications:

In a nutshell, the attack surface of an Ajax application is essentially the complete attack surface of a traditional Web application plus the complete attack surface of a Web service.... Where are all the secret attacks that can instantly destroy any Ajax application? For better or worse, there aren't any. If just being sure to defend against a particular attack was all there was to Ajax security, then this would be a pretty short book.... [D]efending an Ajax application is really just like defending both a Web application and a Web service - all at the same time. This is the price you must pay for expanding the functionality of your site.

Once Hoffman and Sullivan have spelled out this mission statement, the book kicks into high gear with chapters on the business-logic transparency of Ajax applications; the security vulnerabilities of JavaScript, JSON and even CSS; the risk of client-side storage and offline frameworks; and the security considerations of mashups and aggregator sites. I could fill an entire month's worth of blog posts with all of the individual tools, techniques and surprising facts in this book. Here's a random sampling:

• XHR requests that return raw query results as JSON- or XML-formatted data can make classic SQL injection attacks almost effortless.
• Offline frameworks such as Google Gears can store sensitive data on user machines without providing a GUI for ever purging that data. When a Gears-enabled web app isn't sufficiently locked down, this increases the user's vulnerability considerably.
• Public APIs can provide a trojan horse in which an attacker bypasses a web service's built-in security features by posing as or piggybacking on a trusted API consumer.
• JavaScript is hardly the only vector of attack against Ajax applications. CSS files can expose a host of their own unique vulnerabilities. Image paths in global CSS files can reveal the locations of hidden administrative interfaces, while CSS hijacking can enable phishing scams called "look and feel hacks."

Messrs. Sullivan and Hoffman do more than simply list the vulnerabilities of the Ajax programming model. They also conduct hands-on research into the ways existing companies leverage Ajax and offer advice about how to learn from their mistakes. Consider the following three examples:

Review continues on the next page...