Ajax (Asynchronous JavaScript and XML), the technology of choice today for building powerful, interactive Web applications, comes at a price. If developers aren't careful they will pay that price in security.

Most developers writing Ajax applications don't work for software companies, but inside large enterprises. Unfortunately a lot of the Ajax community doesn't understand the potential risks, security experts say.

"We're not seeing security consciousness with developers, we're not seeing it with the people writing frameworks, and we're not seeing it with the quality-assurance testers," says Billy Hoffman, lead researcher at security firm SPIdynamics. "Developers assume the client is going to work a certain way and they don't think about what happens if the client doesn't work like that."

But the bad guys do think about security weaknesses, and they're getting progressively better at exploiting them via Ajax.

For the most part, Ajax doesn't present new security problems—although there are a few new types. Mostly it presents old challenges in new ways, says Pete Lindstrom, a senior analyst for Burton Group. First, Ajax can offer a considerably larger attack surface for old vulnerabilities. Also, the lack of familiarity with Ajax security can compound that problem.

The Trouble with Ajax
As compared to Web 1.0 applications, Web 2.0 technologies such as Ajax in effect split an application, to offload a lot of the processing onto the client browser. This makes for a more flexible, responsive application, but it also exposes functionality previously handled on the server.

"The big change is that the amount of logic and smarts that is no longer on the server can expose immature applications to problems," says Kevin Henrickson, director of engineering at Zimbra, which makes an Ajax-based e-mail application.

This split is the main reason Ajax applications have a much larger attack surface than older Web applications.

"From a security standpoint this [division] introduces two flavors of problems," says Brian Chess, chief scientist and a founder of security firm Fortify Software. "First, it's a more complex system. You've got these two smart things interacting and there's more room for all kinds of errors." What's more, "because we have a much richer client interface we have much harder applications to test," Chess adds.

Top Mistakes to Prevent
The first rule of Ajax security, Henrickson says, is protect the server. "That's where you enforce your security. At the end of the day you can't trust the client," he says. "You have to have a way of ensuring that the person is authenticated and that the person making the request is the one you want getting data."