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Rich data: the dark side to Web 2.0 applications

With great programming comes great responsibility

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Think about HTML for a minute. We don't really "view" web pages anymore. They're programs. We "run" them in our browser. Even a tiny fragment of HTML can contain a script. Even without JavaScript, web pages can be used for a Cross-Site Request Forgery (CSRF) attacks that perform a series of functions for the attacker. That's a kind of program too.

Would you open an HTML document sent to you?

Chain-gang attack

Attackers have now started to chain these attacks and use them in multiple stages. Consider the recent massive bot attacks that used SQL injection to jam JavaScript code into all the strings in a database. The infected data gets used in a web page and the attack redirects the victim's browser to a site that installs malware. You can imagine attacks that are passed from system to system before they are ever executed and their payload is realized.

One factor that makes detecting these attacks difficult is that the web enables so many different types of encoding. There are more than 100 different character encodings, and we've added higher level encodings such as percent-encoding, HTML-entity encoding, and bbcode on top of those.

The real nightmare here is that anywhere downstream, systems may decode this data and reawaken a dormant attack. So, even if your application isn't vulnerable to injection, someone might use the data from your application or service.

As Web 2.0 continues to mashup data from different sources, the likelihood of these attacks increases.

Stamp out injection

You should view untrusted data as though it's malicious code and treat it accordingly: validate, separate, and encode.

Validate: have a whitelist input validation rule for every input - no exceptions. Not just for form fields, but hidden fields, URL parameters, headers, cookies, and all backend systems.

Separate: don't mix up the data into command strings. Wherever possible, you should use parameterized interfaces, such as PreparedStatement in Java, that prevent injection by keeping code and data separate.

Encode: encode untrusted data for the destination. One thing you absolutely have to do, but almost nobody does, is specify the character set you'll be using. Then you'll need a set of methods that apply the proper encoding for the destination, such as an HTML page, HTML attribute, JavaScript, XPath query, LDAP query and so on.

Show you care

Injection is not a new problem - we've known about it for decades. The body of knowledge on XSS and SQL injection is extensive. If your system forwards an attack to an innocent victim, though, you not only make yourself look bad but in the Web 2.0 world there's a chance your software will lead to wider a proliferation of attacks than was possible in the bad old days.

Jeff Williams is the founder and CEO of Aspect Security and the volunteer chair of the Open Web Application Security Project. His latest project is the Enterprise Security API, a free and open set of foundational security building blocks for developers.

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