All web applications allow some form of rich data, but that rich data has become a key part of Web 2.0. Data is "rich" if it allows markup, special characters, images, formatting, and other complex syntax. This richness allows users create new and innovative content and services.
Unfortunately, richness affords attackers an unprecedented opportunity to bury attacks targeting users and systems downstream of the offending application or service supplier.
Even in the early days of Web 2.0, this is a huge problem: at least half the vulnerabilities that plague web applications and web services involve some form of injection.
The software industry already has a poor reputation for delivering software that doesn't work or contains security holes. Imagine how bad things will get in a world where people pick up vulnerabilities and hacks by connecting to dynamic web sites and "mashing up" applications.
Here are some things to bear in mind, to protect both your reputation and your users' systems and data.
Unscramble the egg
One of the oldest security principles in the book is you should always keep code and data separate. Once you mix them together, it's almost impossible separate them again. Unfortunately, most of the data formats and protocols we're using today mixing code and data like a bad DJ hashing up a cross fade. That's why injection is going to be with us for a long time.
There's no simple validation that can detect all the variants of code in all these places. However, you have to have a full security parser to validate HTML data before you can use it.
Untrusted data is code
SQL injection is just an attacker sneaking malicious SQL inside user inputs that gets concatenated into a query. Injected code isn't just a snippet anymore - it might be a huge program.
What's important to remember is that every piece of untrusted data - every form field, every URL parameter, every cookie, and every XML parameter - might contain injected code for some downstream system. If you're not absolutely sure there is no code in the data - and that's pretty much impossible - then for all you know, that data is really a little program. There is no such thing as plain old "data" anymore.