To their credit, they're mandating that merchants protect Web applications by either Web application firewalls (which aim to protect these apps from exploitation) or have Web applications evaluated by security experts.
I'm a fan (and this should come as no surprise to those who know me) of doing both.
But the best way to avoid security worries is to develop secure software from the jump. While that's easier said than done, a good tool in your arsenal is a Web application vulnerability scanner that will help you to find and fix flaws during production of your applications. Be warned: These products aren't perfect, and don't replace eyes skilled at the art of bug finding. But develop Web code without one (or two) at your own risk.
Here are a few pointers to consider when choosing a Web application security scanner:
Relentless, automated bug finder: Any Web application vulnerability scanner you choose needs to be able to find the broad range of Web application vulnerabilities. These include problems such as unvalidated inputs, cracked access controls, cross-site scripting flaws, buffer overflows, and such.
Act like a user: Any scanner you choose should be smart enough to be able to mimic some the actions of a user. It's tough for developers to predict all of the silly things that users will do with their applications. Developers get caught up in how they think users should use the applications. But as any good hacker knows, the fun (and danger) lurks in trying to bend applications in unexpected directions. So let your Web application scanner login and rip through the (hopefully) preproduction version. You could be amazed at what it finds, and the vulnerabilities it finds after the logon.
These are just a few of the things you want to be on the lookout for. But most important, your scanner needs to be easy to use and maintain. Vulnerabilities and attack methods change, so it needs to be kept current. The best way to keep your Web applications as secure as possible is to develop code with as few flaws as possible.
Web application security is complex, even for experienced developers. This Rolling Review, Strategic Security: Web Applications Scanners, is an excellent place to start.