Web security is at the top of customers' minds after many well-publicized personal data breaches, but the people who actually build Web applications aren't paying much attention to security, experts say.

"They're totally ignoring it," says IT consultant Joel Snyder. "When you go to your Web site design team, what you're looking for is people who are creative and able to build these interesting Web sites... That's No. 1, and No. 9 on the list would be that it's a secure Web site." Read the latest WhitePaper - State of Internet Security Report on Protecting Enterprise Systems

The biggest problem is designers aren't building walls within Web applications to partition and validate data moving between parts of the system, he says.

Security is usually something that's considered after a site is built rather than before it is designed, agrees Khalid Kark, senior analyst at Forrester.

"I'd say the majority of Web sites are hackable," Kark says. "The crux of the problem is security isn't thought of at the time of creating the application."

That's a big problem, and it's one the nonprofit Open Web Application Security Project (OWASP) is trying to solve. An OWASP report called "The Ten Most Critical Web Application Security Vulnerabilities" was issued this year to raise awareness about the biggest security challenges facing Web developers.

The first version of the list was released in 2004, but OWASP Chairman Jeff Williams says Web security has barely improved. New technologies such as AJAX and Rich Internet Applications that make Web sites look better also create more attack surfaces, he says. Convincing businesses their Web sites are insecure is no easy task, though.

"It's frustrating to me, because these flaws are so easy to find and so easy to exploit," says Williams, who is also CEO and co-founder of Aspect Security. "It's like missing a wall on a house."

Here is a summary of OWASP's top 10 Web vulnerabilities, including a description of each problem, real-world examples and how to fix the flaws.

1. Cross site scripting (XSS)

The problem: The "most prevalent and pernicious" Web application security vulnerability, XSS flaws happen when an application sends user data to a Web browser without first validating or encoding the content. This lets hackers execute malicious scripts in a browser, letting them hijack user sessions, deface Web sites, insert hostile content and conduct phishing and malware attacks.

Attacks are usually executed with JavaScript, letting hackers manipulate any aspect of a page. In a worst-case scenario, a hacker could steal information and impersonate a user on a bank's Web site, according to Snyder.

Real-world example: PayPal was targeted last year when attackers redirected PayPal visitors to a page warning users their accounts had been compromised. Victims were redirected to a phishing site and prompted to enter PayPal login information, Social Security numbers and credit card details. PayPal said it closed the vulnerability in June 2006.

How to protect users: Use a whitelist to validate all incoming data, which rejects any data that's not specified on the whitelist as being good. This approach is the opposite of blacklisting, which rejects only inputs known to be bad. Additionally, use appropriate encoding of all output data. "Validation allows the detection of attacks, and encoding prevents any successful script injection from running in the browser," OWASP says.