This script is possibly vulnerable to Cross Site Scripting (XSS) attacks.
Quote from The Cross Site Scripting FAQ - http://www.cgisecurity.com/articles/xss-faq.shtml
Websites today are more complex than ever, containing a lot of dynamic content making the experience for the user more enjoyable. Dynamic content is achieved through the use of web applications which can deliver different output to a user depending on their settings and needs. Dynamic websites suffer from a threat that static websites don't, called "Cross Site Scripting" (or XSS dubbed by other security professionals). Currently small informational tidbits about Cross Site Scripting holes exist but none really explain them to an average person or administrator. This FAQ was written to provide a better understanding of this emerging threat, and to give guidance on detection and prevention.
"What is Cross Site Scripting?"
"What does XSS and CSS mean?"
Often people refer to Cross Site Scripting as CSS. There has been a lot of confusion with Cascading Style Sheets (CSS) and cross site scripting. Some security people refer to Cross Site Scripting as XSS. If you hear someone say "I found a XSS hole", they are talking about Cross Site Scripting for certain.
"What are the threats of Cross Site Scripting?"
"What can I do to protect myself as a vendor?"
This is a simple answer. Never trust user input and always filter metacharacters. This will eliminate the majority of XSS attacks. Converting < and > to < and > is also suggested when it comes to script output. Remember XSS holes can be damaging and costly to your business if abused. Often attackers will disclose these holes to the public, which can erode customer and public confidence in the security and privacy of your organization's site. Filtering < and > alone will not solve all cross site scripting attacks and it is suggested you also attempt to filter out ( and ) by translating them to ( and ), and also # and & by translating them to # (#) and & (&).
"What can I do to protect myself as a user?"
"How common are XSS holes?"
Cross site scripting holes are gaining popularity among hackers as easy holes to find in large websites. Websites from FBI.gov, CNN.com, Time.com, Ebay, Yahoo, Apple computer, Microsoft, Zdnet, Wired, and Newsbytes have all had one form or another of XSS bugs.
Every month roughly 10-25 XSS holes are found in commercial products and advisories are published explaining the threat.
"Does encryption protect me?"
Websites that use SSL (https) are in no way more protected than websites that are not encrypted. The web applications work the same way as before, except the attack is taking place in an encrypted connection. People often think that because they see the lock on their browser it means everything is secure. This just isn't the case.
"Can XSS holes allow command execution?"
"What if I don't feel like fixing a CSS/XSS Hole?"
By not fixing an XSS hole this could allow possible user account compromise in portions of your site as they get added or updated. Cross Site Scripting has been found in various large sites recently and have been widely publicized. Left unrepaired, someone may discover it and publish a warning about your company. This may damage your company's reputation, depicting it as being lax on security matters. This of course also sends the message to your clients that you aren't dealing with every problem that arises, which turns into a trust issue. If your client doesn't trust you why would they wish to do business with you?
Your script should filter metacharacters from user input.
Acunetix Cross Site Scripting Attack
VIDEO: How Cross-Site Scripting (XSS) Works
The Cross Site Scripting Faq
OWASP Cross Site Scripting
XSS Filter Evasion Cheat Sheet
Cross site scripting
OWASP PHP Top 5
How To: Prevent Cross-Site Scripting in ASP.NET