A4 2004 Cross Site Scripting
This article is for the OWASP Top 10 2004
Cross-site scripting (sometimes referred to as XSS) vulnerabilities occur when an attacker uses a web application to send malicious code, generally in the form of a script, to a different end user. These flaws are quite widespread and occur anywhere a web application uses input from a user in the output it generates without validating it. An attacker can use cross site scripting to send malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by your browser and used with that site. These scripts can even rewrite the content of the HTML page.
XSS issues can also be present in the underlying web and application servers as well. Most web and application servers generate simple web pages to display in the case of various errors, such as a 404 ‘page not found’ or a 500 ‘internal server error.’ If these pages reflect back any information from the user’s request, such as the URL they were trying to access, they may be vulnerable to a reflected XSS attack. The likelihood that a site contains XSS vulnerabilities is extremely high. There are a wide variety of ways to trick web applications into relaying malicious scripts. Developers that attempt to filter out the malicious parts of these requests are very likely to overlook possible attacks or encodings. Finding these flaws is not tremendously difficult for attackers, as all they need is a browser and some time. There are numerous free tools available that help hackers find these flaws as well as carefully craft and inject XSS attacks into a target site.
All web servers, application servers, and web application environments are susceptible to cross site scripting.
Examples and References
- The Cross Site Scripting FAQ: http://www.cgisecurity.com/articles/xss-faq.shtml
- CERT Advisory on Malicious HTML Tags: http://www.cert.org/advisories/CA-2000-02.html
- CERT “Understanding Malicious Content Mitigation” http://www.cert.org/tech_tips/malicious_code_mitigation.html
- Understanding the cause and effect of CSS Vulnerabilities: http://www.technicalinfo.net/papers/CSS.html
- OWASP Guide to Building Secure Web Applications and Web Services, Data Validation
- How to Build an HTTP Request Validation Engine (J2EE validation with Stinger)
- Have Your Cake and Eat it Too (.NET validation)
How to Determine If You Are Vulnerable
How to Protect Yourself
The OWASP Filters project is producing reusable components in several languages to help prevent many forms of parameter tampering, including the injection of XSS attacks. OWASP has also released CodeSeeker, an application level firewall. In addition, the OWASP WebGoat training program has lessons on Cross Site Scripting and data encoding.