Cross Site Scripting Flaw
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Last revision (mm/dd/yy): 09/21/2009
Cross site Scripting (XSS) attacks are a type of injection problem, in which malicious scripts are injected into otherwise benign and trusted web sites. Cross site scripting flaws are the most prevalent flaw in web applications today. Cross site scripting attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user in the output it generates without validating or encoding it.
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.
How to Determine If You Are Vulnerable
Stored XSS takes hostile data, stores it in a file, a database, or other back end system, and then at a later stage, displays the data to the user, unfiltered. This is extremely dangerous in systems such as CMS, blogs, or forums, where a large number of users will see input from other individuals.
Using XmlHttpRequest (AJAX), it is sometimes possible to get around a browser’s same source origination policy - thus forwarding victim data to hostile sites, and to create complex worms and malicious zombies that last as long as the browser stays open. AJAX attacks do not have to be visible or require user interaction to perform dangerous cross site request forgery (CSRF) attacks (see CSRF).
OWASP's recommended defenses against XSS are documented in the OWASP XSS (Cross Site Scripting) Prevention Cheat Sheet.
- OWASP Guide to Building Secure Web Applications and Web Services, Data Validation
- OWASP Testing Guide, Testing for Cross site scripting
- The Cross Site Scripting FAQ: http://www.cgisecurity.com/articles/xss-faq.shtml
- XSS Cheat Sheet: http://ha.ckers.org/xss.html
- 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
- How to Build an HTTP Request Validation Engine (J2EE validation with Stinger)
- XSSed - Cross-Site Scripting (XSS) Information and Mirror Archive of Vulnerable Websites http://www.xssed.com