Cross-site Scripting (XSS)

Cross-Site Scripting attacks are a type of injection problem, in which malicious scripts are injected into the otherwise benign and trusted web sites. Cross-site scripting (XSS) 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.

An attacker can use XSS to send a 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.

The normal practice is to HTML-escape any user-controlled data during redisplaying in JSP, not during processing the submitted data in servlet nor during storing in DB. In JSP you can use the JSTL (to install it, just drop jstl-1.2.jar in /WEB-INF/lib) <c:out> tag or fn:escapeXml function for this. E.g.

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %> ... <p>Welcome <c:out value="${user.name}" /></p> OR <%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %> ... <input name="username" value="${fn:escapeXml(param.username)}">
This is the classic example: http://127.0.0.1:8088/sitexfer-testing/index.jsp?ERROR_MSG=INVALID_TARGET_DOMAIN&authtype=AUTH_T3%20style=width:expression(alert(1))&appname=DEFAULT_TIER3&TARGET=%24SM%24http%3A%2F%2Fwww.google.com Look carefully to the param authtype, did you see the alert(1)??? If you don't write safe code, you might get an infinite loop in your screen :)

For more details please go to: https://www.owasp.org/index.php/Cross-site_Scripting_(XSS)