Cross-Site Scripting in Java

Alta CWE-79 Draft — pending review

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Explicación simple

Imagine a bulletin board where someone posts an instruction that forces the viewer's computer to secretly send their session cookie to an attacker.

output encoding: Converting special markup characters into safe entities so browsers display them as text rather than script.

Qué está pasando

XSS in Java web applications occurs when JSP, Thymeleaf, or FreeMarker templates render request parameters directly into response streams without encoding.

Impacto en el mundo real

In 2005, the Samy XSS worm infected over 1 million user profiles on MySpace in under 20 hours, forcing the platform offline.

Documented historical AppSec case study.

Vulnerable vs. corregido

VULNERABLE



  Welcome, <%= request.getParameter("name") %>

FIXED



  Welcome,

Por qué funciona la solución

JSTL and Thymeleaf th:text convert sensitive HTML characters (<, >, &, ") into inert HTML entities.

Puntos críticos específicos

Thymeleaf th:utext

th:utext explicitly disables escaping. Use th:text for untrusted data.

Conceptos errneos comunes

"Spring Boot automatically prevents all XSS"

Spring handles URL and JSON encoding, but template rendering depends on proper tag usage.

Cómo comprobar si está afectado

grep -rn "<%=.*getParameter" --include="*.jsp" . grep -rn "th:utext" --include="*.html" .

Scan JSP and template views using FindSecBugs (rule XSS_REQUEST_PARAMETER_TO_JSP_WRITER).

Lista de comprobación de prevención

✓Use in JSP or th:text in Thymeleaf
✓Run FindSecBugs in CI to catch unescaped output

Preguntas frecuentes

How should rich user HTML be handled in Java?

Use OWASP Java HTML Sanitizer to strip dangerous tags and attributes before rendering.

Referencias

Ver en: Java Python JavaScript Java Go PHP C# Rust

Ver también: SQL Injection Command InjectionPath Traversal