Cross-Site Scripting in Java

Alta CWE-79 Draft — pending review

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Explicação simples (ELI5)

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.

O que está acontecendo

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

Impacto no 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.

Vulnerável vs. corrigido

VULNERABLE



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

FIXED



  Welcome,

Por que a correção funciona

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

Armadilhas específicas da linguagem

Thymeleaf th:utext

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

Mitos comuns

"Spring Boot automatically prevents all XSS"

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

Como verificar se você foi afetado

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 verificação de prevenção

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

Perguntas frequentes

How should rich user HTML be handled in Java?

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

Referências

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

Veja também: SQL Injection Command InjectionPath Traversal