Vulnerable Code:
Java Example:
@WebServlet("/search")
public class SearchServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String query = request.getParameter("q");
PrintWriter out = response.getWriter();
out.println("<html><body>");
out.println("<h1>Search Results for: " + query + "</h1>");
out.println("</body></html>");
}
}
Reason for Vulnerability:
The user input (query) is directly embedded into the HTML output without any sanitization, allowing potential injection of malicious scripts.
Fixed Code:
javaCopyimport org.owasp.encoder.Encode;
@WebServlet("/search")
public class SearchServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String query = request.getParameter("q");
PrintWriter out = response.getWriter();
out.println("<html><body>");
out.println("<h1>Search Results for: " + Encode.forHtml(query) + "</h1>");
out.println("</body></html>");
}
}
Reason for Fix:
The fixed code uses the OWASP Java Encoder library to properly encode the user input for HTML context, preventing XSS attacks.
Java Example
Vulnerable Code:
javaCopy@Controller
public class CommentController {
@PostMapping("/addComment")
public String addComment(@RequestParam String comment, Model model) {
model.addAttribute("latestComment", comment);
return "comments";
}
}
<!-- comments.html -->
<div th:utext="${latestComment}"></div>
Reason for Vulnerability:
The Thymeleaf template uses th:utext to render the comment, which does not escape HTML entities, allowing potential XSS attacks.
Fixed Code:
javaCopy@Controller
public class CommentController {
@PostMapping("/addComment")
public String addComment(@RequestParam String comment, Model model) {
model.addAttribute("latestComment", comment);
return "comments";
}
}
<!-- comments.html -->
<div th:text="${latestComment}"></div>
Reason for Fix:
The fixed code uses th:text instead of th:utext in the Thymeleaf template. This automatically escapes HTML entities, preventing XSS attacks.
JavaScript Example
Vulnerable Code:
javascriptCopyconst express = require('express');
const app = express();
app.get('/welcome', (req, res) => {
const name = req.query.name;
res.send(`<h1>Welcome, ${name}!</h1>`);
});
app.listen(3000, () => console.log('Server running on port 3000'));
Reason for Vulnerability:
The user input (name) is directly inserted into the HTML response without any sanitization, allowing potential XSS attacks.
Fixed Code:
javascriptCopyconst express = require('express');
const escapeHtml = require('escape-html');
const app = express();
app.get('/welcome', (req, res) => {
const name = req.query.name;
res.send(`<h1>Welcome, ${escapeHtml(name)}!</h1>`);
});
app.listen(3000, () => console.log('Server running on port 3000'));
Reason for Fix:
The fixed code uses the escape-html package to sanitize the user input before inserting it into the HTML response, preventing XSS attacks.
Reflected XSS in JavaScript
Vulnerability: Reflected Cross-Site Scripting (XSS)
Vulnerable Code:
javascriptCopy codeconst userInput = getParameterByName('input');
document.getElementById('output').innerHTML = userInput;
Reason for vulnerability: Directly inserting user input into the HTML without sanitization or encoding.
Fixed Code:
javascriptCopy codeconst userInput = getParameterByName('input');
document.getElementById('output').innerText = userInput;
Reason for fix: Using innerText instead of innerHTML escapes the input, preventing XSS.
Stored XSS in Java
Vulnerability: Stored Cross-Site Scripting (XSS)
Vulnerable Code:
javaCopy codeString comment = request.getParameter("comment");
response.getWriter().println(comment);
Reason for vulnerability: User input is directly displayed in the response without encoding, allowing malicious scripts to be executed.
Fixed Code:
javaCopy codeString comment = request.getParameter("comment");
response.getWriter().println(StringEscapeUtils.escapeHtml4(comment));
Reason for fix: Escaping the user input using StringEscapeUtils.escapeHtml4 ensures that any HTML or JavaScript code in the input is neutralized.
Python (Flask)
Vulnerable Code:
pythonCopy code@app.route('/greet')
def greet():
name = request.args.get('name')
return f'Hello {name}'
Reason for vulnerability: User input is directly included in the response, allowing XSS.
Fixed Code:
pythonCopy code@app.route('/greet')
def greet():
name = request.args.get('name')
return f'Hello {escape(name)}'
Reason for fix: Escape user input before including it in the response.
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