Unsanitized User Input

Unsanitized User Input

September 11, 2021

example java server servlets post

This example uses an HTML form to create a POST request containing the user’s name.

index.html contains an HTML form:

<!DOCTYPE html>
    <h1>Enter some input:</h1>
    <form action="/user-input-unsanitized/form" method="POST">
      <input type="text" name="data" value="<h1>oh no</h1>">
      <input type="submit" value="Submit">

FormServlet.java handles the POST request by outputting the user’s input directly to the response:

package io.happycoding.servlets;

import java.io.IOException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

public class FormServlet extends HttpServlet {

  public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws IOException {

    String data = request.getParameter("data");

    response.getWriter().println("You entered: " + data);

input form

Because the servlet does not sanitize the data, the HTML is rendered in the output:

html output

See the server libraries tutorial and the sanitizing user input tutorial for more information about why this is bad, and what to do about it.

Sanitizing User Input Examples


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