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| Code Block | ||||
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public class SampleServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
String visible = request.getParameter("visible");
String hidden = request.getParameter("hidden");
if (visible != null || hidden != null) {
out.println("Visible Parameter:");
out.println( sanitize( visible));
out.println("<br>Hidden Parameter:");
out.println( hidden);
} else {
out.println("<p>");
out.print("<form action=\"");
out.print("SampleServlet\" ");
out.println("method=POST>");
out.println("Parameter:");
out.println("<input type=text size=20 name=visible>");
out.println("<br>");
out.println("<input type=hidden name=hidden value=\'a benign value\'>");
out.println("<input type=submit>");
out.println("</form>");
}
}
/**
* Filter the specified message string for characters that are sensitive
* in HTML. This avoids potential attacks caused by including JavaScript
* codes in the request URL that is often reported in error messages.
*
* @param message The message string to be filtered
*/
public static String sanitize(String message) {
if (message == null) {
return null;
}
char content[] = new char[message.length()];
message.getChars(0, message.length(), content, 0);
StringBuilder result = new StringBuilder(content.length + 50);
for (int i = 0; i < content.length; i++) {
switch (content[i]) {
case '<':
result.append("<");
break;
case '>':
result.append(">");
break;
case '&':
result.append("&");
break;
case '"':
result.append(""");
break;
default:
result.append(content[i]);
}
}
return result.toString();
}
}
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...
if (visible != null || hidden != null) {
out.println("Visible Parameter:");
out.println( sanitize( visible));
out.println("<br>Hidden Parameter:");
out.println( sanitize( hidden));
} else {
...
|
Consequently, when the malicious URL is entered into a browser, the servlet produces:
...