xSS vulnerabilities are not just as simple as pop-up windows. This article will first talk about the form of triggering the vulnerability. In another issue, we will talk about how harmful xSS is!
Reflected xSS vulnerability
Reflected xSS vulnerabilities occur when an attacker adds malicious code to a web page request, and the request immediately generates a malicious response in the user’s browser. This attack often requires the user to click on a malicious link first to complete.
A simple Java code example of a reflected xSS vulnerability can be as follows:
Servlet code:
javaCopy Codeimport javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
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public class xSSExampleServlet extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
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//Receive URL parameters
String name = request.getParameter("name");
out.println("<html><body>");
out.println("<h1>Welcome, " + name + "!</h1>"); // The generation of reflected xSS vulnerability
out.println("</body></html>");
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out.close();
}
}
This code example will print the “name” parameter provided by the user in the URL directly to the page, allowing an attacker to insert JavaScript code in the URL.
For example, an attacker could create a link like this: http://yourwebsite.com/xSSExampleServlet?name=.
When a user clicks on this link, the server returns a page containing malicious JavaScript code. This JavaScript code will be executed in the user’s browser, resulting in an xSS attack.
This is just a simple example. In fact, the JavaScript code that the attacker may insert can be more complex and can steal the user’s cookies, session information, etc.
The solution is actually quite simple, all user input should be properly encoded or filtered. In Java, you can use some libraries, such as OWASP’s Java Encoder library for HTML encoding.
Stored xSS vulnerability
Stored xSS is an attack method in which an attacker stores a malicious script on the server. When another user browses to the page with the malicious script stored, the script is executed by the browser.
The following is a simple Java code example of a stored xSS vulnerability:
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.sql.*;
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public class xSSExampleServlet extends HttpServlet {
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public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
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String user = request.getParameter("username");
String comment = request.getParameter("comment");
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// Store comments submitted by users into the database
try {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/testDB", "username", "password");
Statement st = conn.createStatement();
st.executeUpdate("INSERT INTO comments (user, comment) values ('" + user + "', '" + comment + "')");
conn.close();
} catch(Exception e) {
System.out.println(e);
}
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out.println("<html><body>");
out.println("<h1>Thank you, " + user + ". Your comment has been submitted.</h1>");
out.println("</body></html>");
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out.close();
}
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public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
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//Here we will display all user comments
// Without filtering or escaping, comment information taken directly from the database and displayed on the page may trigger a stored xSS attack.
try {
Class.forName("com.mysql.jdbc.Driver");
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/testDB", "username", "password");
Statement st = conn.createStatement();
ResultSet rs = st.executeQuery("SELECT user, comment FROM comments");
out.println("<html><body>");
out.println("<h1>Comments</h1>");
while (rs.next()) {
String user = rs.getString("user");
String comment = rs.getString("comment");
out.println("<h2>" + user + " says:</h2>"); // Display the user's comments in the user's browser
out.println("<p>" + comment + "</p>");
}
out.println("</body></html>");
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conn.close();
} catch(Exception e) {
System.out.println(e);
}
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out.close();
}
}
In this example, an attacker can insert JavaScript code in the comment field. This code is stored in the database, and when other users view the comment, this JavaScript code is executed in their browsers.
To prevent xSS attacks, all user input should be appropriately filtered or escaped. For example, you can use the OWASP Java Encoder class library to encode the text output to HTML to prevent malicious scripts from being executed.
DOM type xSS vulnerability
DOM-type xSS attack is an xSS attack carried out on the client side. It is implemented by manipulating the DOM (Document Object Model) through JavaScript. The exploited vulnerabilities mainly exist in JavaScript code.
<!DOCTYPE html>
<html>
<body>
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<p>Click the button to execute a function that will load a URL from the input field.</p>
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<input type="text" id="myInput" value="http://example.com">
<button onclick="myFunction()">Try it</button>
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<p id="demo"></p>
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<script>
function myFunction() {
let x = document.getElementById("myInput").value;
document.getElementById("demo").innerHTML = "You wrote: " + x;
}
</script>
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</body>
</html>
In the above example, user input is inserted directly into the HTML without filtering/encoding, which provides an opportunity for DOM-type xSS attacks. An attacker can insert malicious JavaScript code into the input field, such as
<script>
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var pos = document.URL.indexOf("#") + 1;
var name = document.URL.substring(pos, document.URL.length);
document.write(name);
eval("var a = " + name);
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</script>
This JavaScript code is vulnerable to DOM-type xSS attacks.
The code works like this:
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var pos = document.URL.indexOf("#") + 1;First, find the position of the “#” character in the URL and obtain the position after it. This step attempts to get the fragment part of the URL, which is everything after the “#”. -
var name = document.URL.substring(pos, document.URL.length);Use the substring method to intercept the part starting after the “#” character and ending at the end of the URL, and store it in the name variable. -
document.write(name);displays the storednamevariable. -
eval("var a = " + name);Finally, create a new variable namedawhose value is the value of thenamevariable . Since theeval()function is used here, this opens the door to DOM-type xSS attacks.
Regarding xSS attacks, the attacker can add a piece of JavaScript code after the “#” in the URL, such as #1;alert('xSS');. When the URL is loaded, the eval() function will execute the var a = 1;alert('xSS'); code and pop up a message containing the text ‘xSS’ dialog box.
Attack payload example
http://example.com#1;alert(‘xSS’);
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