Skip to main content

XPath injection

Need​

Prevention of XPath injection attacks

Context​

  • Usage of Java for building robust and scalable applications
  • Usage of javax.servlet for Java web application development
  • Usage of org.w3c.dom for manipulating and interacting with the Document Object Model (DOM) in web browsers.
  • Usage of javax.xml.parsers for XML parsing and manipulation
  • Usage of javax.xml.xpath for XPath-based XML processing in Java

Description​

Non compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import org.w3c.dom.*;
import javax.xml.parsers.*;
import javax.xml.xpath.*;

public class XPathInjectionServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        try {
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            Document doc = builder.parse(getServletContext().getResourceAsStream("/WEB-INF/users.xml"));

            XPathFactory xPathfactory = XPathFactory.newInstance();
            XPath xpath = xPathfactory.newXPath();
            XPathExpression expr = xpath.compile("//user[username/text()='" + username + "' and password/text()='" + password + "']");

            NodeList nodes = (NodeList) expr.evaluate(doc, XPathConstants.NODESET);
            if (nodes.getLength() > 0) {
                // User exists, proceed with login
            } else {
                // Invalid login attempt
            }
        } catch (Exception e) {
            // Handle error
        }
    }
}

The above code is a simple servlet implementation that takes username and password as input from the HTTP request parameters and uses them to perform an XPath query on an XML document to authenticate the user.

The vulnerability lies in the way the XPath query is constructed. The username and password are directly concatenated into the XPath query without any validation or sanitization. This allows an attacker to inject malicious XPath expressions via the request parameters, leading to an XPath Injection vulnerability.

For example, an attacker could provide a username such as ' or '1'='1 which would make the XPath query return all users, effectively bypassing the authentication.

This vulnerability can lead to unauthorized access, information disclosure, or even data corruption if the XML data store supports XPath-based modifications. It's a serious security flaw that needs to be addressed to prevent potential exploits.

Steps​

  • Validate and sanitize user input: Before using the username and password parameters in the XPath expression, ensure that they are properly validated and sanitized to prevent any malicious input.
  • Use parameterized queries: Instead of concatenating the username and password directly into the XPath expression, use parameterized queries or prepared statements to bind the values securely.
  • Implement input validation: Implement input validation to ensure that the username and password meet the required criteria and do not contain any malicious characters or patterns.
  • Limit privileges: Ensure that the user executing the XPath query has the minimum required privileges to access and manipulate the XML data. Avoid using privileged accounts for this purpose.
  • Implement access controls: Implement access controls to restrict the access to the XML data based on user roles and permissions.
  • Use secure XML parsing libraries: Use secure XML parsing libraries that provide protection against XPath injection attacks, such as libraries that automatically escape special characters in XPath expressions.
  • Regularly update and patch dependencies: Keep the XML parsing libraries and other dependencies up to date with the latest security patches to mitigate any known vulnerabilities.
  • Monitor and log XPath queries: Implement logging and monitoring mechanisms to track and analyze XPath queries executed by the application. This can help detect any suspicious or malicious activities.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import org.w3c.dom.*;
import javax.xml.parsers.*;
import javax.xml.xpath.*;

public class XPathInjectionServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        // Validate and sanitize user input
        if (isValid(username) && isValid(password)) {
            try {
                DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
                DocumentBuilder builder = factory.newDocumentBuilder();
                Document doc = builder.parse(getServletContext().getResourceAsStream("/WEB-INF/users.xml"));

                XPathFactory xPathfactory = XPathFactory.newInstance();
                XPath xpath = xPathfactory.newXPath();
                XPathExpression expr = xpath.compile("//user[username/text()=:username and password/text()=:password]");

                // Use parameterized queries
                ((XPathExpression) expr).setParameter("username", username);
                ((XPathExpression) expr).setParameter("password", password);

                NodeList nodes = (NodeList) expr.evaluate(doc, XPathConstants.NODESET);
                if (nodes.getLength() > 0) {
                    // User exists, proceed with login
                } else {
                    // Invalid login attempt
                }
            } catch (Exception e) {
                // Handle error
            }
        }
    }

    // Implement input validation
    private boolean isValid(String input) {
        // Add validation logic here
        return true;
    }
}

The updated code includes several measures to prevent XPath injection attacks:

  1. Input Validation and Sanitization: The isValid method is used to validate and sanitize the user input. This method should contain the logic to ensure that the input meets the required criteria and does not contain any malicious characters or patterns.

  2. Parameterized Queries: Instead of concatenating the username and password directly into the XPath expression, parameterized queries are used to bind the values securely. This is done using the setParameter method.

Please note that the isValid method in this example is a placeholder and should be replaced with actual validation and sanitization logic. Also, the XPathExpression class does not natively support parameterized queries. You may need to use a library that supports this feature or implement it manually.

Remember to implement access controls, limit privileges, use secure XML parsing libraries, regularly update and patch dependencies, and monitor and log XPath queries as part of your overall security strategy.

References​

Last updated on