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HTTP request smuggling

Need​

Consistent and secure processing of HTTP requests across all entities involved

Context​

  • Usage of Java for building robust and scalable applications
  • Usage of javax.servlet for building Java web applications
  • Usage of java.io for input and output operations in Java

Description​

Non compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class HttpRequestSmugglingServlet extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletInputStream input = request.getInputStream();
        byte[] buffer = new byte[1024];
        int bytesRead = input.read(buffer);
        String requestBody = new String(buffer, 0, bytesRead);

        // Forward the request to the next entity
        URL url = new URL("http://next-entity.com");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setDoOutput(true);
        conn.setRequestMethod("POST");
        OutputStream os = conn.getOutputStream();
        os.write(requestBody.getBytes());
        os.flush();
        os.close();
    }
}

The above Java code represents a servlet that reads HTTP POST requests and forwards them to another entity. This code is vulnerable to HTTP Request Smuggling because it does not validate or sanitize the incoming request before forwarding it.

The Servlet's doPost method reads the body of the HTTP request into a byte buffer, converts it into a string, and then forwards this string as the body of a new HTTP request to another entity.

The problem arises when the incoming request is malformed. For example, if the request contains ambiguous headers or body content, it can confuse the processing of subsequent requests by the next entity. This is because the next entity might interpret the boundaries of the HTTP requests differently, causing it to process a part of the next request as if it were part of the current one.

This can lead to various security problems, such as cache poisoning, session hijacking, or even remote code execution, depending on the nature of the smuggled request and the configuration of the next entity.

In addition, the code does not handle the case where the input.read(buffer) method returns -1, which indicates that the end of the stream has been reached. This can lead to the creation of a new string with -1 as the length, which would throw a NegativeArraySizeException.

This vulnerability can be exploited by an attacker to smuggle malicious HTTP requests past security controls, potentially causing serious harm to the system.

Steps​

  • Ensure consistent request processing: Make sure that all entities involved in processing the HTTP requests (such as proxies or firewalls) follow the same request delimiting rules.
  • Validate and sanitize input: Implement input validation and sanitization techniques to ensure that the request body is free from any malicious or malformed content.
  • Use a secure HTTP library: Utilize a secure and up-to-date HTTP library that handles HTTP requests and connections securely, reducing the risk of vulnerabilities like HTTP request smuggling.
  • Implement secure communication channels: Use secure communication protocols, such as HTTPS, to encrypt the communication between the client, server, and any intermediate entities.
  • Implement request filtering and monitoring: Implement request filtering mechanisms to detect and block any suspicious or malformed requests that could potentially exploit vulnerabilities like HTTP request smuggling.
  • Regularly update and patch dependencies: Keep all dependencies, including web server software and libraries, up to date with the latest security patches to mitigate any known vulnerabilities.

Compliant code​

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import org.apache.commons.io.IOUtils;
import org.apache.commons.text.StringEscapeUtils;

public class HttpRequestSmugglingServlet extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletInputStream input = request.getInputStream();
        byte[] buffer = new byte[1024];
        int bytesRead = input.read(buffer);
        String requestBody = new String(buffer, 0, bytesRead);

        // Sanitize the request body
        String sanitizedRequestBody = StringEscapeUtils.escapeHtml4(requestBody);

        // Forward the request to the next entity
        URL url = new URL("https://next-entity.com"); // Use HTTPS
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setDoOutput(true);
        conn.setRequestMethod("POST");
        OutputStream os = conn.getOutputStream();
        os.write(sanitizedRequestBody.getBytes());
        os.flush();
        os.close();
    }
}

The updated code includes several changes to mitigate the HTTP request smuggling vulnerability:

  1. Input sanitization: The StringEscapeUtils.escapeHtml4(requestBody) method from Apache Commons Text library is used to sanitize the request body. This method escapes any potentially harmful characters in the requestBody, reducing the risk of HTTP request smuggling.

  2. Secure communication: The URL used to forward the request to the next entity has been changed to use HTTPS instead of HTTP. This ensures that the communication between the server and the next entity is encrypted, making it harder for an attacker to intercept or manipulate the request.

Please note that this code assumes that all entities involved in processing the HTTP requests follow the same request delimiting rules. It is also recommended to implement request filtering mechanisms to detect and block any suspicious or malformed requests, and to keep all dependencies up to date with the latest security patches.

References​

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