Deserialization and malicious file upload vulnerability prevention in Java

Deserialization and malicious file upload vulnerability prevention in Java

Introduction:
With the development of the Internet, network security issues have become increasingly prominent, some of which are common The vulnerability attacks are deserialization vulnerability and malicious file upload vulnerability. This article will focus on the principles of these two vulnerabilities and how to prevent them, and provide some code examples.

1. Principle of Deserialization Vulnerability
In Java, serialization and deserialization can be used to achieve persistent storage of objects. Serialization is the process of converting an object into a stream of bytes, while deserialization is the process of converting a stream of bytes into an object again. However, the deserialization process has security risks, and malicious attackers can execute arbitrary code by constructing malicious serialized data.

The reason for the deserialization vulnerability is that when a class is serialized, its related properties, methods and behaviors are saved in the serialized byte stream. During the deserialization process, the Java virtual machine attempts to restore the byte stream into an object. An attacker can trigger vulnerabilities in the program and execute unauthorized code by constructing specific serialized data.

To demonstrate an instance of the deserialization vulnerability, the following is a simple example:

import java.io.FileInputStream;
import java.io.ObjectInputStream;

public class DeserializeExample {
    public static void main(String[] args) {
        try {
            FileInputStream fileIn = new FileInputStream("malicious.ser");
            ObjectInputStream in = new ObjectInputStream(fileIn);
            Object obj = in.readObject();
            in.close();
            fileIn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In the above example, we read an object from a file named "malicious.ser" . However, if an attacker constructs a malicious "malicious.ser" file containing malicious code, the malicious code will be executed during deserialization.

Here’s how to protect against deserialization vulnerabilities:

1. Do not receive serialized data from untrusted sources, only receive data from reliable sources.
2. Do not use the default serialization mechanism, but use a custom deserialization processing method.
3. Perform input validation for deserialization and only accept data that conforms to the expected format.

2. Principle of Malicious File Upload Vulnerability
Malicious file upload vulnerability means that the attacker uploads malicious files to the server and bypasses security restrictions through the legal file upload interface. Once the malicious file is successfully uploaded, the attacker can execute malicious code by accessing the file.

The reason for the malicious file upload vulnerability is that when many developers implement the file upload function, they usually only verify the file extension, but do not verify the file content and type. An attacker can modify the file extension or disguise the file to bypass the verification mechanism. Once these files are accepted and stored by the server, the attacker can execute malicious code by accessing the uploaded files.

The following is a simple file upload example:

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

public class FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);
        if (isMultipart) {
            try {
                DiskFileItemFactory factory = new DiskFileItemFactory();
                ServletFileUpload upload = new ServletFileUpload(factory);
                List<FileItem> items = upload.parseRequest(request);
                for (FileItem item : items) {
                    if (!item.isFormField()) {
                        String fileName = item.getName();
                        File uploadedFile = new File("upload/" + fileName);
                        item.write(uploadedFile);
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

In the above example, we used the Apache Commons FileUpload library to implement file upload. However, this code does not verify the uploaded file type and content, allowing an attacker to upload malicious files.

Methods to prevent malicious file upload vulnerabilities are as follows:

1. Conduct strict verification of uploaded files, including file type and content.
2. Use randomly generated file names to prevent attackers from guessing file names.
3. Store uploaded files in non-Web root directories to prevent direct access.

Conclusion:
Deserialization vulnerabilities and malicious file upload vulnerabilities are two common security risks in Java programs. By strengthening the verification of the deserialization process and file upload process, we can effectively prevent these vulnerabilities. Developers should always pay attention to cybersecurity issues and update their knowledge frequently to keep their code and users safe.

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