单例模式安全之序列化攻击
什么是序列化攻击呢？
简单说，一个单例对象经过序列化再反序列化后，内存中会存在两个对象，这样单例模式就被破坏。
序列化攻击复现
序列化攻击复过程
- 获取到单例对象
- 对象序列化持久到磁盘
- 反序列化成对象
这里采用JDK的自带的序列化方式
单例实现Serializable接口
package com.fine.serialize;
import java.io.Serializable;
/**
* 单例
* volatile 双重校验
*
* @author finefine at: 2019-05-03 21:43
*/
public class Singleton implements Serializable {
private static final long serialVersionUID = 1L;
private volatile static Singleton INSTANCE;
private Singleton() {
}
public static Singleton getInstance() {
if (INSTANCE==null){
//同步代码块
synchronized (Singleton.class){
if (INSTANCE == null) {
INSTANCE = new Singleton();
}
}
}
return INSTANCE;
}
}
测试代码
package com.fine.serialize;
import java.io.*;
/**
* @author finefine at: 2019-05-03 21:50
*/
public class DeSerailizeAttackTest {
public static void main(String[] args) {
Singleton singleton = Singleton.getInstance();
try (ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("object"));
ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream("object"))) {
//将对象持久化到磁盘中
outputStream.writeObject(singleton);
outputStream.flush();
//从磁盘中反序列化成对象
Singleton singleton1 = (Singleton) inputStream.readObject();
if (singleton == singleton1) {
System.out.println("是同一个对象");
} else {
System.out.println("是两个不同的对象");
}
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
结果输出了两个不同的对象
image-20190503221321745
通过debug 可以看到确实是两个不同的对象
image-20190503221436080
反序列化攻击源码分析
反序列化攻击的问题代码在此
//默认情况下 该方法重新new对象
private Object readOrdinaryObject(boolean unshared)
throws IOException
{
if (bin.readByte() != TC_OBJECT) {
throw new InternalError();
}
ObjectStreamClass desc = readClassDesc(false);
desc.checkDeserialize();
Class<?> cl = desc.forClass();
if (cl == String.class || cl == Class.class
|| cl == ObjectStreamClass.class) {
throw new InvalidClassException("invalid class descriptor");
}
Object obj;
try {
obj = desc.isInstantiable() ? desc.newInstance() : null;
} catch (Exception ex) {
throw (IOException) new InvalidClassException(
desc.forClass().getName(),
"unable to create instance").initCause(ex);
}
passHandle = handles.assign(unshared ? unsharedMarker : obj);
ClassNotFoundException resolveEx = desc.getResolveException();
if (resolveEx != null) {
handles.markException(passHandle, resolveEx);
}
if (desc.isExternalizable()) {
readExternalData((Externalizable) obj, desc);
} else {
readSerialData(obj, desc);
}
handles.finish(passHandle);
//经过上面的代码，新对象已经被new 出来了, hasReadResolveMethod这个方法很关键
//下面的逻辑就是说 如果该类存在一个readResolve 方法就会调用该方法，并重新替换新的对象，如果不存在就直接把new出来的对象返回出去
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
if (unshared && rep.getClass().isArray()) {
rep = cloneArray(rep);
}
if (rep != obj) {
// Filter the replacement object
if (rep != null) {
if (rep.getClass().isArray()) {
filterCheck(rep.getClass(), Array.getLength(rep));
} else {
filterCheck(rep.getClass(), -1);
}
}
handles.setObject(passHandle, obj = rep);
}
}
return obj;
}
/**
*
* 返回该类是否有readResolve方法
*/
boolean hasReadResolveMethod() {
requireInitialized();
return (readResolveMethod != null);
}
由于Singleton 类中不存在readResolve ，所以也就导致反序列化出新的对象了。
解决方法
- 添加readResolve方法
- 使用枚举类
添加readResolve
Singleton 代码
package com.fine.serialize;
import java.io.Serializable;
/**
* 单例
* volatile 双重校验
*
* @author finefine at: 2019-05-03 21:43
*/
public class Singleton implements Serializable {
private static final long serialVersionUID = 1L;
private volatile static Singleton INSTANCE;
private Singleton() {
}
public static Singleton getInstance() {
if (INSTANCE==null){
//同步代码块
synchronized (Singleton.class){
if (INSTANCE == null) {
INSTANCE = new Singleton();
}
}
}
return INSTANCE;
}
//添加的readResolve方法
private Object readResolve() {
return INSTANCE;
}
}
这里测试代码不用更改，看看测试结果
image-20190503225944248
使用枚举类
单例代码
package com.fine.serialize;
import java.io.Serializable;
/**
* @author finefine at: 2019-05-03 23:00
*/
public enum SingletonEnum implements Serializable {
INSTANCE;
private static final long serialVersionUID = 2L;
}
测试代码
package com.fine.serialize;
import java.io.*;
/**
* @author finefine at: 2019-05-03 23:02
*/
public class DeSerailizeEnumAttackTest {
public static void main(String[] args) {
SingletonEnum singleton = SingletonEnum.INSTANCE;
try (ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("object"));
ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream("object"))) {
//将对象持久化到磁盘中
outputStream.writeObject(singleton);
outputStream.flush();
//从磁盘中反序列化成对象
SingletonEnum singleton1 = (SingletonEnum) inputStream.readObject();
if (singleton == singleton1) {
System.out.println("是同一个对象");
} else {
System.out.println("是两个不同的对象");
}
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
测试结果
image-20190504000713071
那为什么这里使用枚举类就可以避免反射攻击呢？深入源码分析
JDK 的反序列化都会调用到这个方法，readObject0对每种类型反序列化都做了不同的实现，当对枚举类进行反序列化时进入到TC_ENUM分支，最终调用readEnum方法
private Object readObject0(boolean unshared) throws IOException {
boolean oldMode = bin.getBlockDataMode();
if (oldMode) {
int remain = bin.currentBlockRemaining();
if (remain > 0) {
throw new OptionalDataException(remain);
} else if (defaultDataEnd) {
/*
* Fix for 4360508: stream is currently at the end of a field
* value block written via default serialization; since there
* is no terminating TC_ENDBLOCKDATA tag, simulate
* end-of-custom-data behavior explicitly.
*/
throw new OptionalDataException(true);
}
bin.setBlockDataMode(false);
}
byte tc;
while ((tc = bin.peekByte()) == TC_RESET) {
bin.readByte();
handleReset();
}
depth++;
totalObjectRefs++;
try {
switch (tc) {
case TC_NULL:
return readNull();
case TC_REFERENCE:
return readHandle(unshared);
case TC_CLASS:
return readClass(unshared);
case TC_CLASSDESC:
case TC_PROXYCLASSDESC:
return readClassDesc(unshared);
case TC_STRING:
case TC_LONGSTRING:
return checkResolve(readString(unshared));
case TC_ARRAY:
return checkResolve(readArray(unshared));
//枚举类
case TC_ENUM:
return checkResolve(readEnum(unshared));
//Object
case TC_OBJECT:
return checkResolve(readOrdinaryObject(unshared));
case TC_EXCEPTION:
IOException ex = readFatalException();
throw new WriteAbortedException("writing aborted", ex);
case TC_BLOCKDATA:
case TC_BLOCKDATALONG:
if (oldMode) {
bin.setBlockDataMode(true);
bin.peek(); // force header read
throw new OptionalDataException(
bin.currentBlockRemaining());
} else {
throw new StreamCorruptedException(
"unexpected block data");
}
case TC_ENDBLOCKDATA:
if (oldMode) {
throw new OptionalDataException(true);
} else {
throw new StreamCorruptedException(
"unexpected end of block data");
}
default:
throw new StreamCorruptedException(
String.format("invalid type code: %02X", tc));
}
} finally {
depth--;
bin.setBlockDataMode(oldMode);
}
}
readEnum 最终调用了Enum.valueOf 返回实例对象
private Enum<?> readEnum(boolean unshared) throws IOException {
if (bin.readByte() != TC_ENUM) {
throw new InternalError();
}
ObjectStreamClass desc = readClassDesc(false);
if (!desc.isEnum()) {
throw new InvalidClassException("non-enum class: " + desc);
}
int enumHandle = handles.assign(unshared ? unsharedMarker : null);
ClassNotFoundException resolveEx = desc.getResolveException();
if (resolveEx != null) {
handles.markException(enumHandle, resolveEx);
}
String name = readString(false);
Enum<?> result = null;
Class<?> cl = desc.forClass();
if (cl != null) {
try {
@SuppressWarnings("unchecked")
//这里找到了实例
Enum<?> en = Enum.valueOf((Class)cl, name);
result = en;
} catch (IllegalArgumentException ex) {
throw (IOException) new InvalidObjectException(
"enum constant " + name + " does not exist in " +
cl).initCause(ex);
}
if (!unshared) {
handles.setObject(enumHandle, result);
}
}
handles.finish(enumHandle);
passHandle = enumHandle;
return result;
}
总结
经过单例模式安全之反射攻击和本片文章的内容，可以发现，使用枚举类实现单例是非常有利的，不用开发者考虑太多其他的因素。从单例的角度及安全的角度来看，枚举单例模式有以下三个特点：
- jvm 底层保证线程安全。
- jvm 底层抑制了反射攻击。
- jdk 序列化方式的特殊处理，防止了反序列化攻击。
